Evict old files from the cache prior to saving

[planetmarshall]

uses the --evict-older-than option to evict any files from the cache that were not used during the job run. This prevents the cache from growing indefinitely as otherwise old files from previously restored caches are never deleted.

For a test I performed the following -

1. Built CMake 3.30.6
   • evictOldFiles=true - cache Size 39Mb
   • evictOldFiles=false - cache Size 39Mb
2. Then Built CMake 3.31.2, reusing the cache from 1. Minimal cache hits in both cases due to change in sources.
   • evictOldFiles=true - cache Size 39Mb
   • evictOldFiles=false - cache Size 78Mb
3. Then Built CMake 3.31.2 again
   • evictOldFiles=true - cache Size 39Mb - Cache hits 100%
   • evictOldFiles=false - cache Size 78Mb - Cache hits 100%

[hendrikmuhs]

Thanks for the PR.

I thought a bit about it. First a comment:

This prevents the cache from growing indefinitely as otherwise old files from previously restored caches are never deleted.

There is already a max-size option with a default of 500M, ccache should respect that and keep the total cache size below that boundary.

Nevertheless, I think a 2nd option to evict based on age makes sense to me. I have some questions:

• Did you think about simply exposing the --evict-older-than option as is?
• What is reason to turn it into a bool?

I am suggesting the following:

We could support both cases, the use case of using evict-older-than as ccache designed it and your case of evicting everything that didn't hit in the current job run.

If you define evict-older-than as string, you could support additional magic values for the special cases:

• ``(empty) for no eviction(default, current behavior)
• job to evict everything older than this run.
• everything else passes the value as is, an int with either d for days or s for seconds

That way we keep the option as in ccache and just add job for this special case.

[planetmarshall]

There is already a max-size option with a default of 500M, ccache should respect that and keep the total cache size below that boundary.

Sure, but with the CI cache limited at 10Gb (this can be quickly exhausted if you're working on something like, say, LLVM) I think it makes sense to be able to trim the cache where possible.

Nevertheless, I think a 2nd option to evict based on age makes sense to me. I have some questions:

* Did you think about simply exposing the `--evict-older-than` option as is?

I did - but I explicitly wanted to use the job time for this parameter which is dynamic. I also thought about using the namespace feature for this - maybe something for a future enhancement.

* What is reason to turn it into a bool?

Just because the job time is an internal calculation - so you can't supply it as a static value.

I am suggesting the following:

We could support both cases, the use case of using evict-older-than as ccache designed it and your case of evicting everything that didn't hit in the current job run.

If you define evict-older-than as string, you could support additional magic values for the special cases:

* ``(empty) for no eviction(default, current behavior)

* `job` to evict everything older than this run.

* everything else passes the value as is, an int with either `d` for days or `s` for seconds

That way we keep the option as in ccache and just add job for this special case.

Yes I like this suggestion - it works for my use case. I will update the PR.

[hendrikmuhs]

👍 great! LGTM