# Plan to test parallel rustc

**URL:** https://internals.rust-lang.org/t/plan-to-test-parallel-rustc/11487
**Category:** announcements
**Created:** [December 16, 2019, 9:08pm UTC](https://internals.rust-lang.org/t/plan-to-test-parallel-rustc/11487 "2019-12-16T21:08:56Z")
**Posts on this page:** 1
**Showing post:** 16

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### Author: ![josh](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/josh/32/5934_2.png) [@josh](https://internals.rust-lang.org/u/josh)
#### Post date: [December 18, 2019, 6:34am UTC](https://internals.rust-lang.org/t/plan-to-test-parallel-rustc/11487/16 "2019-12-18T06:34:35Z")

</div>

I ran another test, with a [much smaller crate](https://github.com/git-series/git-series/) (commit hash c570a015e15214be46a7fd06ba08526622738e20), and got less consistent results, more in the noise.

Non-parallel rustc:

```nohighlight
real	0m19.857s
user	2m33.522s
sys	0m9.018s

```

```nohighlight
real	0m19.849s
user	2m30.636s
sys	0m8.092s

```

Parallel rustc (EDIT: ignore the first time measurement, see below for how this happened):

```nohighlight
real	0m20.149s
user	3m30.696s
sys	0m15.814s

```

```nohighlight
real	0m18.522s
user	2m42.483s
sys	0m10.206s

```

Because those two builds with the parallel rustc gave such wildly different results, I tried a few more builds with the parallel rustc:

```nohighlight
real	0m18.837s
user	2m43.548s
sys	0m10.142s

```

```nohighlight
real	0m18.509s
user	2m43.081s
sys	0m10.904s

```

```nohighlight
real	0m18.613s
user	2m43.926s
sys	0m10.744s

```

Those three runs seem _far_ more consistent, both in user time and in wall-clock time.

EDIT: oh, I just realized the problem, and it has _nothing_ to do with rustc. The build of git2 (and potentially other libraries) compiled C code, and that compilation used ccache, missing the cache the first time and hitting the cache the remaining times. Nevermind, please ignore that first build. Leaving this up to document a potential pitfall for others doing build benchmarking.

Looks like smaller crates get a decent improvement in build time as well, though not quite as substantial.

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_[View the full topic](https://internals.rust-lang.org/t/plan-to-test-parallel-rustc/11487)._