linux-kernel - Re: efficient access to "rotational"; new fcntl?

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Message-ID: <20090919133013.5a27290c@infradead.org>
Date:	Sat, 19 Sep 2009 13:30:13 +0200
From:	Arjan van de Ven <arjan@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Jim Meyering <jim@...ering.net>, Theodore Tso <tytso@....edu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: efficient access to "rotational";  new fcntl?

On Sat, 19 Sep 2009 14:11:38 +0300
Avi Kivity <avi@...hat.com> wrote:

> On 09/19/2009 12:19 PM, Arjan van de Ven wrote:
> >> However, sort *would* benefit, and some UCLA students implemented
> >> that for a term project.  Unfortunately, the project is stalled
> >> because the implementation was not efficient enough, and no one
> >> has found the time to improve it since.
> >>      
> > parallel sort... call me skeptical. My gut feeling is that you'll
> > get killed by communication overhead.
> > (sort seems to be more communication than raw cpu use)
> >
> >    
> 
> Why?  a sort that fits in memory is purely cpu and memory access.

memory access == communication due to cache line bounces....

> 
> Instead of O(N log N) you'd get K * O(N/K log N/K) followed by an
> O(N) merge.  For large N and small K, you get a speedup of roughly K
> (since the O(N) merge is dominated by the preceding sort.

doing big-O arithmatic and then add constant divisions/multipliers is
rather.. dangerous ;)

You actually get K * C1 * O(N/K log N/K) + C2 * O(N)
where C1/C2 is the actual cost of the extra intra-cpu communication.
(and for most sorting, I suspect the communication costs dominate over
CPU cost)

I'd be happy to be proven wrong... 


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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