linux-kernel - Re: Who does determine the number of requests that can be serving simultaneously in a storage?

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Message-ID: <4D2710DC.9050908@kernel.dk>
Date:	Fri, 07 Jan 2011 14:10:52 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Yuehai Xu <yuehaixu@...il.com>
CC:	linux-kernel@...r.kernel.org, cmm@...ibm.com, rwheeler@...hat.com,
	vgoyal@...hat.com, czoccolo@...il.com, yhxu@...ne.edu
Subject: Re: Who does determine the number of requests that can be serving
  simultaneously in a storage?

Please don't top-post, thanks.

On 2011-01-07 14:00, Yuehai Xu wrote:
> I add a tracepoint so that I can get nr_sorted and in_flight[0/1] of
> request_queue when request is completed, I consider nr_sorted as the
> number of pending requests and in_flight[0/1] represent the number
> serving in the storage. Does these two parameters stand for what I
> mean?

nr_sorted is the number of requests that reside in the IO scheduler.
That means requests that are not on the dispatch list yet. in_flight is
the number that the driver is currently handling. So I think your
understanding is correct.

If you look at where you added your trace point, there are already a
trace point right there. I would recommend that you use blktrace, and
then use btt to parse it. That will give you all sorts of queueing
information.

> The test benchmark I use is postmark which simulates the email server
> system, over 90% requests are small random write. The storage is Intel
> M SSD. Generally, I think the number of in_flight[0/1] should be much
> greater than 1, but the result shows that this value is almost 1 no
> matter what I/O scheduler(CFQ/DEADLINE/NOOP) or
> filesystem(EXT4/EXT3/BTRFS) it is. Is it normal?

Depends, do you have more requests pending in the IO scheduler? I'm
assuming you already verified that NCQ is active and working for your
drive.

-- 
Jens Axboe

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