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From: Matteo Frigo <Matteo.Frigo@qrclab.com>
To: John Carr <jfc@MIT.EDU>
Cc: ygrek <ygrekheretix@gmail.com>,  caml-list@inria.fr
Subject: Re: [Caml-list] ackermann microbenchmark strange results
Date: Wed, 24 Apr 2013 13:35:22 -0400	[thread overview]
Message-ID: <87ppxjvnl1.fsf@matteo.qrclab.com> (raw)
In-Reply-To: <201304241557.r3OFvT9a012995@outgoing.mit.edu> (John Carr's message of "Wed, 24 Apr 2013 11:57:29 -0400")

John Carr <jfc@MIT.EDU> writes:

> Try changing loop alignment by editing assembly code.  The address of
> ack is different in the different versions.  Modern Intel processors are
> sensitive to code alignment.  There is a limit on the number of branch
> prediction table entries per cache line.  An instruction that crosses a
> cache line boundary may be slower than an instruction within a cache
> line.  I am not surprised to see a 20% difference caused by an apparently
> irrelevant code change.

See also

    http://people.csail.mit.edu/nkushman/papers/ms-thesis.pdf

for a detailed analysis of a related problem on older (1998) hardware.


  parent reply	other threads:[~2013-04-24 17:35 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-04-24 10:35 ygrek
2013-04-24 15:57 ` John Carr
2013-04-24 16:08   ` Alain Frisch
2013-04-24 16:57     ` Anthony Tavener
2013-04-24 17:26     ` rixed
2013-04-24 17:31       ` Török Edwin
2013-04-24 17:35   ` Matteo Frigo [this message]
2013-04-26  3:31   ` ygrek
2013-04-24 17:40 ` Xavier Leroy

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