Re: qemu CI & ccache: cache size is too small

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, May 27, 2024 at 01:49:41PM +0300, Michael Tokarev wrote:
> Hi!
> 
> Noticed today that a rebuild of basically the same tree (a few commits apart)
> in CI result in just 11% hit rate of ccache:
> 
> https://gitlab.com/mjt0k/qemu/-/jobs/6947445337#L5054
> 
> while it should be near 100%.  What's interesting in there is:
> 
> 1) cache size is close to max cache size,
> and more important,
> 2) cleanups performed 78
> 
> so it has to remove old entries before it finished the build.
> 
> So effectively, our ccache usage is an extra burden, not help.

I think this ends up being different per job. If I try the
'build-system-fedora' job, for example, I get a 99% cache
hit rate, and 0.2 GB usage of cache storage

https://gitlab.com/berrange/qemu/-/jobs/6876054586

$ ccache --show-stats
Cacheable calls:   3018 / 3208 (94.08%)
  Hits:              49 / 3018 ( 1.62%)
    Direct:           0 /   49 ( 0.00%)
    Preprocessed:    49 /   49 (100.0%)
  Misses:          2969 / 3018 (98.38%)
Uncacheable calls:  190 / 3208 ( 5.92%)
Local storage:
  Cache size (GB):  0.2 /  0.5 (30.55%)
  Hits:              49 / 3018 ( 1.62%)
  Misses:          2969 / 3018 (98.38%)

If I compare the jobs, the big differences are the target lists:

  CentOS: '--target-list=ppc64-softmmu or1k-softmmu s390x-softmmu x86_64-softmmu rx-softmmu sh4-softmmu'

  Fedora: '--target-list=microblaze-softmmu mips-softmmu xtensa-softmmu m68k-softmmu riscv32-softmmu ppc-softmmu sparc64-softmmu'

And  then a few minor things:

  CentOS: '--disable-nettle' '--enable-gcrypt' '--enable-vfio-user-server' '--enable-modules' '--enable-trace-backends=dtrace'

  Fedora: '--disable-gcrypt' '--enable-nettle'

the crypto won't make a diffeernce to caching. Modules ought not to make a
difference either, as that's just moving some .o files from the exe to a
so, not adding many more exes.

The trace backends will add quite a few .o files, but I'm not sure that
will impact cache.

IOW, I bet the target list has the big difference on the amount of data
that needs to be cached, to explain the different cache usage.

I wonder what the picture looks like for cache hits / cache disk usage
across all the other jobs. Is CentOS an outlier or is FEdora an outlier?

We do want cache to be in the 90+% mark if possible as it has a big impact
on build time.



> I should be increased at least, I think.  But it's actually difficult
> to say really, - is the cache shared between all builds or is it unique
> for each build config?  Because if it the former, it shouldn't even
> work since different ccache versions use different format of the files
> in cache.

It is unique per job per buildtest-template.yml:

  cache:
    paths:
      - ccache
    key: "$CI_JOB_NAME"
    when: always


> What's unique in my pipeline run - I ran just a single build job
> in two pipelines, nothing more.

In my test I ran a job, then re-ran it in the same pipeline.


With regards,
Daniel
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