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?

[codyps]

Note that setting target-dir or CARGO_TARGET_DIR (as suggested in #5931 (comment)) right now to a fixed location to have a per-user target-dir will break things badly in some cases due to #12516 (edit: apparently already linked above in the comments)

[idelvall]

• When expecting to compile a package

  • If in cache, copy it into CARGO_TARGET_DIR
  • If not in cache, compile it and write it to cache

@epage could you avoid the copy to CARGO_TARGET_DIR and read the binaries directly from the new cache?

That would help us having three mount caches at earthly/lib/rust without duplicated entries: One for CARGO_HOME, other for CARGO_TARGET_DIR and other for the new cache.

[epage]

Yes, we could have locks on a per-cached item basis and read directly from it. Whether we do depends on how much we trust the end-to-end process.

[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!