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To: "Andi Kleen" <andi@firstfloor.org>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>, djwong@us.ibm.com,
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Subject: Re: [PATCH v1 00/16] ext4: Add metadata checksumming
From: "Martin K. Petersen" <martin.petersen@oracle.com>
Organization: Oracle
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Date: Sun, 04 Sep 2011 18:19:16 -0400
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	(Andi Kleen's message of "Sun, 4 Sep 2011 19:44:55 +0200")
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>>>>> "Andi" == Andi Kleen <andi@firstfloor.org> writes:

Andi> Doesn't have any performance numbers.

It's been a while since I read them. I thought they had some compelling
numbers. Anyway, made a big difference in real life testing here. For
sustained I/O we're talking an order of magnitude.


Andi> You need to keep in mind that PCLMULQDQ uses FPU state, so any
Andi> speedup for the kernel must be large enough to amortize the cost
Andi> of saving the FPU state.

Yeah, my test cases were for bulk database I/O, not for writing a
handful of fs metadata blocks. Plus for the DB tests the CRC was
generated in userland.

I seem to recall Joel picking something other than the hw-accelerated
CRC32C for ocfs2 metadata and that didn't cause any problems.

That said, I do see a difference between IP checksum and CRC on normal
FS workloads with DIX enabled here.


Andi> Typically that only works out for quite large buffers, but kernel
Andi> buffers are relatively small.

*nod*

-- 
Martin K. Petersen	Oracle Linux Engineering