Re: [PATCH] MM: implement MADV_FREE lazy freeing of anonymous memory

[Rik van Riel <riel@redhat.com>]

Nick Piggin wrote:
> Rik van Riel wrote:
>> With lazy freeing of anonymous pages through MADV_FREE, performance of
>> the MySQL sysbench workload more than doubles on my quad-core system.
> 
> OK, I've run some tests on a 16 core Opteron system, both sysbench with
> MySQL 5.33 (set up as described in the freebsd vs linux page), and with
> ebizzy.
> 
> What I found is that, on this system, MADV_FREE performance improvement
> was in the noise when you look at it on top of the MADV_DONTNEED glibc
> and down_read(mmap_sem) patch in sysbench.

It turns out that setting the pte accessed bit in hardware
can apparently take a few thousand CPU cycles - 3000 cycles
is the number I've heard for one CPU family.

This is a similar number of cycles as is needed to zero out
a page.  Giving a cache hot page to userspace could cancel
out the rest of the cost of the page fault handling.

Lets stick with the simpler MADV_DONTNEED code for now and
save the page flag for something else...

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