linux-kernel - Re: CFQ: async queue blocks the whole system

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Date:	Mon, 13 Jun 2011 17:41:54 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tao Ma <tm@....ma>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: CFQ: async queue blocks the whole system

On Mon, Jun 13, 2011 at 06:08:40PM +0800, Tao Ma wrote:

[..]
> > You can also run iostat on disk and should be able to see that with
> > the patch you are dispatching writes more often than before.
> Sorry, the patch doesn't work.
> 
> I used trace event to capture all the blktraces since it doesn't
> interfere with the tests, hope it helps.

Actually I was looking for CFQ traces. This seems to be generic block
layer trace points. May be you can use "blktrace -d /dev/<device>"
and then blkparse. It also gives the aggregate view which is helpful.

> 
> Please downloaded it from http://blog.coly.li/tmp/blktrace.tar.bz2

What concerns me is following.

5255.521353: block_rq_issue: 8,0 W 0 () 571137153 + 8 [attr_set]
5578.863871: block_rq_issue: 8,0 W 0 () 512950473 + 48 [kworker/0:1]

IIUC, we dispatched second write more than 300 seconds after dispatching
1 write. What happened in between. We should have dispatched more writes.

CFQ traces might give better idea in terms of whether wl_type for async
queues was scheduled or not at all.

Thanks
Vivek
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