| 9ec02f9d | 29-Aug-2023 |
Christopher Serr <[email protected]> |
Decouple `serde` from its `derive` crate (#6917)
By not activating the `derive` feature on `serde`, the compilation speed
can be improved by a lot. This is because `serde` can then compile in
parall
Decouple `serde` from its `derive` crate (#6917)
By not activating the `derive` feature on `serde`, the compilation speed
can be improved by a lot. This is because `serde` can then compile in
parallel to `serde_derive`, allowing it to finish compilation possibly
even before `serde_derive`, unblocking all the crates waiting for
`serde` to start compiling much sooner.
As it turns out the main deciding factor for how long the compile time of a
project is, is primarly determined by the depth of dependencies rather
than the width. In other words, a crate's compile times aren't affected
by how many crates it depends on, but rather by the longest chain of
dependencies that it needs to wait on. In many cases `serde` is part of
that long chain, as it is part of a long chain if the `derive` feature
is active:
`proc-macro2` compile build script > `proc-macro2` run build script >
`proc-macro2` > `quote` > `syn` > `serde_derive` > `serde` >
`serde_json` (or any crate that depends on serde)
By decoupling it from `serde_derive`, the chain is shortened and compile
times get much better.
Check this issue for a deeper elaboration:
https://github.com/serde-rs/serde/issues/2584
For `wasmtime` I'm seeing a reduction from 24.75s to 22.45s when
compiling in `release` mode. This is because wasmtime through `gimli`
has a dependency on `indexmap` which can only start compiling when
`serde` is finished, which you want to happen as early as possible so
some of wasmtime's dependencies can start compiling.
To measure the full effect, the dependencies can't by themselves
activate the `derive` feature. I've upstreamed a patch for
`fxprof-processed-profile` which was the only dependency that activated
it for `wasmtime` (not yet published to crates.io). `wasmtime-cli` and
co. may need patches for their dependencies to see a similar
improvement.
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