linux-ext4 - Re: [PATCH 3/6 -v3] libext2fs: add ext2fs

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Date:	Tue, 27 Nov 2012 09:50:23 -0800
From:	Zach Brown <zab@...bo.net>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 3/6 -v3] libext2fs: add ext2fs_bitcount() function

On Tue, Nov 27, 2012 at 12:16:17AM -0500, Theodore Ts'o wrote:
> I did some quick benchmarking, and the difference it makes when
> checking 4TB's worth of bitmaps is negligble:
> 
> slow popcount: 0.2623
> fast popcount: 0.0700
> 
> For a 128TB's worth of bitmaps, the time difference is:
> 
> slow popcount: 8.0185
> fast popcount: 2.2066
> 
> I measured running e2fsck on an empty 128TB file system, and that took
> 202 CPU seconds (assuming all of the fs metadata blocks are in cache),
> so with this optimization we would save at most 3%.  (For comparison,
> using an unmodified 1.42.6 e2fsck, it burned 392.7 CPU seconds.)

Nice, thanks for taking the time to get numbers.

> My conclusion is that using __builtin_popcnt() is a nice-to-have, and
> if someone sends me patches I'll probably take them as a optimization,
> but it's not super high priority for me.

Agreed.  I'll chuck it at the end of my fun-projects-some-day list as
well, but getting it right for all the platforms that e2fsprogs
supports.. meh :).

- z
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