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

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Date:	Sat, 05 May 2007 09:47:56 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ulrich Drepper <drepper@...hat.com>
CC:	Rik van Riel <riel@...hat.com>,
	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>,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: [PATCH] MM: implement MADV_FREE lazy freeing of anonymous memory

Ulrich Drepper wrote:
> Nick Piggin wrote:
> 
>>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.
> 
> 
> I don't want to judge the numbers since I cannot but I want to make an
> observations: even if in the SMP case MADV_FREE turns out to not be a
> bigger boost then there is still the UP case to keep in mind where Rik
> measured a significant speed-up.  As long as the SMP case isn't hurt
> this is reaosn enough to use the patch.  With more and more cores on one
> processor SMP systems are pushed evermore to the high-end side.  You'll
> find many installations which today use SMP will be happy enough with
> many-core UP machines.

OK, sure. I think we need more numbers though.

And even if this was a patch with _no_ possibility for regressions and it
was a completely trivial one that improves performance in some cases...
one big problem is that it uses another page flag.

I literally have about 4 or 5 new page flags I'd like to add today :) I
can't of course, because we have very few spare ones left.

 From the MySQL numbers on this system, it seems like performance is in the
noise, and MADV_DONTNEED makes the _vast_ majority of the improvement.
This is also the case with Rik's benchmarks, and while he did see some
improvement, I found the runs to be quite variable, so it would be ideal
to get a larger sample.

And the fact that the poor behaviour of the old style malloc/free went
unnoticed for so long indicates that it won't be the end of the world if
we didn't merge MADV_FREE right now.

-- 
SUSE Labs, Novell Inc.
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