linux-kernel - Re: [PATCH] AIO: Don't plug the I/O queue in do_io

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Date:	Tue, 13 Dec 2011 17:26:07 -0600
From:	Dave Kleikamp <dave.kleikamp@...cle.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	linux-aio@...ck.org, linux-kernel@...r.kernel.org,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <axboe@...nel.dk>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] AIO: Don't plug the I/O queue in do_io_submit()

On 12/13/2011 04:18 PM, Jeff Moyer wrote:
> Dave Kleikamp <dave.kleikamp@...cle.com> writes:
> 
>> Asynchronous I/O latency to a solid-state disk greatly increased
>> between the 2.6.32 and 3.0 kernels. By removing the plug from
>> do_io_submit(), we observed a 34% improvement in the I/O latency.
>>
>> Unfortunately, at this level, we don't know if the request is to
>> a rotating disk or not.
> 
> I'm guessing I know the answer to this, but what workload were you
> testing, and can you provide more concrete evidence than "latency
> greatly increased?"

It is a piece of a larger industry-standard benchmark and you're
probably guessing correctly. The "greatly increased" latency was
actually slightly higher the improvement I get with this patch. So the
patch brought the latency nearly down to where it was before.

 I will try a microbenchmark to see if I get similar behavior, but I
wanted to throw this out here to get input.

I also failed to mention that the earlier kernel was a vendor kernel
(similar results on both Redhat and Oracle kernels). The 3.0 kernel is
much closer to mainline, but I haven't played with mainline kernels yet.
I expect similar results, but I can verify that.

> Also, have you tested the effects this has when
> using traditional storage for whatever your workload is?

That may be difficult, but hopefully, I can demonstrate it with a
simpler benchmark which I could test on traditional storage.

> I don't feel
> comfortable essentially reverting a performance patch without knowing
> the entire picture.  I will certainly do some testing on my end, too.

Understood. Thanks,

Shaggy

> Cheers,
> Jeff
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