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

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Message-ID: <465C5BE0.4090903@redhat.com>
Date:	Tue, 29 May 2007 12:59:12 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ulrich Drepper <drepper@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: [PATCH] MM: implement MADV_FREE lazy freeing of anonymous memory

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...

-- 
Politics is the struggle between those who want to make their country
the best in the world, and those who believe it already is.  Each group
calls the other unpatriotic.
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