linux-kernel - Re: [PATCH v2] bitmap: speedup in bitmap_find_free

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Date:	Tue, 9 Apr 2013 13:22:02 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Chanho Min" <chanho.min@....com>
Cc:	"'Nadia Yvette Chambers'" <nyc@...omorphy.com>,
	"'Jiri Kosina'" <jkosina@...e.cz>,
	"'Guennadi Liakhovetski'" <lg@...x.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] bitmap: speedup in bitmap_find_free_region when
 order is 0

On Tue, 9 Apr 2013 15:34:44 +0900 "Chanho Min" <chanho.min@....com> wrote:

> >It seems excessively complicated to me.  Why not change
> >bitmap_find_free_region() to skip the leading all-ones words and when
> >it finds a not-all-ones word, adjust `pos' then fall into the existing
> >bit-at-a-time search?
> 
> Do we need the additional 'if' inside the for loop for implementing it?

It was just a concept - there are various ways of optimizing a real
implementation.  Move the code into userspace then run some
microbenchmarks.

> I thought the separation of the routine for order=0 is the way to avoid
> the impact on the existing codes and its performance.

The existing code doesn't have any performance ;) It's so darn slow
that even a minor effort here will yield large (ie order-of-magnitude)
performance improvements.
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