linux-kernel - Re: The difference of request dir between AS and Deadline I/O scheduler?

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Message-ID: <20090302131313.GU11787@kernel.dk>
Date:	Mon, 2 Mar 2009 14:13:13 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	???? <xiegang112@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: The difference of request dir between AS and Deadline I/O
	scheduler?

On Mon, Mar 02 2009, ???? wrote:
> Do you mean that the same process tends to have the same behavior of
> I/O in the way of sycn?
> Does this mean AS works better when requests are distincted by sync
> mode  (the success rate of anticipation is higher when requests are
> grouped by the mode of sync)?

Please don't top post, put your reply beneath the text you are replying
to.

It means that there's a key distinction between requests. Sync requests
often have dependencies on each other, and AS thus enables idling for
these types of requests. Async io is usually background activity. I
don't understand what you mean by higher success rate. Anticipation is
only for sync requests. The grouping is done because it makes
behavioural sense.


> 
> Thanks,
> 
> 
> On Mon, Mar 2, 2009 at 8:25 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
> > On Mon, Mar 02 2009, ???? wrote:
> >> Hi,
> >>
> >> I'm little confused about the defination of request dir in AS and
> >> Deadline I/O scheduler.
> >> In AS, the request dir is defined by wheher it's sync:
> >>
> >> data_dir = rq_is_sync(rq);
> >>
> >> But in Deadline, the requests are grouped by read and write.
> >>
> >> Why is there the difference since AS is an extension of Deadline?
> >> what's the consideration?
> >
> > Because AS uses the sync vs async distinction to decide whether to
> > anticipate a new request from that process. 'sync' is then reads or sync
> > writes, whereas deadline does not distinguish between sync and async
> > writes.
> >
> > --
> > Jens Axboe
> >
> >
> 
> 
> 
> -- 
> Xie Gang

-- 
Jens Axboe

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