Sch.jl

module Sch

using Distributed
import MemPool
import MemPool: DRef, StorageResource
import MemPool: poolset, storage_available, storage_capacity, storage_utilized, externally_varying
import Statistics: mean
import Random: randperm
import Base: @invokelatest

import ..Dagger
import ..Dagger: Context, Processor, Thunk, WeakThunk, ThunkFuture, ThunkFailedException, Chunk, WeakChunk, OSProc, AnyScope, DefaultScope, LockedObject
import ..Dagger: order, dependents, noffspring, istask, inputs, unwrap_weak_checked, affinity, tochunk, timespan_start, timespan_finish, procs, move, chunktype, processor, default_enabled, get_processors, get_parent, execute!, rmprocs!, addprocs!, thunk_processor, constrain, cputhreadtime
import ..Dagger: @dagdebug
import DataStructures: PriorityQueue, enqueue!, dequeue_pair!, peek

import ..Dagger

const OneToMany = Dict{Thunk, Set{Thunk}}

include("util.jl")
include("fault-handler.jl")
include("dynamic.jl")

mutable struct ProcessorCacheEntry
    gproc::OSProc
    proc::Processor
    next::ProcessorCacheEntry

    ProcessorCacheEntry(gproc::OSProc, proc::Processor) = new(gproc, proc)
end
Base.isequal(p1::ProcessorCacheEntry, p2::ProcessorCacheEntry) =
    p1.proc === p2.proc
function Base.show(io::IO, entry::ProcessorCacheEntry)
    entries = 1
    next = entry.next
    while next !== entry
        entries += 1
        next = next.next
    end
    print(io, "ProcessorCacheEntry(pid $(entry.gproc.pid), $(entry.proc), $entries entries)")
end

const Signature = Vector{Any}

"""
    ComputeState

The internal state-holding struct of the scheduler.

Fields:
- `uid::UInt64` - Unique identifier for this scheduler instance
- `waiting::OneToMany` - Map from downstream `Thunk` to upstream `Thunk`s that still need to execute
- `waiting_data::Dict{Union{Thunk,Chunk},Set{Thunk}}` - Map from input `Chunk`/upstream `Thunk` to all unfinished downstream `Thunk`s, to retain caches
- `ready::Vector{Thunk}` - The list of `Thunk`s that are ready to execute
- `cache::WeakKeyDict{Thunk, Any}` - Maps from a finished `Thunk` to it's cached result, often a DRef
- `valid::WeakKeyDict{Thunk, Nothing}` - Tracks all `Thunk`s that are in a valid scheduling state
- `running::Set{Thunk}` - The set of currently-running `Thunk`s
- `running_on::Dict{Thunk,OSProc}` - Map from `Thunk` to the OS process executing it
- `thunk_dict::Dict{Int, WeakThunk}` - Maps from thunk IDs to a `Thunk`
- `node_order::Any` - Function that returns the order of a thunk
- `worker_time_pressure::Dict{Int,Dict{Processor,UInt64}}` - Maps from worker ID to processor pressure
- `worker_storage_pressure::Dict{Int,Dict{Union{StorageResource,Nothing},UInt64}}` - Maps from worker ID to storage resource pressure
- `worker_storage_capacity::Dict{Int,Dict{Union{StorageResource,Nothing},UInt64}}` - Maps from worker ID to storage resource capacity
- `worker_loadavg::Dict{Int,NTuple{3,Float64}}` - Worker load average
- `worker_chans::Dict{Int, Tuple{RemoteChannel,RemoteChannel}}` - Communication channels between the scheduler and each worker
- `procs_cache_list::Base.RefValue{Union{ProcessorCacheEntry,Nothing}}` - Cached linked list of processors ready to be used
- `signature_time_cost::Dict{Signature,UInt64}` - Cache of estimated CPU time (in nanoseconds) required to compute calls with the given signature
- `signature_alloc_cost::Dict{Signature,UInt64}` - Cache of estimated CPU RAM (in bytes) required to compute calls with the given signature
- `transfer_rate::Ref{UInt64}` - Estimate of the network transfer rate in bytes per second
- `halt::Base.Event` - Event indicating that the scheduler is halting
- `lock::ReentrantLock` - Lock around operations which modify the state
- `futures::Dict{Thunk, Vector{ThunkFuture}}` - Futures registered for waiting on the result of a thunk.
- `errored::WeakKeyDict{Thunk,Bool}` - Indicates if a thunk's result is an error.
- `chan::RemoteChannel{Channel{Any}}` - Channel for receiving completed thunks.
"""
struct ComputeState
    uid::UInt64
    waiting::OneToMany
    waiting_data::Dict{Union{Thunk,Chunk},Set{Thunk}}
    ready::Vector{Thunk}
    cache::WeakKeyDict{Thunk, Any}
    valid::WeakKeyDict{Thunk, Nothing}
    running::Set{Thunk}
    running_on::Dict{Thunk,OSProc}
    thunk_dict::Dict{Int, WeakThunk}
    node_order::Any
    worker_time_pressure::Dict{Int,Dict{Processor,UInt64}}
    worker_storage_pressure::Dict{Int,Dict{Union{StorageResource,Nothing},UInt64}}
    worker_storage_capacity::Dict{Int,Dict{Union{StorageResource,Nothing},UInt64}}
    worker_loadavg::Dict{Int,NTuple{3,Float64}}
    worker_chans::Dict{Int, Tuple{RemoteChannel,RemoteChannel}}
    procs_cache_list::Base.RefValue{Union{ProcessorCacheEntry,Nothing}}
    signature_time_cost::Dict{Signature,UInt64}
    signature_alloc_cost::Dict{Signature,UInt64}
    transfer_rate::Ref{UInt64}
    halt::Base.Event
    lock::ReentrantLock
    futures::Dict{Thunk, Vector{ThunkFuture}}
    errored::WeakKeyDict{Thunk,Bool}
    chan::RemoteChannel{Channel{Any}}
end

const UID_COUNTER = Threads.Atomic{UInt64}(1)

function start_state(deps::Dict, node_order, chan)
    state = ComputeState(Threads.atomic_add!(UID_COUNTER, UInt64(1)),
                         OneToMany(),
                         deps,
                         Vector{Thunk}(undef, 0),
                         WeakKeyDict{Thunk, Any}(),
                         WeakKeyDict{Thunk, Nothing}(),
                         Set{Thunk}(),
                         Dict{Thunk,OSProc}(),
                         Dict{Int, WeakThunk}(),
                         node_order,
                         Dict{Int,Dict{Processor,UInt64}}(),
                         Dict{Int,Dict{Union{StorageResource,Nothing},UInt64}}(),
                         Dict{Int,Dict{Union{StorageResource,Nothing},UInt64}}(),
                         Dict{Int,NTuple{3,Float64}}(),
                         Dict{Int, Tuple{RemoteChannel,RemoteChannel}}(),
                         Ref{Union{ProcessorCacheEntry,Nothing}}(nothing),
                         Dict{Signature,UInt64}(),
                         Dict{Signature,UInt64}(),
                         Ref{UInt64}(1_000_000),
                         Base.Event(),
                         ReentrantLock(),
                         Dict{Thunk, Vector{ThunkFuture}}(),
                         WeakKeyDict{Thunk,Bool}(),
                         chan)

    for k in sort(collect(keys(deps)), by=node_order)
        if istask(k)
            waiting = Set{Thunk}(Iterators.filter(istask, k.syncdeps))
            if isempty(waiting)
                push!(state.ready, k)
            else
                state.waiting[k] = waiting
            end
            state.valid[k] = nothing
        end
    end
    state
end

"""
    SchedulerOptions

Stores DAG-global options to be passed to the Dagger.Sch scheduler.

# Arguments
- `single::Int=0`: (Deprecated) Force all work onto worker with specified id.
`0` disables this option.
- `proclist=nothing`: (Deprecated) Force scheduler to use one or more
processors that are instances/subtypes of a contained type. Alternatively, a
function can be supplied, and the function will be called with a processor as
the sole argument and should return a `Bool` result to indicate whether or not
to use the given processor. `nothing` enables all default processors.
- `allow_errors::Bool=true`: Allow thunks to error without affecting
non-dependent thunks.
- `checkpoint=nothing`: If not `nothing`, uses the provided function to save
the final result of the current scheduler invocation to persistent storage, for
later retrieval by `restore`.
- `restore=nothing`: If not `nothing`, uses the provided function to return the
(cached) final result of the current scheduler invocation, were it to execute.
If this returns a `Chunk`, all thunks will be skipped, and the `Chunk` will be
returned.  If `nothing` is returned, restoring is skipped, and the scheduler
will execute as usual. If this function throws an error, restoring will be
skipped, and the error will be displayed.
"""
Base.@kwdef struct SchedulerOptions
    single::Union{Int,Nothing} = nothing
    proclist = nothing
    allow_errors::Union{Bool,Nothing} = false
    checkpoint = nothing
    restore = nothing
end

"""
    ThunkOptions

Stores Thunk-local options to be passed to the Dagger.Sch scheduler.

# Arguments
- `single::Int=0`: (Deprecated) Force thunk onto worker with specified id. `0`
disables this option.
- `proclist=nothing`: (Deprecated) Force thunk to use one or more processors
that are instances/subtypes of a contained type. Alternatively, a function can
be supplied, and the function will be called with a processor as the sole
argument and should return a `Bool` result to indicate whether or not to use
the given processor. `nothing` enables all default processors.
- `time_util::Dict{Type,Any}`: Indicates the maximum expected time utilization
for this thunk. Each keypair maps a processor type to the utilization, where
the value can be a real (approximately the number of nanoseconds taken), or
`MaxUtilization()` (utilizes all processors of this type). By default, the
scheduler assumes that this thunk only uses one processor.
- `alloc_util::Dict{Type,UInt64}`: Indicates the maximum expected memory
utilization for this thunk. Each keypair maps a processor type to the
utilization, where the value is an integer representing approximately the
maximum number of bytes allocated at any one time.
- `occupancy::Dict{Type,Real}`: Indicates the maximum expected processor
occupancy for this thunk. Each keypair maps a processor type to the
utilization, where the value can be a real between 0 and 1 (the occupancy
ratio, where 1 is full occupancy). By default, the scheduler assumes that this
thunk has full occupancy.
- `allow_errors::Bool=true`: Allow this thunk to error without affecting
non-dependent thunks.
- `checkpoint=nothing`: If not `nothing`, uses the provided function to save
the result of the thunk to persistent storage, for later retrieval by
`restore`.
- `restore=nothing`: If not `nothing`, uses the provided function to return the
(cached) result of this thunk, were it to execute.  If this returns a `Chunk`,
this thunk will be skipped, and its result will be set to the `Chunk`.  If
`nothing` is returned, restoring is skipped, and the thunk will execute as
usual. If this function throws an error, restoring will be skipped, and the
error will be displayed.
- `storage::Union{Chunk,Nothing}=nothing`: If not `nothing`, references a
`MemPool.StorageDevice` which will be passed to `MemPool.poolset` internally
when constructing `Chunk`s (such as when constructing the return value). The
device must support `MemPool.CPURAMResource`. When `nothing`, uses
`MemPool.GLOBAL_DEVICE[]`.
- `storage_root_tag::Any=nothing`: If not `nothing`,
specifies the MemPool storage leaf tag to associate with the thunk's result.
This tag can be used by MemPool's storage devices to manipulate their behavior,
such as the file name used to store data on disk."
- `storage_leaf_tag::MemPool.Tag,Nothing}=nothing`: If not `nothing`,
specifies the MemPool storage leaf tag to associate with the thunk's result.
This tag can be used by MemPool's storage devices to manipulate their behavior,
such as the file name used to store data on disk."
- `storage_retain::Bool=false`: The value of `retain` to pass to
`MemPool.poolset` when constructing the result `Chunk`.
"""
Base.@kwdef struct ThunkOptions
    single::Union{Int,Nothing} = nothing
    proclist = nothing
    time_util::Union{Dict{Type,Any},Nothing} = nothing
    alloc_util::Union{Dict{Type,UInt64},Nothing} = nothing
    occupancy::Union{Dict{Type,Real},Nothing} = nothing
    allow_errors::Union{Bool,Nothing} = nothing
    checkpoint = nothing
    restore = nothing
    storage::Union{Chunk,Nothing} = nothing
    storage_root_tag = nothing
    storage_leaf_tag::Union{MemPool.Tag,Nothing} = nothing
    storage_retain::Bool = false
end

"""
    Base.merge(sopts::SchedulerOptions, topts::ThunkOptions) -> ThunkOptions

Combine `SchedulerOptions` and `ThunkOptions` into a new `ThunkOptions`.
"""
function Base.merge(sopts::SchedulerOptions, topts::ThunkOptions)
    single = topts.single !== nothing ? topts.single : sopts.single
    allow_errors = topts.allow_errors !== nothing ? topts.allow_errors : sopts.allow_errors
    proclist = topts.proclist !== nothing ? topts.proclist : sopts.proclist
    ThunkOptions(single,
                 proclist,
                 topts.time_util,
                 topts.alloc_util,
                 topts.occupancy,
                 allow_errors,
                 topts.checkpoint,
                 topts.restore,
                 topts.storage,
                 topts.storage_root_tag,
                 topts.storage_leaf_tag,
                 topts.storage_retain)
end
Base.merge(sopts::SchedulerOptions, ::Nothing) =
    ThunkOptions(sopts.single,
                 sopts.proclist,
                 nothing,
                 nothing,
                 sopts.allow_errors)
"""
    populate_defaults(opts::ThunkOptions, Tf, Targs) -> ThunkOptions

Returns a `ThunkOptions` with default values filled in for a function of type
`Tf` with argument types `Targs`, if the option was previously unspecified in
`opts`.
"""
function populate_defaults(opts::ThunkOptions, Tf, Targs)
    function maybe_default(opt::Symbol)
        old_opt = getproperty(opts, opt)
        if old_opt !== nothing
            return old_opt
        else
            return Dagger.default_option(Val(opt), Tf, Targs...)
        end
    end
    ThunkOptions(
        maybe_default(:single),
        maybe_default(:proclist),
        maybe_default(:time_util),
        maybe_default(:alloc_util),
        maybe_default(:occupancy),
        maybe_default(:allow_errors),
        maybe_default(:checkpoint),
        maybe_default(:restore),
        maybe_default(:storage),
        maybe_default(:storage_root_tag),
        maybe_default(:storage_leaf_tag),
        maybe_default(:storage_retain),
    )
end

function cleanup(ctx)
end

# Eager scheduling
include("eager.jl")

const WORKER_MONITOR_LOCK = Threads.ReentrantLock()
const WORKER_MONITOR_TASKS = Dict{Int,Task}()
const WORKER_MONITOR_CHANS = Dict{Int,Dict{UInt64,RemoteChannel}}()
function init_proc(state, p, log_sink)
    ctx = Context(Int[]; log_sink)
    timespan_start(ctx, :init_proc, p.pid, 0)
    # Initialize pressure and capacity
    gproc = OSProc(p.pid)
    lock(state.lock) do
        state.worker_time_pressure[p.pid] = Dict{Processor,UInt64}()
        for proc in get_processors(gproc)
            state.worker_time_pressure[p.pid][proc] = 0
        end

        state.worker_storage_pressure[p.pid] = Dict{Union{StorageResource,Nothing},UInt64}()
        state.worker_storage_capacity[p.pid] = Dict{Union{StorageResource,Nothing},UInt64}()
        #= FIXME
        for storage in get_storage_resources(gproc)
            pressure, capacity = remotecall_fetch(gproc.pid, storage) do storage
                storage_pressure(storage), storage_capacity(storage)
            end
            state.worker_storage_pressure[p.pid][storage] = pressure
            state.worker_storage_capacity[p.pid][storage] = capacity
        end
        =#

        state.worker_loadavg[p.pid] = (0.0, 0.0, 0.0)
    end
    if p.pid != 1
        lock(WORKER_MONITOR_LOCK) do
            wid = p.pid
            if !haskey(WORKER_MONITOR_TASKS, wid)
                t = @async begin
                    try
                        # Wait until this connection is terminated
                        remotecall_fetch(sleep, wid, typemax(UInt64))
                    catch err
                        # TODO: Report other kinds of errors? IOError, etc.
                        #if !(err isa ProcessExitedException)
                        #end
                    finally
                        lock(WORKER_MONITOR_LOCK) do
                            d = WORKER_MONITOR_CHANS[wid]
                            for uid in keys(d)
                                try
                                    put!(d[uid], (wid, OSProc(wid), nothing, (ProcessExitedException(wid), nothing)))
                                catch
                                end
                            end
                            empty!(d)
                            delete!(WORKER_MONITOR_CHANS, wid)
                            delete!(WORKER_MONITOR_TASKS, wid)
                        end
                    end
                end
                errormonitor_tracked(t)
                WORKER_MONITOR_TASKS[wid] = t
                WORKER_MONITOR_CHANS[wid] = Dict{UInt64,RemoteChannel}()
            end
            WORKER_MONITOR_CHANS[wid][state.uid] = state.chan
        end
    end

    # Setup worker-to-scheduler channels
    inp_chan = RemoteChannel(p.pid)
    out_chan = RemoteChannel(p.pid)
    lock(state.lock) do
        state.worker_chans[p.pid] = (inp_chan, out_chan)
    end

    # Setup dynamic listener
    dynamic_listener!(ctx, state, p.pid)

    timespan_finish(ctx, :init_proc, p.pid, 0)
end
function _cleanup_proc(uid, log_sink)
    empty!(CHUNK_CACHE) # FIXME: Should be keyed on uid!
    proc_states(uid) do states
        for (proc, state) in states
            istate = state.state
            istate.done[] = true
            notify(istate.reschedule)
        end
        empty!(states)
    end
end
function cleanup_proc(state, p, log_sink)
    ctx = Context(Int[]; log_sink)
    wid = p.pid
    timespan_start(ctx, :cleanup_proc, wid, 0)
    lock(WORKER_MONITOR_LOCK) do
        if haskey(WORKER_MONITOR_CHANS, wid)
            delete!(WORKER_MONITOR_CHANS[wid], state.uid)
        end
    end
    remote_do(_cleanup_proc, wid, state.uid, log_sink)
    timespan_finish(ctx, :cleanup_proc, wid, 0)
end

"Process-local condition variable (and lock) indicating task completion."
const TASK_SYNC = Threads.Condition()

"Process-local set of running task IDs."
const TASKS_RUNNING = Set{Int}()

"Process-local dictionary tracking per-processor total time utilization."
const PROCESSOR_TIME_UTILIZATION = Dict{UInt64,Dict{Processor,Ref{UInt64}}}()

# TODO: "Process-local count of actively-executing Dagger tasks per processor type."

"""
    MaxUtilization

Indicates a thunk that uses all processors of a given type.
"""
struct MaxUtilization end

function compute_dag(ctx, d::Thunk; options=SchedulerOptions())
    if options === nothing
        options = SchedulerOptions()
    end
    ctx.options = options
    if options.restore !== nothing
        try
            result = options.restore()
            if result isa Chunk
                return result
            elseif result !== nothing
                throw(ArgumentError("Invalid restore return type: $(typeof(result))"))
            end
        catch err
            report_catch_error(err, "Scheduler restore failed")
        end
    end

    chan = RemoteChannel(()->Channel(typemax(Int)))
    deps = dependents(d)
    ord = order(d, noffspring(deps))

    node_order = x -> -get(ord, x, 0)
    state = start_state(deps, node_order, chan)

    master = OSProc(myid())

    timespan_start(ctx, :scheduler_init, 0, master)
    try
        scheduler_init(ctx, state, d, options, deps)
    finally
        timespan_finish(ctx, :scheduler_init, 0, master)
    end

    value, errored = try
        scheduler_run(ctx, state, d, options)
    finally
        # Always try to tear down the scheduler
        timespan_start(ctx, :scheduler_exit, 0, master)
        try
            scheduler_exit(ctx, state, options)
        catch err
            @error "Error when tearing down scheduler" exception=(err,catch_backtrace())
        finally
            timespan_finish(ctx, :scheduler_exit, 0, master)
        end
    end

    if errored
        throw(value)
    end
    return value
end

function scheduler_init(ctx, state::ComputeState, d::Thunk, options, deps)
    # setup thunk_dict mappings
    for node in filter(istask, keys(deps))
        state.thunk_dict[node.id] = WeakThunk(node)
        for dep in deps[node]
            state.thunk_dict[dep.id] = WeakThunk(dep)
        end
    end

    # Initialize workers
    @sync for p in procs_to_use(ctx)
        @async begin
            try
                init_proc(state, p, ctx.log_sink)
            catch err
                @error "Error initializing worker $p" exception=(err,catch_backtrace())
                remove_dead_proc!(ctx, state, p)
            end
        end
    end

    # Listen for new workers
    @async begin
        try
            monitor_procs_changed!(ctx, state)
        catch err
            @error "Error assigning workers" exception=(err,catch_backtrace())
        end
    end
end

function scheduler_run(ctx, state::ComputeState, d::Thunk, options)
    @dagdebug nothing :global "Initializing scheduler" uid=state.uid

    safepoint(state)

    # Loop while we still have thunks to execute
    while !isempty(state.ready) || !isempty(state.running)
        if !isempty(state.ready)
            # Nothing running, so schedule up to N thunks, 1 per N workers
            schedule!(ctx, state)
        end

        check_integrity(ctx)

        isempty(state.running) && continue
        timespan_start(ctx, :take, 0, 0)
        @dagdebug nothing :take "Waiting for results"
        chan_value = take!(state.chan) # get result of completed thunk
        timespan_finish(ctx, :take, 0, 0)
        if chan_value isa RescheduleSignal
            continue
        end
        pid, proc, thunk_id, (res, metadata) = chan_value
        @dagdebug thunk_id :take "Got finished task"
        gproc = OSProc(pid)
        safepoint(state)
        lock(state.lock) do
            thunk_failed = false
            if res isa Exception
                if unwrap_nested_exception(res) isa ProcessExitedException
                    @warn "Worker $(pid) died, rescheduling work"

                    # Remove dead worker from procs list
                    timespan_start(ctx, :remove_procs, 0, 0)
                    remove_dead_proc!(ctx, state, gproc)
                    timespan_finish(ctx, :remove_procs, 0, 0)

                    timespan_start(ctx, :handle_fault, 0, 0)
                    handle_fault(ctx, state, gproc)
                    timespan_finish(ctx, :handle_fault, 0, 0)
                    return # effectively `continue`
                else
                    if something(ctx.options.allow_errors, false) ||
                       something(unwrap_weak_checked(state.thunk_dict[thunk_id]).options.allow_errors, false)
                        thunk_failed = true
                    else
                        throw(res)
                    end
                end
            end
            node = unwrap_weak_checked(state.thunk_dict[thunk_id])
            if metadata !== nothing
                state.worker_time_pressure[pid][proc] = metadata.time_pressure
                #to_storage = fetch(node.options.storage)
                #state.worker_storage_pressure[pid][to_storage] = metadata.storage_pressure
                #state.worker_storage_capacity[pid][to_storage] = metadata.storage_capacity
                state.worker_loadavg[pid] = metadata.loadavg
                sig = signature(node, state)
                state.signature_time_cost[sig] = (metadata.threadtime + get(state.signature_time_cost, sig, 0)) ÷ 2
                state.signature_alloc_cost[sig] = (metadata.gc_allocd + get(state.signature_alloc_cost, sig, 0)) ÷ 2
                if metadata.transfer_rate !== nothing
                    state.transfer_rate[] = (state.transfer_rate[] + metadata.transfer_rate) ÷ 2
                end
            end
            state.cache[node] = res
            state.errored[node] = thunk_failed
            if node.options !== nothing && node.options.checkpoint !== nothing
                try
                    @invokelatest node.options.checkpoint(node, res)
                catch err
                    report_catch_error(err, "Thunk checkpoint failed")
                end
            end

            timespan_start(ctx, :finish, thunk_id, (;thunk_id))
            finish_task!(ctx, state, node, thunk_failed)
            timespan_finish(ctx, :finish, thunk_id, (;thunk_id))
        end

        safepoint(state)
    end

    # Final value is ready
    value = state.cache[d]
    errored = get(state.errored, d, false)
    if !errored
        if options.checkpoint !== nothing
            try
                options.checkpoint(value)
            catch err
                report_catch_error(err, "Scheduler checkpoint failed")
            end
        end
    end
    return value, errored
end
function scheduler_exit(ctx, state::ComputeState, options)
    @dagdebug nothing :global "Tearing down scheduler" uid=state.uid

    @sync for p in procs_to_use(ctx)
        @async cleanup_proc(state, p, ctx.log_sink)
    end

    lock(state.lock) do
        close(state.chan)
        notify(state.halt)

        # Notify any waiting tasks
        for (_, futures) in state.futures
            for future in futures
                put!(future, SchedulingException("Scheduler exited"); error=true)
            end
        end
        empty!(state.futures)
    end

    # Let the context procs handler clean itself up
    lock(ctx.proc_notify) do
        notify(ctx.proc_notify)
    end
end

function procs_to_use(ctx, options=ctx.options)
    return if options.single !== nothing
        @assert options.single in vcat(1, workers()) "Sch option `single` must specify an active worker ID."
        OSProc[OSProc(options.single)]
    else
        procs(ctx)
    end
end

check_integrity(ctx) = @assert !isempty(procs_to_use(ctx)) "No suitable workers available in context."

struct SchedulingException <: Exception
    reason::String
end
function Base.show(io::IO, se::SchedulingException)
    print(io, "SchedulingException ($(se.reason))")
end

const CHUNK_CACHE = Dict{Chunk,Dict{Processor,Any}}()

function schedule!(ctx, state, procs=procs_to_use(ctx))
    lock(state.lock) do
        safepoint(state)
        @assert length(procs) > 0

        populate_processor_cache_list!(state, procs)

        # Schedule tasks
        to_fire = Dict{Tuple{OSProc,<:Processor},Vector{Tuple{Thunk,<:Any,<:Any,UInt64,UInt32}}}()
        failed_scheduling = Thunk[]

        # Select a new task and get its options
        task = nothing
        @label pop_task
        if task !== nothing
            timespan_finish(ctx, :schedule, task.id, (;thunk_id=task.id))
        end
        if isempty(state.ready)
            @goto fire_tasks
        end
        task = pop!(state.ready)
        timespan_start(ctx, :schedule, task.id, (;thunk_id=task.id))
        if haskey(state.cache, task)
            if haskey(state.errored, task)
                # An error was eagerly propagated to this task
                finish_failed!(state, task)
            else
                # This shouldn't have happened
                iob = IOBuffer()
                println(iob, "Scheduling inconsistency: Task being scheduled is already cached!")
                println(iob, "  Task: $(task.id)")
                println(iob, "  Cache Entry: $(typeof(state.cache[task]))")
                ex = SchedulingException(String(take!(iob)))
                state.cache[task] = ex
                state.errored[task] = true
            end
            @goto pop_task
        end
        opts = merge(ctx.options, task.options)
        sig = signature(task, state)

        # Calculate scope
        scope = if task.f isa Chunk
            task.f.scope
        else
            if task.options.proclist !== nothing
                # proclist overrides scope selection
                AnyScope()
            else
                DefaultScope()
            end
        end
        for (_,input) in task.inputs
            input = unwrap_weak_checked(input)
            chunk = if istask(input)
                state.cache[input]
            elseif input isa Chunk
                input
            else
                nothing
            end
            chunk isa Chunk || continue
            scope = constrain(scope, chunk.scope)
            if scope isa Dagger.InvalidScope
                ex = SchedulingException("Scopes are not compatible: $(scope.x), $(scope.y)")
                state.cache[task] = ex
                state.errored[task] = true
                set_failed!(state, task)
                @goto pop_task
            end
        end

        fallback_threshold = 1024 # TODO: Parameterize this threshold
        if length(procs) > fallback_threshold
            @goto fallback
        end
        local_procs = unique(vcat([collect(Dagger.get_processors(gp)) for gp in procs]...))
        if length(local_procs) > fallback_threshold
            @goto fallback
        end

        inputs = map(last, collect_task_inputs(state, task))
        opts = populate_defaults(opts, chunktype(task.f), map(chunktype, inputs))
        local_procs, costs = estimate_task_costs(state, local_procs, task, inputs)
        scheduled = false

        # Move our corresponding ThreadProc to be the last considered
        if length(local_procs) > 1
            sch_threadproc = Dagger.ThreadProc(myid(), Threads.threadid())
            sch_thread_idx = findfirst(proc->proc==sch_threadproc, local_procs)
            if sch_thread_idx !== nothing
                deleteat!(local_procs, sch_thread_idx)
                push!(local_procs, sch_threadproc)
            end
        end

        for proc in local_procs
            gproc = get_parent(proc)
            can_use, scope = can_use_proc(task, gproc, proc, opts, scope)
            if can_use
                has_cap, est_time_util, est_alloc_util, est_occupancy =
                    has_capacity(state, proc, gproc.pid, opts.time_util, opts.alloc_util, opts.occupancy, sig)
                if has_cap
                    # Schedule task onto proc
                    # FIXME: est_time_util = est_time_util isa MaxUtilization ? cap : est_time_util
                    proc_tasks = get!(to_fire, (gproc, proc)) do
                        Vector{Tuple{Thunk,<:Any,<:Any,UInt64,UInt32}}()
                    end
                    push!(proc_tasks, (task, scope, est_time_util, est_alloc_util, est_occupancy))
                    state.worker_time_pressure[gproc.pid][proc] += est_time_util
                    @dagdebug task :schedule "Scheduling to $gproc -> $proc"
                    @goto pop_task
                end
            end
        end
        state.cache[task] = SchedulingException("No processors available, try widening scope")
        state.errored[task] = true
        set_failed!(state, task)
        @goto pop_task

        # Fast fallback algorithm, used when the smarter cost model algorithm
        # would be too expensive
        @label fallback
        selected_entry = nothing
        entry = state.procs_cache_list[]
        cap, extra_util = nothing, nothing
        procs_found = false
        # N.B. if we only have one processor, we need to select it now
        can_use, scope = can_use_proc(task, entry.gproc, entry.proc, opts, scope)
        if can_use
            has_cap, est_time_util, est_alloc_util, est_occupancy =
                has_capacity(state, entry.proc, entry.gproc.pid, opts.time_util, opts.alloc_util, opts.occupancy, sig)
            if has_cap
                selected_entry = entry
            else
                procs_found = true
                entry = entry.next
            end
        else
            entry = entry.next
        end
        while selected_entry === nothing
            if entry === state.procs_cache_list[]
                # Exhausted all procs
                if procs_found
                    push!(failed_scheduling, task)
                else
                    state.cache[task] = SchedulingException("No processors available, try widening scope")
                    state.errored[task] = true
                    set_failed!(state, task)
                end
                @goto pop_task
            end

            can_use, scope = can_use_proc(task, entry.gproc, entry.proc, opts, scope)
            if can_use
                has_cap, est_time_util, est_alloc_util, est_occupancy =
                    has_capacity(state, entry.proc, entry.gproc.pid, opts.time_util, opts.alloc_util, opts.occupancy, sig)
                if has_cap
                    # Select this processor
                    selected_entry = entry
                else
                    # We could have selected it otherwise
                    procs_found = true
                    entry = entry.next
                end
            else
                # Try next processor
                entry = entry.next
            end
        end
        @assert selected_entry !== nothing

        # Schedule task onto proc
        gproc, proc = entry.gproc, entry.proc
        est_time_util = est_time_util isa MaxUtilization ? cap : est_time_util
        proc_tasks = get!(to_fire, (gproc, proc)) do
            Vector{Tuple{Thunk,<:Any,<:Any,UInt64,UInt32}}()
        end
        push!(proc_tasks, (task, scope, est_time_util, est_alloc_util, est_occupancy))

        # Proceed to next entry to spread work
        state.procs_cache_list[] = state.procs_cache_list[].next
        @goto pop_task

        # Fire all newly-scheduled tasks
        @label fire_tasks
        for gpp in keys(to_fire)
            fire_tasks!(ctx, to_fire[gpp], gpp, state)
        end

        append!(state.ready, failed_scheduling)
    end
end

"""
Monitors for workers being added/removed to/from `ctx`, sets up or tears down
per-worker state, and notifies the scheduler so that work can be reassigned.
"""
function monitor_procs_changed!(ctx, state)
    # Load current set of procs
    old_ps = procs_to_use(ctx)

    while !state.halt.set
        # Wait for the notification that procs have changed
        lock(ctx.proc_notify) do
            wait(ctx.proc_notify)
        end

        timespan_start(ctx, :assign_procs, 0, 0)

        # Load new set of procs
        new_ps = procs_to_use(ctx)

        # Initialize new procs
        diffps = setdiff(new_ps, old_ps)
        for p in diffps
            init_proc(state, p, ctx.log_sink)

            # Empty the processor cache list and force reschedule
            lock(state.lock) do
                state.procs_cache_list[] = nothing
            end
            put!(state.chan, RescheduleSignal())
        end

        # Cleanup removed procs
        diffps = setdiff(old_ps, new_ps)
        for p in diffps
            cleanup_proc(state, p, ctx.log_sink)

            # Empty the processor cache list
            lock(state.lock) do
                state.procs_cache_list[] = nothing
            end
        end

        timespan_finish(ctx, :assign_procs, 0, 0)
        old_ps = new_ps
    end
end

function remove_dead_proc!(ctx, state, proc, options=ctx.options)
    @assert options.single !== proc.pid "Single worker failed, cannot continue."
    rmprocs!(ctx, [proc])
    delete!(state.worker_time_pressure, proc.pid)
    delete!(state.worker_storage_pressure, proc.pid)
    delete!(state.worker_storage_capacity, proc.pid)
    delete!(state.worker_loadavg, proc.pid)
    delete!(state.worker_chans, proc.pid)
    state.procs_cache_list[] = nothing
end

function finish_task!(ctx, state, node, thunk_failed)
    pop!(state.running, node)
    delete!(state.running_on, node)
    if thunk_failed
        set_failed!(state, node)
    end
    if node.cache
        node.cache_ref = state.cache[node]
    end
    schedule_dependents!(state, node, thunk_failed)
    fill_registered_futures!(state, node, thunk_failed)

    to_evict = cleanup_syncdeps!(state, node)
    if node.f isa Chunk
        # FIXME: Check the graph for matching chunks
        push!(to_evict, node.f)
    end
    if haskey(state.waiting_data, node) && isempty(state.waiting_data[node])
        delete!(state.waiting_data, node)
    end
    evict_all_chunks!(ctx, to_evict)
end

function evict_all_chunks!(ctx, to_evict)
    if !isempty(to_evict)
        @sync for w in map(p->p.pid, procs_to_use(ctx))
            @async remote_do(evict_chunks!, w, ctx.log_sink, to_evict)
        end
    end
end
function evict_chunks!(log_sink, chunks::Set{Chunk})
    # Need worker id or else Context might use Processors which user does not want us to use.
    # In particular workers which have not yet run using Dagger will cause the call below to throw an exception
    ctx = Context([myid()];log_sink) 
    for chunk in chunks
        lock(TASK_SYNC) do
            timespan_start(ctx, :evict, myid(), (;data=chunk))
            haskey(CHUNK_CACHE, chunk) && delete!(CHUNK_CACHE, chunk)
            timespan_finish(ctx, :evict, myid(), (;data=chunk))
        end
    end
    nothing
end

fire_task!(ctx, thunk::Thunk, p, state; scope=AnyScope(), time_util=10^9, alloc_util=10^6, occupancy=typemax(UInt32)) =
    fire_task!(ctx, (thunk, scope, time_util, alloc_util, occupancy), p, state)
fire_task!(ctx, (thunk, scope, time_util, alloc_util, occupancy)::Tuple{Thunk,<:Any}, p, state) =
    fire_tasks!(ctx, [(thunk, scope, time_util, alloc_util, occupancy)], p, state)
function fire_tasks!(ctx, thunks::Vector{<:Tuple}, (gproc, proc), state)
    to_send = []
    for (thunk, scope, time_util, alloc_util, occupancy) in thunks
        push!(state.running, thunk)
        state.running_on[thunk] = gproc
        if thunk.cache && thunk.cache_ref !== nothing
            # the result might be already cached
            data = thunk.cache_ref
            if data !== nothing
                # cache hit
                state.cache[thunk] = data
                thunk_failed = get(state.errored, thunk, false)
                finish_task!(ctx, state, thunk, thunk_failed)
                continue
            else
                # cache miss
                thunk.cache_ref = nothing
            end
        end
        if thunk.options !== nothing && thunk.options.restore !== nothing
            try
                result = @invokelatest thunk.options.restore(thunk)
                if result isa Chunk
                    state.cache[thunk] = result
                    state.errored[thunk] = false
                    finish_task!(ctx, state, thunk, false)
                    continue
                elseif result !== nothing
                    throw(ArgumentError("Invalid restore return type: $(typeof(result))"))
                end
            catch err
                report_catch_error(err, "Thunk restore failed")
            end
        end

        ids = Int[0]
        data = Any[thunk.f]
        positions = Union{Symbol,Nothing}[]
        for (idx, pos_x) in enumerate(thunk.inputs)
            pos, x = pos_x
            x = unwrap_weak_checked(x)
            push!(ids, istask(x) ? x.id : -idx)
            push!(data, istask(x) ? state.cache[x] : x)
            push!(positions, pos)
        end
        toptions = thunk.options !== nothing ? thunk.options : ThunkOptions()
        options = merge(ctx.options, toptions)
        propagated = get_propagated_options(thunk)
        @assert (options.single === nothing) || (gproc.pid == options.single)
        # TODO: Set `sch_handle.tid.ref` to the right `DRef`
        sch_handle = SchedulerHandle(ThunkID(thunk.id, nothing), state.worker_chans[gproc.pid]...)

        # TODO: De-dup common fields (log_sink, uid, etc.)
        push!(to_send, Any[thunk.id, time_util, alloc_util, occupancy,
                           scope, chunktype(thunk.f), data,
                           thunk.get_result, thunk.persist, thunk.cache, thunk.meta, options,
                           propagated, ids, positions,
                           (log_sink=ctx.log_sink, profile=ctx.profile),
                           sch_handle, state.uid])
    end
    # N.B. We don't batch these because we might get a deserialization
    # error due to something not being defined on the worker, and then we don't
    # know which task failed.
    tasks = Task[]
    for ts in to_send
        # TODO: errormonitor
        @async begin
            timespan_start(ctx, :fire, gproc.pid, 0)
            try
                remotecall_wait(do_tasks, gproc.pid, proc, state.chan, [ts])
            catch err
                bt = catch_backtrace()
                thunk_id = ts[1]
                put!(state.chan, (gproc.pid, proc, thunk_id, (CapturedException(err, bt), nothing)))
            finally
                timespan_finish(ctx, :fire, gproc.pid, 0)
            end
        end
    end
end

@static if VERSION >= v"1.9"
const Doorbell = Base.Event
else
# We need a sticky, resetable signal
mutable struct Doorbell
    waiter::Union{Task,Nothing}
    @atomic sleeping::Int
    Doorbell() = new(nothing, 0)
end
function Base.wait(db::Doorbell)
    db.waiter = current_task()
    while true
        _, succ = @atomicreplace db.sleeping 0 => 1
        if succ
            # No messages, wait for someone to wake us
            wait()
        end
        _, succ = @atomicreplace db.sleeping 2 => 0
        if succ
            # We had a notification
            return
        end
    end
end
function Base.notify(db::Doorbell)
    while true
        if (@atomic db.sleeping) == 2
            # Doorbell already rung
            return
        end

        _, succ = @atomicreplace db.sleeping 0 => 2
        if succ
            # Task was definitely busy, we're done
            return
        end

        _, succ = @atomicreplace db.sleeping 1 => 2
        if succ
            # Task was sleeping, wake it and wait for it to awaken
            waiter = db.waiter
            @assert waiter !== nothing
            waiter::Task
            schedule(waiter)
            while true
                sleep_value = @atomic db.sleeping
                if sleep_value == 0 || sleep_value == 2
                    return
                end
                #if waiter._state === Base.task_state_runnable && t.queue === nothing
                #    schedule(waiter)
                #else
                    yield()
                #end
            end
        end
    end
end
end

struct TaskSpecKey
    task_id::Int
    task_spec::Vector{Any}
    TaskSpecKey(task_spec::Vector{Any}) = new(task_spec[1], task_spec)
end
Base.getindex(key::TaskSpecKey) = key.task_spec
Base.hash(key::TaskSpecKey, h::UInt) = hash(key.task_id, hash(TaskSpecKey, h))

struct ProcessorInternalState
    ctx::Context
    proc::Processor
    queue::LockedObject{PriorityQueue{TaskSpecKey, UInt32, Base.Order.ForwardOrdering}}
    reschedule::Doorbell
    tasks::Dict{Int,Task}
    proc_occupancy::Base.RefValue{UInt32}
    time_pressure::Base.RefValue{UInt64}
    done::Base.RefValue{Bool}
end
struct ProcessorState
    state::ProcessorInternalState
    runner::Task
end

const PROCESSOR_TASK_STATE = LockedObject(Dict{UInt64,Dict{Processor,ProcessorState}}())

function proc_states(f::Base.Callable, uid::UInt64)
    lock(PROCESSOR_TASK_STATE) do all_states
        if !haskey(all_states, uid)
            all_states[uid] = Dict{Processor,ProcessorState}()
        end
        our_states = all_states[uid]
        return f(our_states)
    end
end
proc_states(f::Base.Callable) =
    proc_states(f, task_local_storage(:_dagger_sch_uid)::UInt64)

task_tid_for_processor(::Processor) = nothing
task_tid_for_processor(proc::Dagger.ThreadProc) = proc.tid

stealing_permitted(::Processor) = true
stealing_permitted(proc::Dagger.ThreadProc) = proc.owner != 1 || proc.tid != 1

proc_has_occupancy(proc_occupancy, task_occupancy) =
    UInt64(task_occupancy) + UInt64(proc_occupancy) <= typemax(UInt32)

function start_processor_runner!(istate::ProcessorInternalState, uid::UInt64, return_queue::RemoteChannel)
    to_proc = istate.proc
    proc_run_task = @task begin
        ctx = istate.ctx
        tasks = istate.tasks
        proc_occupancy = istate.proc_occupancy
        time_pressure = istate.time_pressure

        work_to_do = false
        while isopen(return_queue)
            # Wait for new tasks
            if !work_to_do
                @dagdebug nothing :processor "Waiting for tasks"
                timespan_start(ctx, :proc_run_wait, to_proc, nothing)
                wait(istate.reschedule)
                @static if VERSION >= v"1.9"
                    reset(istate.reschedule)
                end
                timespan_finish(ctx, :proc_run_wait, to_proc, nothing)
                if istate.done[]
                    return
                end
            end

            # Fetch a new task to execute
            @dagdebug nothing :processor "Trying to dequeue"
            timespan_start(ctx, :proc_run_fetch, to_proc, nothing)
            work_to_do = false
            task_and_occupancy = lock(istate.queue) do queue
                # Only steal if there are multiple queued tasks, to prevent
                # ping-pong of tasks between empty queues
                if length(queue) == 0
                    @dagdebug nothing :processor "Nothing to dequeue"
                    return nothing
                end
                _, occupancy = peek(queue)
                if !proc_has_occupancy(proc_occupancy[], occupancy)
                    @dagdebug nothing :processor "Insufficient occupancy" proc_occupancy=proc_occupancy[] task_occupancy=occupancy
                    return nothing
                end
                queue_result = dequeue_pair!(queue)
                work_to_do = length(queue) > 0
                return queue_result
            end
            if task_and_occupancy === nothing
                timespan_finish(ctx, :proc_run_fetch, to_proc, nothing)

                @dagdebug nothing :processor "Failed to dequeue"

                if !stealing_permitted(to_proc)
                    continue
                end

                if proc_occupancy[] == typemax(UInt32)
                    continue
                end

                @dagdebug nothing :processor "Trying to steal"

                # Try to steal a task
                timespan_start(ctx, :steal_local, to_proc, nothing)

                # Try to steal from local queues randomly
                # TODO: Prioritize stealing from busiest processors
                states = collect(proc_states(values, uid))
                # TODO: Try to pre-allocate this
                P = randperm(length(states))
                for state in getindex.(Ref(states), P)
                    other_istate = state.state
                    if other_istate.proc === to_proc
                        continue
                    end
                    # FIXME: We need to lock two queues to compare occupancies
                    proc_occupancy_cached = lock(istate.queue) do _
                        proc_occupancy[]
                    end
                    task_and_occupancy = lock(other_istate.queue) do queue
                        if length(queue) == 0
                            return nothing
                        end
                        task_spec, occupancy = peek(queue)
                        task = task_spec[]
                        scope = task[5]
                        if !isa(constrain(scope, Dagger.ExactScope(to_proc)),
                                Dagger.InvalidScope) &&
                           typemax(UInt32) - proc_occupancy_cached >= occupancy
                            # Compatible, steal this task
                            return dequeue_pair!(queue)
                        end
                        return nothing
                    end
                    if task_and_occupancy !== nothing
                        from_proc = other_istate.proc
                        thunk_id = task[1]
                        @dagdebug thunk_id :processor "Stolen from $from_proc by $to_proc"
                        timespan_finish(ctx, :steal_local, to_proc, (;from_proc, thunk_id))
                        # TODO: Keep stealing until we hit full occupancy?
                        @goto execute
                    end
                end
                timespan_finish(ctx, :steal_local, to_proc, nothing)

                # TODO: Try to steal from remote queues

                continue
            end

            @label execute
            task_spec, task_occupancy = task_and_occupancy
            task = task_spec[]
            thunk_id = task[1]
            time_util = task[2]
            timespan_finish(ctx, :proc_run_fetch, to_proc, (;thunk_id, proc_occupancy=proc_occupancy[], task_occupancy))
            @dagdebug thunk_id :processor "Dequeued task"

            # Execute the task and return its result
            t = @task begin
                result = try
                    do_task(to_proc, task)
                catch err
                    bt = catch_backtrace()
                    (CapturedException(err, bt), nothing)
                finally
                    lock(istate.queue) do _
                        delete!(tasks, thunk_id)
                        proc_occupancy[] -= task_occupancy
                        time_pressure[] -= time_util
                    end
                    notify(istate.reschedule)
                end
                try
                    put!(return_queue, (myid(), to_proc, thunk_id, result))
                catch err
                    if unwrap_nested_exception(err) isa InvalidStateException || !isopen(return_queue)
                        @dagdebug thunk_id :execute "Return queue is closed, failing to put result" chan=return_queue exception=(err, catch_backtrace())
                    else
                        rethrow(err)
                    end
                end
            end
            lock(istate.queue) do _
                tid = task_tid_for_processor(to_proc)
                if tid !== nothing
                    t.sticky = true
                    ret = ccall(:jl_set_task_tid, Cint, (Any, Cint), t, tid-1)
                else
                    t.sticky = false
                end
                tasks[thunk_id] = errormonitor_tracked(schedule(t))
                proc_occupancy[] += task_occupancy
                time_pressure[] += time_util
            end
        end
    end
    tid = task_tid_for_processor(to_proc)
    if tid !== nothing
        proc_run_task.sticky = true
        ret = ccall(:jl_set_task_tid, Cint, (Any, Cint), proc_run_task, tid-1)
    else
        proc_run_task.sticky = false
    end
    return errormonitor_tracked(schedule(proc_run_task))
end

"""
    do_tasks(to_proc, return_queue, tasks)

Executes a batch of tasks on `to_proc`, returning their results through
`return_queue`.
"""
function do_tasks(to_proc, return_queue, tasks)
    @dagdebug nothing :processor "Enqueuing task batch" batch_size=length(tasks)

    # FIXME: This is terrible
    ctx_vars = first(tasks)[16]
    ctx = Context(Processor[]; log_sink=ctx_vars.log_sink, profile=ctx_vars.profile)
    uid = first(tasks)[18]
    state = proc_states(uid) do states
        get!(states, to_proc) do
            queue = PriorityQueue{TaskSpecKey, UInt32}()
            queue_locked = LockedObject(queue)
            reschedule = Doorbell()
            istate = ProcessorInternalState(ctx, to_proc,
                                            queue_locked, reschedule,
                                            Dict{Int,Task}(),
                                            Ref(UInt32(0)), Ref(UInt64(0)),
                                            Ref(false))
            runner = start_processor_runner!(istate, uid, return_queue)
            @static if VERSION < v"1.9"
                reschedule.waiter = runner
            end
            return ProcessorState(istate, runner)
        end
    end
    istate = state.state
    lock(istate.queue) do queue
        for task in tasks
            thunk_id = task[1]
            occupancy = task[4]
            timespan_start(ctx, :enqueue, (;to_proc, thunk_id), nothing)
            should_launch = lock(TASK_SYNC) do
                # Already running; don't try to re-launch
                if !(thunk_id in TASKS_RUNNING)
                    push!(TASKS_RUNNING, thunk_id)
                    true
                else
                    false
                end
            end
            should_launch || continue
            enqueue!(queue, TaskSpecKey(task), occupancy)
            timespan_finish(ctx, :enqueue, (;to_proc, thunk_id), nothing)
            @dagdebug thunk_id :processor "Enqueued task"
        end
    end
    notify(istate.reschedule)

    # Kick other processors to make them steal
    # TODO: Alternatively, automatically balance work instead of blindly enqueueing
    states = collect(proc_states(values, uid))
    P = randperm(length(states))
    for other_state in getindex.(Ref(states), P)
        other_istate = other_state.state
        if other_istate.proc === to_proc
            continue
        end
        notify(other_istate.reschedule)
    end
    @dagdebug nothing :processor "Kicked processors"
end

"""
    do_task(to_proc, task_desc) -> Any

Executes a single task specified by `task_desc` on `to_proc`.
"""
function do_task(to_proc, task_desc)
    thunk_id, est_time_util, est_alloc_util, est_occupancy,
        scope, Tf, data,
        send_result, persist, cache, meta,
        options, propagated, ids, positions,
        ctx_vars, sch_handle, sch_uid = task_desc
    ctx = Context(Processor[]; log_sink=ctx_vars.log_sink, profile=ctx_vars.profile)

    from_proc = OSProc()
    Tdata = Any[]
    for x in data
        push!(Tdata, chunktype(x))
    end
    f = isdefined(Tf, :instance) ? Tf.instance : nothing

    # Wait for required resources to become available
    to_storage = options.storage !== nothing ? fetch(options.storage) : MemPool.GLOBAL_DEVICE[]
    to_storage_name = nameof(typeof(to_storage))
    storage_cap = storage_capacity(to_storage)

    timespan_start(ctx, :storage_wait, thunk_id, (;f, to_proc, device=typeof(to_storage)))
    real_time_util = Ref{UInt64}(0)
    real_alloc_util = UInt64(0)
    if !meta
        # Factor in the memory costs for our lazy arguments
        for arg in data[2:end]
            if arg isa Chunk
                est_alloc_util += arg.handle.size
            end
        end
    end

    debug_storage(msg::String) = @debug begin
        let est_alloc_util=Base.format_bytes(est_alloc_util),
            real_alloc_util=Base.format_bytes(real_alloc_util),
            storage_cap=Base.format_bytes(storage_cap)
            "[$(myid()), $thunk_id] $f($Tdata) $msg: $est_alloc_util | $real_alloc_util/$storage_cap"
        end
    end
    lock(TASK_SYNC) do
        while true
            # Get current time utilization for the selected processor
            time_dict = get!(()->Dict{Processor,Ref{UInt64}}(), PROCESSOR_TIME_UTILIZATION, sch_uid)
            real_time_util = get!(()->Ref{UInt64}(UInt64(0)), time_dict, to_proc)

            # Get current allocation utilization and capacity
            real_alloc_util = storage_utilized(to_storage)
            storage_cap = storage_capacity(to_storage)

            # Check if we'll go over memory capacity from running this thunk
            # Waits for free storage, if necessary
            #= TODO: Implement a priority queue, ordered by est_alloc_util
            if est_alloc_util > storage_cap
                debug_storage("WARN: Estimated utilization above storage capacity on $to_storage_name, proceeding anyway")
                break
            end
            if est_alloc_util + real_alloc_util > storage_cap
                if externally_varying(to_storage)
                    debug_storage("WARN: Insufficient space and allocation behavior is externally varying on $to_storage_name, proceeding anyway")
                    break
                end
                if length(TASKS_RUNNING) <= 2 # This task + eager submission task
                    debug_storage("WARN: Insufficient space and no other running tasks on $to_storage_name, proceeding anyway")
                    break
                end
                # Fully utilized, wait and re-check
                debug_storage("Waiting for free $to_storage_name")
                wait(TASK_SYNC)
            else
                # Sufficient free storage is available, prepare for execution
                debug_storage("Using available $to_storage_name")
                break
            end
            =#
            # FIXME
            break
        end
    end
    timespan_finish(ctx, :storage_wait, thunk_id, (;f, to_proc, device=typeof(to_storage)))

    @dagdebug thunk_id :execute "Moving data"

    # Initiate data transfers for function and arguments
    transfer_time = Threads.Atomic{UInt64}(0)
    transfer_size = Threads.Atomic{UInt64}(0)
    _data, _ids = if meta
        (Any[first(data)], Int[first(ids)]) # always fetch function
    else
        (data, ids)
    end
    fetch_tasks = map(Iterators.zip(_data,_ids)) do (x, id)
        @async begin
            timespan_start(ctx, :move, (;thunk_id, id), (;f, id, data=x))
            x = if x isa Chunk
                value = lock(TASK_SYNC) do
                    if haskey(CHUNK_CACHE, x)
                        Some{Any}(get!(CHUNK_CACHE[x], to_proc) do
                            # Convert from cached value
                            # TODO: Choose "closest" processor of same type first
                            some_proc = first(keys(CHUNK_CACHE[x]))
                            some_x = CHUNK_CACHE[x][some_proc]
                            @dagdebug thunk_id :move "Cache hit for argument $id at $some_proc: $some_x"
                            @invokelatest move(some_proc, to_proc, some_x)
                        end)
                    else
                        nothing
                    end
                end

                if value !== nothing
                    something(value)
                else
                    # Fetch it
                    time_start = time_ns()
                    from_proc = processor(x)
                    _x = @invokelatest move(from_proc, to_proc, x)
                    time_finish = time_ns()
                    if x.handle.size !== nothing
                        Threads.atomic_add!(transfer_time, time_finish - time_start)
                        Threads.atomic_add!(transfer_size, x.handle.size)
                    end

                    @dagdebug thunk_id :move "Cache miss for argument $id at $from_proc"

                    # Update cache
                    lock(TASK_SYNC) do
                        CHUNK_CACHE[x] = Dict{Processor,Any}()
                        CHUNK_CACHE[x][to_proc] = _x
                    end

                    _x
                end
            else
                @invokelatest move(to_proc, x)
            end
            @dagdebug thunk_id :move "Moved argument $id to $to_proc: $x"
            timespan_finish(ctx, :move, (;thunk_id, id), (;f, id, data=x); tasks=[Base.current_task()])
            return x
        end
    end
    fetched = Any[]
    for task in fetch_tasks
        push!(fetched, fetch_report(task))
    end
    if meta
        append!(fetched, data[2:end])
    end

    f = popfirst!(fetched)
    @assert !(f isa Chunk) "Failed to unwrap thunk function"
    fetched_args = Any[]
    fetched_kwargs = Pair{Symbol,Any}[]
    for (idx, x) in enumerate(fetched)
        pos = positions[idx]
        if pos === nothing
            push!(fetched_args, x)
        else
            push!(fetched_kwargs, pos => x)
        end
    end

    #= FIXME: If MaxUtilization, stop processors and wait
    if (est_time_util isa MaxUtilization) && (real_time_util > 0)
        # FIXME: Stop processors
        # FIXME: Wait on processors to stop
        est_time_util = count(c->typeof(c)===typeof(to_proc), children(from_proc))
    end
    =#

    real_time_util[] += est_time_util
    timespan_start(ctx, :compute, thunk_id, (;f, to_proc))
    res = nothing

    # Start counting time and GC allocations
    threadtime_start = cputhreadtime()
    # FIXME
    #gcnum_start = Base.gc_num()

    @dagdebug thunk_id :execute "Executing"

    result_meta = try
        # Set TLS variables
        Dagger.set_tls!((
            sch_uid,
            sch_handle,
            processor=to_proc,
            task_spec=task_desc,
        ))

        res = Dagger.with_options(propagated) do
            # Execute
            execute!(to_proc, f, fetched_args...; fetched_kwargs...)
        end

        # Check if result is safe to store
        device = nothing
        if !(res isa Chunk)
            timespan_start(ctx, :storage_safe_scan, thunk_id, (;T=typeof(res)))
            device = if walk_storage_safe(res)
                to_storage
            else
                MemPool.CPURAMDevice()
            end
            timespan_finish(ctx, :storage_safe_scan, thunk_id, (;T=typeof(res)))
        end

        # Construct result
        # TODO: We should cache this locally
        send_result || meta ? res : tochunk(res, to_proc; device, persist, cache=persist ? true : cache,
                                            tag=options.storage_root_tag,
                                            leaf_tag=something(options.storage_leaf_tag, MemPool.Tag()),
                                            retain=options.storage_retain)
    catch ex
        bt = catch_backtrace()
        RemoteException(myid(), CapturedException(ex, bt))
    end

    threadtime = cputhreadtime() - threadtime_start
    # FIXME: This is not a realistic measure of max. required memory
    #gc_allocd = min(max(UInt64(Base.gc_num().allocd) - UInt64(gcnum_start.allocd), UInt64(0)), UInt64(1024^4))
    timespan_finish(ctx, :compute, thunk_id, (;f, to_proc))
    lock(TASK_SYNC) do
        real_time_util[] -= est_time_util
        pop!(TASKS_RUNNING, thunk_id)
        notify(TASK_SYNC)
    end

    @dagdebug thunk_id :execute "Returning"

    # TODO: debug_storage("Releasing $to_storage_name")
    metadata = (
        time_pressure=real_time_util[],
        storage_pressure=real_alloc_util,
        storage_capacity=storage_cap,
        loadavg=((Sys.loadavg()...,) ./ Sys.CPU_THREADS),
        threadtime=threadtime,
        # FIXME: Add runtime allocation tracking
        gc_allocd=(isa(result_meta, Chunk) ? result_meta.handle.size : 0),
        transfer_rate=(transfer_size[] > 0 && transfer_time[] > 0) ? round(UInt64, transfer_size[] / (transfer_time[] / 10^9)) : nothing,
    )
    return (result_meta, metadata)
end

end # module Sch