Re: [PATCH v2 01/12] gitlab: enable ccache for many build jobs

[Michael Tokarev]

24.08.2023 19:38, Alex Bennée wrote:
> From: Daniel P. Berrangé <berrange@redhat.com>
>
> The `ccache` tool can be very effective at reducing compilation times
> when re-running pipelines with only minor changes each time. For example
> a fresh 'build-system-fedora' job will typically take 20 minutes on the
> gitlab.com shared runners. With ccache this is reduced to as little as
> 6 minutes.

I've been using ccache when building qemu in debian, for quite a while.

The problem here, in the way qemu build system works, is that the cache
is hugely dependent on the path to the source. You change just one char
in there (/build/qemu/v8.1.0/ => /build/qemu/v8.1.1) and whole cache becomes
unusable, it all gets compiled anew.  This is because qemu build sys uses
absolute file names when building, and this is detected by ccache, so
the source dir gets mixed into the hash together with gcc version and
other things.

Dunno how well this will work in the qemu ci though.

/mjt