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Date:	Wed, 18 Jan 2012 15:01:14 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Darrick J. Wong" <djwong@...ibm.com>
Cc:	Herbert Xu <herbert@...dor.hengli.com.au>,
	Theodore Tso <tytso@....edu>,
	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>,
	Bob Pearson <rpearson@...temfabricworks.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-crypto <linux-crypto@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org,
	linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH v5.4 00/13] crc32c: Add faster algorithm and self-test
 code

On Wed, 18 Jan 2012 14:27:33 -0800
"Darrick J. Wong" <djwong@...ibm.com> wrote:

> 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.
> This latest submission combines Bob's patches from late August 2011 with mine
> so that they can be one coherent patch set.  Please excuse my inability to
> combine some of the patches; I've been advised to leave Bob's patches alone and
> build atop them instead. :/
> 
> Since the last posting, I've also collected some crc32c test results on a bunch
> of different x86/powerpc/sparc platforms.  The results can be viewed here:
> http://goo.gl/sgt3i ; the "crc32-kern-le" and "crc32c" columns describe the
> performance of the kernel's current crc32 and crc32c software implementations.
> The "crc32c-by8-le" column shows crc32c performance with this patchset applied.
> I expect crc32 performance to be roughly the same.
> 
> The two _boost columns at the right side of the spreadsheet shows how much
> faster the new implementation is over the old one.  As you can see, crc32 rises
> substantially, and crc32c experiences a huge increase.

I grabbed this lot, so it will turn up in linux-next soon.

There are quite a lot of clients of the crc32 code.  net, netfilter and
wireless are prominent, but there are others everywhere.

So I cc'ed various other mailing lists just as a heads-up that this
work is inbound and will hopefully yield performance improvements.  But
of course, it might also yield bugs.

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