On Tue, May 08, 2007 at 04:12:00PM +1000, Nick Piggin wrote:
> I didn't actually check system and user times for the mysql
> benchmark, but that's exactly what I had in mind when I
> mentioned the poor cache behaviour this patch could cause. I
> definitely did see user times go up in benchmarks where I
> measured.
>
> We have percpu and cache affine page allocators, so when
> userspace just frees a page, it is likely to be cache hot, so
> we want to free it up so it can be reused by this CPU ASAP.
> Likewise, when we newly allocate a page, we want it to be one
> that is cache hot on this CPU.
malloc has per-thread arenas, so when using MADV_FREE the pages
should be local to the thread as well (unless the thread has switched
to a different CPU also to the CPU) and in case of sysbench should
be cache hot as well (it is reused RSN). With MADV_DONTNEED you need to
clear the pages while that is not necessary with MADV_FREE.
Jakub
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