linux-ext4 - Re: [PATCH v2 0/3] crc32c: Add faster algorithm and self-test code

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Date:	Tue, 27 Sep 2011 16:07:37 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Andreas Dilger <adilger.kernel@...ger.ca>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Theodore Tso <tytso@....edu>,
	David Miller <davem@...emloft.net>
Cc:	Bob Pearson <rpearson@...temfabricworks.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mingming Cao <cmm@...ibm.com>,
	linux-crypto <linux-crypto@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 0/3] crc32c: Add faster algorithm and self-test code

On Tue, Sep 27, 2011 at 03:12:39PM -0700, Darrick J. Wong wrote:
> Hi all,
> 
> This patchset replaces the current crc32c software implementation, which uses a
> slow per-byte lookup table algorithm, with a faster implementation that uses an
> adaptation of the slice-by-8 algorithm that Bob Pearson has been pushing for
> crc32.
> 
> The motivation for this patchset is that I am working on adding full metadata
> checksumming to ext4[1].  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.
> 
> Please have a look at the patches, and please feel free to suggest any changes.
> I will be at LPC next week if anyone wishes to discuss, debate, or protest.

Oops, ignore that sentence, since LPC has long passed. :(

--D
> 
> 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.
> 
> v2: Use the crypto test manager code to check crc32c operation.
> 
> --D
> 
> [1] https://ext4.wiki.kernel.org/index.php/Ext4_Metadata_Checksums
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