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task-queues.md

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Task Queues

By default, @spawn/spawn submit tasks immediately and directly into Dagger's scheduler without modifications. However, sometimes you want to be able to tweak this behavior for a region of code; for example, when working with GPUs or other operations which operate in-place, you might want to emulate CUDA's stream semantics by ensuring that tasks execute sequentially (to avoid one kernel reading from an array while another kernel is actively writing to it). Or, you might want to ensure that a set of Dagger tasks are submitted into the scheduler all at once for benchmarking purposes or to emulate the behavior of delayed. This and more is possible through a mechanism called "task queues".

A task queue in Dagger is an object that can be configured to accept unlaunched tasks from @spawn/spawn and either modify them or delay their launching arbitrarily. By default, Dagger tasks are enqueued through the DefaultTaskQueue, which submits tasks directly into the scheduler before @spawn/spawn returns. However, Dagger also has an InOrderTaskQueue, which ensures that tasks enqueued through it execute sequentially with respect to each other. This queue can be allocated with Dagger.spawn_sequential:

A = rand(16)
B = zeros(16)
C = zeros(16)
function vcopy!(B, A)
    B .= A .+ 1.0
    return
end
function vadd!(C, A, B)
    C .+= A .+ B
    return
end
wait(Dagger.spawn_sequential() do
    Dagger.@spawn vcopy!(B, A)
    Dagger.@spawn vadd!(C, A, B)
end)

In the above example, vadd! is guaranteed to wait until vcopy! is completed, even though vadd! isn't taking the result of vcopy! as an argument (which is how tasks are normally ordered).

What if we wanted to launch multiple vcopy! calls within a spawn_sequential region and allow them to execute in parallel, but still ensure that the vadd! happens after they all finish? In this case, we want to switch to another kind of task queue: the LazyTaskQueue. This task queue batches up task submissions into groups, so that all tasks enqueued with it are placed in the scheduler all at once. But what would happen if we used this task queue (via spawn_bulk) within a region using spawn_sequential:

A = rand(16)
B1 = zeros(16)
B2 = zeros(16)
C = zeros(16)
wait(Dagger.spawn_sequential() do
    Dagger.spawn_bulk() do
        Dagger.@spawn vcopy!(B1, A)
        Dagger.@spawn vcopy!(B2, A)
    end
    Dagger.@spawn vadd!(C, B1, B2)
end)

Conveniently, Dagger's task queues can be nested to get the expected behavior; the above example will submit the two vcopy! tasks as a group (and they can execute concurrently), while still ensuring that those two tasks finish before the vadd! task executes.

!!! warn Task queues do not propagate to nested tasks; if a Dagger task launches another task internally, the child task doesn't inherit the task queue that the parent task was enqueued in.