linux-kernel - RE: cfq performance gap

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Message-ID: <000001c71e76$d4930e90$bb89030a@amr.corp.intel.com>
Date:	Tue, 12 Dec 2006 21:23:31 -0800
From:	"Chen, Kenneth W" <kenneth.w.chen@...el.com>
To:	"'AVANTIKA R. MATHUR'" <mathur@...ux.vnet.ibm.com>,
	"Jens Axboe" <jens.axboe@...cle.com>
Cc:	"Avantika Mathur" <mathur@...ibm.com>,
	<linux-kernel@...r.kernel.org>
Subject: RE: cfq performance gap

AVANTIKA R. MATHUR wrote on Tuesday, December 12, 2006 5:33 PM
> >> rawio is actually performing sequential reads, but I don't believe it is
> >> purely sequential with the multiple processes.
> >> I am currently running the test with longer runtimes and will post
> >> results once it is complete.
> >> I've also attached the rawio source.
> >>     
> >
> > It's certainly the slice and idling hurting here. But at the same time,
> > I don't really think your test case is very interesting. The test area
> > is very small and you have 16 threads trying to read the same thing,
> > optimizing for that would be silly as I don't think it has much real
> > world relevance.
> 
> Could a database have similar workload to this test?

No.

Not what I have seen with db workloads exhibits such pattern.  There are
basically two types of db workloads: one does transaction processing, and
I/O pattern are truly random with large stride, both in the context of
process and overall I/O seen at device level.  A second one is decision
making type of db queries.  They does large sequential I/O within one
process context.

This rawio test plows through sequential I/O and modulo each small record
over number of threads.  So each thread appears to be non-contiguous within
its own process context, overall request hitting the device are sequential.
I can't see how any application does that kind of I/O pattern.

- Ken
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