linux-kernel - Re: missing madvise functionality

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Message-ID: <46149D13.4000108@cosmosbay.com>
Date:	Thu, 05 Apr 2007 08:54:11 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Ulrich Drepper <drepper@...hat.com>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jakub Jelinek <jakub@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: missing madvise functionality

Ulrich Drepper a écrit :
> Eric Dumazet wrote:
>> Database workload, where the user multi threaded app is constantly
>> accessing GBytes of data, so L2 cache hit is very small. If you want to
>> oprofile it, with say a CPU_CLK_UNHALTED:5000 event, then find_vma() is
>> in the top 5.
> 
> We did have a workload with lots of Java and databases at some point
> when many VMAs were the issue.  I brought this up here one, maybe two
> years ago and I think Blaisorblade went on and looked into avoiding VMA
> splits by having mprotect() not split VMAs and instead store the flags
> in the page table somewhere.  I don't remember the details.
> 
> Nothing came out of this but if this is possible it would be yet another
> way to avoid mmap_sem locking, right?
> 

I was speaking about oprofile needs, that may interfere with target process 
needs, since oprofile calls find_vma() on the target process mm and thus zap 
its mmap_cache.

oprofile is yet another mmap_sem user, but also a mmap_cache destroyer.

We could at least have a separate cache, only for oprofile.

If done correctly we might avoid taking mmap_sem when the same vm_area_struct 
contains EIP/RIP snapshots.


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