linux-kernel - Re: [v2.6.26] what's brewing in x86.git for v2.6.26

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Message-Id: <1208426793.10305.1248377703@webmail.messagingengine.com>
Date:	Thu, 17 Apr 2008 12:06:33 +0200
From:	"Alexander van Heukelum" <heukelum@...tmail.fm>
To:	"Andi Kleen" <andi@...stfloor.org>, "Ingo Molnar" <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [v2.6.26] what's brewing in x86.git for v2.6.26


On Wed, 16 Apr 2008 22:50:51 +0200, "Andi Kleen" <andi@...stfloor.org>
said:
> Ingo Molnar <mingo@...e.hu> writes:
> 
> >  - generalized bitops - they are faster and smaller.
> 
> Faster in a 100% bogus benchmark with the most unrealistic input data
> set one can imagine with some effort. They might be faster or they
> might be slower, nobody really knows currently.

Hello Andi, Ingo,

The input for the first 'benchmark' was indeed completely unrealistic.
They did show a very convincing speedup, though. This program was
really written to verify the implementation and was later converted
to a benchmark. Many benchmarks are unrealistic. I also wrote a
benchmark for find_first_bit and find_next_bit:
        http://heukelum.fastmail.fm/find_first_bit

My conclusion would be: the speed of the generic bitmap implementation
is either better than or at least comparable to the current private
implementations in i386/x86_64. The generic version is out-of-line,
while the private implementation of i386 was inlined: this causes a
regression for very small bitmaps. However, if the bitmap size is
a constant and fits a long integer, the updated generic code should
inline an optimized version, like x86_64 currently does it.

I think the change is a good one.

Greetings,
    Alexander
-- 
  Alexander van Heukelum
  heukelum@...tmail.fm

-- 
http://www.fastmail.fm - The professional email service

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