[Intel-gfx] gem cache size vs performance

[Li, Shaohua]

[Sent to external maillist this time, sorry if you receive two copies]

Hi,
I recently did some benchmarks to verify the relationship about gem cache
size and performance. The workload is x11perf -rgb10text. I use attached
patch to gather statistics about gem cache.

max gem cache size is 64, the performance is best.
Max gem cache size is >= 32, the cache hit times reach max and not change
any more if adjusting cache size higher.
When cache size increase, order 0 object allocation reduced which is
expected, but interesting is order 3 object allocation increase.

Since the cache is never freed, the memory used could be very big. From my
testing, it appears just a 64 max cache size is already good and can save
a lot of memory, but this might be workload dependent.

Thanks,
Shaohua
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