linux-kernel - Re: high-speed disk I/O is CPU-bound?

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Message-ID: <51910436.6070509@ll.mit.edu>
Date:	Mon, 13 May 2013 11:18:14 -0400
From:	David Oostdyk <daveo@...mit.edu>
To:	Rob Landley <rob@...dley.net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: high-speed disk I/O is CPU-bound?

On 05/12/13 12:53, Rob Landley wrote:
> On 05/10/2013 09:04:44 AM, David Oostdyk wrote:
>> Hello,
>>
>> I have a few relatively high-end systems with hardware RAIDs which
>> are being used for recording systems, and I'm trying to get a better
>> understanding of contiguous write performance.
> ...
>> The question is, is it possible that high-speed I/O to these hardware
>> RAIDs could
>> actually be CPU-bound above ~1400MB/sec?
> In some setups your processor is calculating CRCs for the data. It's a
> fairly cheap operation, but a cheap operation on gigabytes of data can
> still saturate your memory bus.
>
> Rob

At what level would you say this calculation is being applied? Somewhere 
in the block/filesystem layer, or in the device driver, or at the 
hardware level?  I'm seeing write speeds that are about 1/4 the memory 
bandwidth of a single thread, which would suggest at least one 
"additional" pass through the data before it gets DMA'd out.

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