linux-kernel - Re: missing madvise functionality

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Date:	Thu, 05 Apr 2007 19:52:09 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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

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.

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.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖

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