linux-kernel - Re: missing madvise functionality

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Date:	Fri, 06 Apr 2007 12:59:42 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ulrich Drepper <drepper@...hat.com>
CC:	Rik van Riel <riel@...hat.com>, Jakub Jelinek <jakub@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: missing madvise functionality

Ulrich Drepper wrote:
> Nick Piggin wrote:
> 
>>Cool. According to my thinking, madvise(MADV_DONTNEED) even in today's
>>kernels using down_write(mmap_sem) for MADV_DONTNEED is better than
>>mmap/mprotect, which have more fundamental locking requirements, more
>>overhead and no benefits (except debugging, I suppose).
> 
> 
> It's a tiny bit faster, see
> 
>   http://people.redhat.com/drepper/dontneed.png
> 
> I just ran it once so the graph is not smooth.  This is on a UP dual
> core machine.  Maybe tomorrow I'll turn on the big 4p machine.

Hmm, I saw an improvement, but that was just on a raw syscall test
with a single page chunk. Real-world use I guess will get progressively
less dramatic as other overheads start being introduced.

Multi-thread performance probably won't get a whole lot better (it does
eliminate 1 down_write(mmap_sem), but one remains) until you use my
madvise patch.


> I would have to see dramatically different results on the big machine to
> make me change the libc code.  The reason is that there is a big drawback.
> 
> So far, when we allocate a new arena, we allocate address space with
> PROT_NONE and only when we need memory the protection is changed to
> PROT_READ|PROT_WRITE.  This is the advantage of catching wild pointer
> accesses.

Sure, yes. And I guess you'd always want to keep that options around as
a debugging aid.

-- 
SUSE Labs, Novell Inc.
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