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

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Date:	Tue, 14 Jun 2011 15:03:24 +0800
From:	Tao Ma <tm@....ma>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: CFQ: async queue blocks the whole system

Hi Vivek,
On 06/14/2011 05:41 AM, Vivek Goyal wrote:
> 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.
I tried several times today, but it looks like that if I enable
blktrace, the hung_task will not show up in the message. So do you think
the blktrace at that time is still useful? If yes, I can capture 1
minute for you. Thanks.

Regards,
Tao
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