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docs contributing.md: document using mold linker with nextest

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On my machine, this halves the time taken by `cargo insta test --workspace
--test-runner nextest`.

I don't believe there would be a worthwhile improvement when not
using `nextest`. I also documented how to tell whether using `mold`
is worthwhile.

I was reminded of this by https://github.com/martinvonz/jj/pull/2858.
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Ilya Grigoriev 2024-01-12 22:28:15 -08:00
parent 0af1a15162
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37
docs/contributing.md
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@ -164,8 +164,41 @@ These are listed roughly in order of decreasing importance.
every `jj log`.
6. To run tests more quickly, use `cargo nextest run --workspace`. To
use `nextest` with `insta`,
use `cargo insta test --workspace --test-runner nextest`.
use `nextest` with `insta`, use `cargo insta test --workspace
--test-runner nextest`.
On Linux, you may be able to speed up `nextest` even further by using
the `mold` linker, as explained below.
### Using `mold` for faster tests on Linux
On a machine with a multi-core CPU, one way to speed up
`cargo nextest` on Linux is to use the multi-threaded [`mold`
linker](https://github.com/rui314/mold). This linker may help
if, currently, your CPU is underused while Rust is linking test
binaries. Before proceeding with `mold`, you can check whether this is
an issue worth solving using a system monitoring tool such as `htop`.
`mold` is packaged for many distributions. On Debian, for example,
`sudo apt install mold` should just work.
A simple way to use `mold` is via the `-run` option, e.g.:
```shell
mold -run cargo insta test --workspace --test-runner nextest
```
There will be no indication that a different linker is used, except for
higher CPU usage while linking and, hopefully, faster completion. You
can verify that `mold` was indeed used by running
`readelf -p .comment target/debug/jj`.
There are also ways of having Rust use `mold` by default, see the ["How
to use" instructions](https://github.com/rui314/mold#how-to-use).
On recent versions of MacOS, the default linker Rust uses is already
multi-threaded. It should use all the CPU cores without any configuration.
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