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To: Rob Landley <landley@trommello.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Rik van Riel <riel@conectiva.com.br>,
        linux-kernel@vger.kernel.org
Subject: Re: can we make anonymous memory non-EXECUTABLE?
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From: Doug McNaught <doug@wireboard.com>
Date: 09 Jan 2002 14:47:32 -0500
In-Reply-To: Rob Landley's message of "Wed, 9 Jan 2002 06:32:39 -0500"
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Rob Landley <landley@trommello.org> writes:

> Glibc does mmap instead of brk because theoretically brk can leave wasted 
> memory between fragments, although apparently nobody's ever seen more than 
> 10% waste in a live program, and the speed penality of taking a soft page 
> fault at access time to muck about with the page tables is a LOT bigger than 
> 10%...

The other reason glibc uses mmap() is because your shared libraries
are (usually) mapped smack dab in the middle of your address space.
brk() assumes a contiguous heap, so when it hits your libraries, it
has to stop, even if there is a gig of VM above the libs.  mmap() can
give you an arbitrary chunk of the address space, so glibc uses it for
'large' allocations.

-Doug
-- 
Let us cross over the river, and rest under the shade of the trees.
   --T. J. Jackson, 1863
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