Re: [PATCH v5.2 00/14] crc32c: Add faster algorithm and self-test code

[Joel Becker <jlbec@evilplan.org>]

On Thu, Dec 01, 2011 at 12:13:41PM -0800, Darrick J. Wong wrote:
> Hi all,
> 
> This patchset (re)uses Bob Pearson's crc32 slice-by-8 code to stamp out a
> software crc32c implementation.  It removes the crc32c implementation in
> crypto/ in favor of using the stamped-out one in lib/.  There is also a change
> to Kconfig so that the kernel builder can pick an implementation best suited
> for the hardware.
> 
> The motivation for this patchset is that I am working on adding full metadata
> checksumming to ext4.  As far as performance impact of adding checksumming
> goes, I see nearly no change with a standard mail server ffsb simulation.  On a
> test that involves only file creation and deletion and extent tree writes, I
> see a drop of about 50 pcercent with the current kernel crc32c implementation;
> this improves to a drop of about 20 percent with the enclosed crc32c code.
> 
> When metadata is usually a small fraction of total IO, this new implementation
> doesn't help much because metadata is usually a small fraction of total IO.
> However, when we are doing IO that is almost all metadata (such as rm -rf'ing a
> tree), then this patch speeds up the operation substantially.
> 
> Incidentally, given that iscsi, sctp, and btrfs also use crc32c, this patchset
> should improve their speed as well.  I have not yet quantified that, however.

	I thought they usually used the SSE instruction for crc32 or
equivalent.

Joel

-- 

"I almost ran over an angel
 He had a nice big fat cigar.
 'In a sense,' he said, 'You're alone here
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			jlbec@evilplan.org