Per-user compiled artifact cache

[djc]

I was wondering if anyone has contemplated somehow sharing compiled crates. If I have a number of projects on disk that often have similar dependencies, I'm spending a lot of time recompiling the same packages. (Even correcting for features, compiler flags and compilation profiles.) Would it make sense to store symlinks in ~/.cargo or equivalent pointing to compiled artefacts?

[alexcrichton]

There's been musings about this historically but never any degree of serious consideration. I've always wanted to explore it though! (I think it's definitely plausible)

[aidanhs]

sccache is one option here - it has a local disk cache in addition to the more exotic options to store compiled artifacts in the cloud.

[djc]

sccache would be good for the compilation time part, but it'd be nice to also get a handle on the disk size part of it.

[Eh2406]

cc #6229

[Vlad-Shcherbina]

I think you can put

[build]
target-dir = "/my/shared/target/dir"

in ~/.cargo/config.

But I have no idea if this mode is officially supported. Is it?

[Eh2406]

Yes it is, as is setting it with the corresponding environment variable. However the problems with cargo never deleting the unused artifacts gets to be dramatic quickly. Hence the connection to #6229

[epage]

@joshtriplett and I had a brainstorming session on this at RustNL last week.

It'd be great if cargo could have a very small subset of sccache's logic: per-user caching of intermediate build artifacts. By building this into cargo, we can tie it into all that cargo knows and cane make extensions to better support it.

Risks

• Poisoning the cache from
  • Broken builds (e.g. from incremental compilation bugs)
  • Non-deterministic builds
  • mtime bugs
• Races with parallel builds
  • Performance hits from locking
• Running out of disk space

To mitigate problems with cache poisoning

• Packages must advertise that they are deterministic
  • Long term: Wasm build scripts and proc-macros to ensure determinism
• Initially limit the caching to packages from immutable sources
  • No mtime bugs
  • No need for incremental compilation

As a contingency for if the cache is poisoned, we need a way to clear the cache (see also #3289)

To mitigate running out of disk space, we need a GC / prune (see also #6509)

• One strategy is to clear caches for a rust version that is no longer installed
• We could manually track atime

Locking strategy to mitigate race conditions / locking performance

• Assumptions:
  • Parallel builds are likely
  • Parallel builds of the same package fingerprint are unlikely
• Design:
  1. Does it exist (without lock)
  2. Build outside the cache
  3. If can't do atomic rename, move into a temp dir, then do an atomic rename. If it now exists, just delete the tmpdir
• Read/prune lock
  • Multi-Reader/single-writer lock
  • What about reading it?
    • Hold lock wile building, blocking prunes
    • Could copy out while holding lock to minimize lock time
    • Hold a lock for copy it
    • Or don't bother copying, just block the prune until lock is available
  • Prune by renaming and then deleting

Transition plan (modeled off of sparse registry)

• Steps:
  1. Unstable
  2. Hacky env variable to opt-in (unstable)
  • On stable, warn if env variable is set so people can set it globally and use versions of Rust from Step 2 and Step 3
  4. Hacky env variable to opt-in (stable)
  5. Stable
• Don't need full cleanup/pruning strategy until it is stable

[epage]

Wonder if something like reflink would be useful

[epage]

See also #7150

[epage]

Some complications that came up when discussing this this with ehuss.

First, some background. We track rebuilds in two ways. The first is we have an external fingerprint that is a hash that we use to tell when to rebuild. The second is we have the hash of build inputs we pass to rustc with -Cmetadata that is used to keep symbols unique. We include this in the file name, so if -Cmetadata changes, then the filename changes. If the file doesn't exist, that is a sure sign it needs to be built.

Problems

• We co-mingle files in the target directory.
  • This makes it easy for us to pass a single directory for rustc to slurp up rlibs
  • Some other tools depend on this for slurping up rlibs or for asm output from rustc
  • This is a problem because we'll need all artifacts for an immutable package to be in isolated directories, for capturing the files and reading from the cache
• RUSTFLAGS is only present in the fingerprint and not in -Cmetadata (Reconsider RUSTFLAGS artifact caching. #8716).
  • We don't want PGO related RUSTFLAGS to change symbols
  • We don't want remap related RUSTFLAGS to change symbols
  • However, this introduces mutable data into the immutable package, making it so we can't cache it

I guess the first question is whether the per-user cache should be organized around fingerprint, -Cmetadata, or something else. Well, -Cmetadata isn't an option so long as it doesn't have RUSTFLAGS, which it shouldn't, so it would more be fingerprint or us adding a new hash type, one that maybe we reuse with the file names and ensure doesn't cause problems with rustc.

[weihanglo]

Cargo uses relative paths to workspace root for path dependencies to generate stable hashes. This causes an issue (#12516) when sharing target directories between package with the same name and version and relative path to workspace.

[epage]

For me, the biggest thing that needs to be figured out before any other progress is worth it is how to get a reasonable amount of value out of this cache.

Take my system

• 74 repos with a Cargo.lock in the root
• 65 of those repos have syn in the lockfile
• Among those 65 repos, 44 different versions of syn are used (there is a mix of v1 and v2)

This is a "bottom of the stack" package. As you go up the stack, the impact of version combinations grows dramatically.

I worry a per-user cache's value will only be slightly more than making cargo clean && cargo build faster and that doesn't feel worth the complexity to me.

[jplatte]

How did you do that analysis? I'd be interested in running it on my own system.

Also. re caching and RUSTFLAGS, could it be an optional to simply fall back to the existing caching scheme (project-specific target dir) if RUSTFLAGS is set at all? Personally, I work on very few projects that utilize RUSTFLAGS (AFAIK.. although I also have it configured globally right now, with -C link-arg=-fuse-ld=mold, which I'd have to disable or find an alternative solution for), so everything else benefitting from a shared cache dir might already be useful.

[epage]

How did you do that analysis? I'd be interested in running it on my own system.

Pre-req: I keep all repos in a single folder.

$ ls */Cargo.lock | wc -l
$ rg 'name = "syn"' */Cargo.lock -l | wc -l
$ rg 'name = "syn"' */Cargo.lock -A 1 | rg version | rg -o '".*"' | sort -u | wc -l

(and yes, there are likely rg features I can use to code-golf this)

[jplatte]

Thanks! I keep my projects in two dirs (approximated active and inactive projects), but running this separately on both I get the following. Also included futures-util as another commonly-used crate, one that does not get released as often.

stat	active	inactive
number of crates / workspaces	34	91
workspaces pulling in syn	29	72
different versions of syn	15	33
workspaces pulling in futures-util	20	44
different versions of futures-util	3	14

[djc]

Also syn is part of a set of crates that gets a lot of little bumps. This is common for dtolnay crates, but not so much for a whole host of other crates -- so I'm not sure this particular test is very representative. (Note that I'm definitely not disagreeing that the utility of a per-user compiled artifact cache might not be as great as hoped.)

FWIW, given what I see Rust-Analyzer doing in a large workspace at work (some 670 crates are involved) it seems to be doing a lot of recompilation even with only weekly updates to the dependencies so even within a single workspace there might be some wins?

[lu-zero]

Somebody with a deduplicating file system could share their statistics for a theoretical upper bound?

[RobJellinghaus]

Hi folks, chiming in here to merge two streams: some of us at Microsoft did a hackathon project to prototype a per-user Cargo cache, late last September.

Here's the Zulip chat: https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/Per-user.20build.20caches
Our HackMD with status as of our last discussion of this with the Cargo team: https://hackmd.io/R64ykWblRr-y9-jeWNLGtQ?view
Comparison of our branch's changes: https://github.com/rust-lang/cargo/compare/master...arlosi:cargo:hackathon?expand=1

Our initial testing of this generally showed surprisingly small speedup, even when things were entirely cached. It seems that rustc is just really damn fast at crate compilation :-O And that (as we know) the long pole is always the final LLVM binary build.

This change took the approach of creating a user-shared cache using the cacache crate, which worked very well and allowed us to get something functioning quickly, but which was generally considered by the Cargo team to perpetuate the anti-pattern of having a not very human-readable filesystem layout. @arlosi had a background goal of creating a more human-manageable filesystem cache provider implementation, but I know other priorities have supervened since then.

I do think this change's approach of having a very narrow cache interface is a good design direction that was reasonably proven by this experiment.

Happy to discuss our approach on any level, hope it is useful to people wanting to move this further forwards.

[mydoghasfleas]

Our initial testing of this generally showed surprisingly small speedup, even when things were entirely cached. It seems that rustc is just really damn fast at crate compilation :-O And that (as we know) the long pole is always the final LLVM binary build.

But the problem being solved here is not only the speed, but the amount of storage space being consumed. When you have dozens of projects each compiling the same crates and each easily taking up 2GB, your drive starts filling up very quickly!

[ssokolow]

Our initial testing of this generally showed surprisingly small speedup, even when things were entirely cached. It seems that rustc is just really damn fast at crate compilation :-O And that (as we know) the long pole is always the final LLVM binary build.

But the problem being solved here is not only the speed, but the amount of storage space being consumed. When you have dozens of projects each compiling the same crates and each easily taking up 2GB, your drive starts filling up very quickly!

Exactly.

I've now got a Ryzen 5 7600. Combined with mold, cargo clean; cargo build --release takes almost no time... but I've only got so much disk space and, even without deleting and re-building things to limit space consumption, I'd still rather avoid unnecessary write cycles on my SSD.

[mydoghasfleas]

Our initial testing of this generally showed surprisingly small speedup, even when things were entirely cached. It seems that rustc is just really damn fast at crate compilation :-O And that (as we know) the long pole is always the final LLVM binary build.

But the problem being solved here is not only the speed, but the amount of storage space being consumed. When you have dozens of projects each compiling the same crates and each easily taking up 2GB, your drive starts filling up very quickly!

Exactly.

I've now got a Ryzen 5 7600. Combined with mold, cargo clean; cargo build --release takes almost no time... but I've only got so much disk space and, even without deleting and re-building things to limit space consumption, I'd still rather avoid unnecessary write cycles on my SSD.

The write cycles issue is very pertinent!

[jaskij]

Came across this, reading the design document , and decided to share my thoughts here. Hopefully this will be good input.

In CI, users generally have to declare what directory is should be cached between jobs. This directory will be compressed and uploaded at the end of the job. If the next job's cache key matches, the tarball will be downloaded and decompressed. If too much is cached, the time for managing the cache can dwarf the benefits of the cache. Some third-party projects exist to help manage cache size.

This makes several assumption about CI behavior, probably based on how GH Actions behaves. Note that other runners will behave differently, for example by default GitLab does not upload the cache anywhere. I'm not even sure if the cache is compressed. This shouldn't be an issue, but it is a set of bad assumptions in the design doc and could theoretically could lead to bad design.

I would also like to add that CI interactions should have supporting multiple CI runners from the get go. GitHub is, for better or worse, dominant, especially in mindshare, and the Rust project shouldn't be furthering it's monopoly.

---

Hashes and fingerprinting

Stale caches are a major pain, to the point that I have learned to recognize the signs working with at least two build systems. Currently, Cargo does not even use the whole hash, only 64 bits (sixteen digits) of it. I'm worried that with user-wide caches, collisions may happen. To that end, I'd prefer there was a plaintext copy of all the data going into the cache, to be verified that it is indeed the correct cache. Or at least use the full length of the hash.

---

Garbage collection

I've seen someone mention clearing old stuff by access time. This would work, in theory, but is also something to be careful around. For example, a lot of desktop systems use relatime, and some people set noatime for performance reasons. Are we even sure that the data is always available?

Personally, I would love to see something more advanced, with data tracking.

• Cargo tracks which dependencies in the global cache are used by which project, including:
  • which projects use which dependencies
  • projects' filesystem paths
• cargo clean:
  • removes connections between projects and it's dependencies
  • deletes all cached dependencies with no connections left (configurable via config.toml)
• connections are restored automatically whenever a project is built
• a new command with subcommands:
  • go through dependencies and removes all that have no project connections
  • go through all projects in the cache tracking database and verify that the projects indeed still exist in the system, removing stale project entries and their connections
  • a combination of both

---

Setting the cache directory

It is a feature that would greatly improve flexibility, while being fairly simple. I know people who would put the build cache in a ramdisk. I can envision a situation where the cache is put on a network share, to provide rudimentary sharing between people. Just allowing the user to configure the cache directory would make it much easier to set up. The default should be under $CARGO_HOME, but it should be possible to change independently. I know all the stuff I described above should be possible under Linux using just filesystem manipulation, but this variable makes it much more user friendly.

[epage]

This makes several assumption about CI behavior, probably based on how GH Actions behaves. Note that other runners will behave differently, for example by default GitLab does not upload the cache anywhere. I'm not even sure if the cache is compressed. This shouldn't be an issue, but it is a set of bad assumptions in the design doc and could theoretically could lead to bad design.

I would also like to add that CI interactions should have supporting multiple CI runners from the get go. GitHub is, for better or worse, dominant, especially in mindshare, and the Rust project shouldn't be furthering it's monopoly.

This is mentioning a use case. Nothing in the design is "github specific"

I've seen someone mention clearing old stuff by access time. This would work, in theory, but is also something to be careful around. For example, a lot of desktop systems use relatime, and some people set noatime for performance reasons. Are we even sure that the data is always available?

We are doing our own access time tracking in an sqlite database, see https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#gc

[CosmicHorrorDev]

Currently, Cargo does not even use the whole hash, only 64 bits (sixteen digits) of it. I'm worried that with user-wide caches, collisions may happen

Looks like with 64 bits the probability of getting a collision starts becoming more of a possibility when you start getting close to a billion entries

From: https://en.wikipedia.org/wiki/Birthday_problem#Probability_table

[luser]

I read through your design document (and some of the discussion here) and just wanted to chime in with a few things that might be interesting or useful.

First, this includes an effort to allow packages to declare themselves as idempotent. Longer term, we'll have sandboxing to help infer / verify idempotence.

When I initially implemented Rust compile caching in sccache I tested it by compiling servo. You might find the things that I uncovered in the process that were causing cache misses due to builds not being reproducible interesting: mozilla/sccache#97

The cache lookup will be extended with plugins to read and/or write to different sources. Open source projects and companies can have their CI read from and write to their cache. Individuals who trust the CI can then configure their plugin to read from the CI cache.

I like this! I really don't like the "save and restore entire directories" cache model that CI systems have foisted on us. It's extremely inefficient. The motivating factor for sccache's existence was the move from dedicated build machines to ephemeral builders in the cloud. Using object storage as the cache backend makes way more sense than trying to make the square peg disk-based cache fit in the circular hole of an ephemeral build environment. If you haven't looked at Bazel's remote caching design you might find it interesting:

• https://bazel.build/remote/caching - kind of a high-level overview
• https://bitrise.io/blog/post/bazel-remote-caching-api - more of an explainer
• https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/execution/v2/remote_execution.proto - the gRPC protocol definitions

This is a milestone on the way to remote caches. Remote caches allows access to CI build caches for the same project you are developing on, allowing full reuse at the cost of network access.

This is certainly a noble goal, but when we discussed this in the context of sccache the security concerns were a bit concerning. Cache poisoning attacks that can result in code execution on developer machines are a terrifying prospect. (I wish I had something more useful to say about this!)

I would be interested to find out what the maximum theoretical cache hit rate possible from this effort would be, assuming all crates had reproducible builds and cargo could calculate hash keys with perfect accuracy. My take was that you could probably do OK for crates with zero dependencies, but anything with a dependency is going to make it extremely hard to cache unless all of the projects you compile are using the exact same versions of all of their transitive dependencies. (As if they were in one big workspace.) Crate features would similarly need to be identical across projects to enable cache hits.

I didn't see anything in the design document about providing feedback to users on why they aren't getting cache hits—is that something you've considered? sccache can't tell you why you got a cache miss instead of a hit, but it does have a facility to track compiles that it was not able to cache for various reasons which has been helpful.

[epage]

Note that in our case, full rustc flag determinism is not needed as we fingerprint at a higher level.

As for user read access to CI cache, again this comes down to trust. I see this most likely of interest to companies. However, if you isolate PR caches from push caches then I'd be willing to trust cargo, as someone who sees most changes going in.

As for misses from dependency version differences in a tree, if you're only pulling from the project you cloned and it commits a lockfile, this shouldn't be a problem. However, this does greatly hamper local-only reuse. That, and we're unsure if other efforts might significantly reduce the value of this, is why we've not put as much emphasis on this (e.g. carrying the project goal forward to 2025h1).