linux-kernel - Re: [PATCH] Optimize bitmap

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Date:	Sat, 26 May 2012 07:57:55 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] Optimize bitmap_weight

2012/5/11 Akinobu Mita <akinobu.mita@...il.com>:
> The current implementation of bitmap_weight simply evaluates the
> population count for each long word of the array, and adds.
>
> The subsection "Counting 1-bits in an Array" in the revisions of
> the book 'Hacker's Delight' explains more superior methods than
> the naive method.
>
> http://www.hackersdelight.org/revisions.pdf
> http://www.hackersdelight.org/HDcode/newCode/pop_arrayHS.c.txt
>
> My benchmark results on Intel Core i3 CPU with 32-bit kernel
> showed 50% faster for 8192 bits bitmap.  However, it is not faster
> for small bitmap (< BITS_PER_LONG * 8) than the naive method.
> So if the bitmap size is known to be small at compile time,
> use the naive method.

My benchmark was bogus.  I used __sw_hweight32 for hweight_long.
It is much slower on x86 with X86_FEATURE_POPCNT.

This patch slows bitmap_weight().  So please drop this patch
(bitmap_weight-optimize-for-large-bitmaps.patch) from -mm tree.
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