linux-kernel - Re: [rfc] no ZERO

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Date:	Thu, 05 Apr 2007 13:23:04 -0400
From:	Valdis.Kletnieks@...edu
To:	William Lee Irwin III <wli@...omorphy.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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, 04 Apr 2007 22:37:29 PDT, William Lee Irwin III said:

> The actual phenomenon of concern here is dense matrix code with sparse
> matrix inputs. The matrices will typically not be vast but may span 1MB
> or so of RAM (1024x1024 is 1M*sizeof(double), and various dense matrix
> algorithms target ca. 300x300). Most of the time this will arise from
> the use of dense matrix code as black box solvers called as a library
> by programs not terribly concerned about efficiency until something
> gets explosively inefficient (and maybe not even then), or otherwise
> numerically naive programs. This, however, is arguably the majority of
> the usage cases by end-user invocations, so beware, though not too much.

Amen, brother! :)

At least in my environment, the vast majority of matrix code is actually run by
graduate students under the direction of whatever professor is the Principal
Investigator on the grant. As a rule, you can expect the grad student to know
about rounding errors and convergence issues and similar program *correctness*
factors.  But it's the rare one that has much interest in program *efficiency*.
If it takes 2 days to run, that's 2 days they can go get another few pages of
thesis written while they wait. :)

The code that gets on our SystemX (a top-50 supercomputer still) is usually
well-tweaked for efficiency.  However, that's just one system - there's on the
order of several hundred smaller compute clusters and boxen and SGI-en on
campus where "protect the system from cargo-cult programming by grad students"
is a valid kernel goal. ;)

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