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Subject[PATCH v5.1 00/14] crc32c: Add faster algorithm and self-test code
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.
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: ; 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'm hoping this patch
set meets with everyone's approval and can go in soon. Herbert Xu didn't
appear to have any strong objections to last month's posting, so I'm wondering
if Andrew has an opinion?


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