linux-kernel - Re: [rfc] no ZERO

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Message-ID: <20070405023026.GE11192@wotan.suse.de>
Date:	Thu, 5 Apr 2007 04:30:26 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Valdis.Kletnieks@...edu, Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	tee@....com, holt@....com, Andrea Arcangeli <andrea@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] no ZERO_PAGE?

On Wed, Apr 04, 2007 at 05:27:31PM -0700, Linus Torvalds wrote:
> 
> 
> On Wed, 4 Apr 2007, Valdis.Kletnieks@...edu wrote:
> > 
> > I'd not be surprised if there's sparse-matrix code out there that wants to
> > malloc a *huge* array (like a 1025x1025 array of numbers) that then only
> > actually *writes* to several hundred locations, and relies on the fact that
> > all the untouched pages read back all-zeros.
> 
> Good point. In fact, it doesn't need to be a malloc() - I remember people 
> doing this with Fortran programs and just having an absolutely incredibly 
> big BSS (with traditional Fortran, dymic memory allocations are just not 
> done).

Sparse matrices are one thing I worry about. I don't know enough about
HPC code to know whether they will be a problem. I know there exist
data structures to optimise sparse matrix storage...

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