linux-kernel - Re: Slow file transfer speeds with CFQ IO scheduler in some cases

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Message-ID: <x498wr7de6u.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 25 Nov 2008 10:21:13 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vladislav Bolkhovitin <vst@...b.net>
Cc:	Wu Fengguang <wfg@...ux.intel.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	"Vitaly V. Bursov" <vitalyb@...enet.dn.ua>,
	linux-kernel@...r.kernel.org
Subject: Re: Slow file transfer speeds with CFQ IO scheduler in some cases

Vladislav Bolkhovitin <vst@...b.net> writes:

> Wu Fengguang wrote:
>> Another scheme is to detect the sequential pattern via looking up
>> the page cache, which provides one single and consistent view of the
>> pages recently accessed. That makes sequential detection possible.
>>
>> The cost will be one extra page cache lookup per random read.
>> If it's not acceptable, the corresponding code could be disabled
>> by default. 
>
> I think, this should be the best and the simplest way to go. Since in
> most case data from the cache should be later copied to user, one more
> page cache lookup should be negligible.

I haven't thought about your suggestion in any detail, but it seems to
me that you will not handle O_DIRECT I/O with this scheme.

Cheers,
Jeff
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