linux-kernel - Re: [PATCH v1 00/16] ext4: Add metadata checksumming

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Date:	Sun, 04 Sep 2011 18:19:16 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	djwong@...ibm.com, "Greg Freemyer" <greg.freemyer@...il.com>,
	"Andreas Dilger" <adilger.kernel@...ger.ca>,
	"Theodore Tso" <tytso@....edu>,
	"Sunil Mushran" <sunil.mushran@...cle.com>,
	"Amir Goldstein" <amir73il@...il.com>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"Mingming Cao" <cmm@...ibm.com>,
	"Joel Becker" <jlbec@...lplan.org>,
	"linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
	linux-ext4@...r.kernel.org, "Coly Li" <colyli@...il.com>
Subject: Re: [PATCH v1 00/16] ext4: Add metadata checksumming

>>>>> "Andi" == Andi Kleen <andi@...stfloor.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
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