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

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Message-ID: <4AB4BC6A.3020104@redhat.com>
Date:	Sat, 19 Sep 2009 14:11:38 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Arjan van de Ven <arjan@...radead.org>
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 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.

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.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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