linux-kernel - Re: Quad core CPUs loaded at only 50% when running a CPU and mmap intensive multi-threaded task

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Message-ID: <20080825134801.GN1408@mit.edu>
Date:	Mon, 25 Aug 2008 09:48:01 -0400
From:	Theodore Tso <tytso@....edu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	edwin <edwintorok@...il.com>, Ingo Molnar <mingo@...e.hu>,
	rml@...h9.net, Linux Kernel <linux-kernel@...r.kernel.org>,
	"Thomas Gleixner mingo@...hat.com" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: Quad core CPUs loaded at only 50% when running a CPU and mmap
	intensive multi-threaded task

On Mon, Aug 25, 2008 at 01:41:17PM +0200, Peter Zijlstra wrote:
> 
> I would certainly consider this for small (< 1M?) files. With mmap the
> faults and pte overhead aren't free either, and the extra memcpy from
> pread() isn't that much.
> 

Even for very big files, if you're only doing a single sequential pass
over a very large file (for example when converting a Canon raw image
file to TIFF format --- I know because I was trying to optimize dcraw
a while aback), you take the page fault for each 4k page, and so
simply using read/pread is faster.  And that's on a single-threded
program.  With a multithreaded program, the locking issues come on top
of that.

Maybe if I had used hugepages it would have been a win, I suppose, but
I never tried the experiment.  And this was several years ago, on much
older hardware, so maybe the relative times of doing the memory copy
versus the page fault, but I wouldn't be surprised if it's even more
expensive to do the mmap, relatively speaking.

							- Ted
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