linux-kernel - Re: [PATCH] block: loop: avoiding too many pending per work I/O

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Message-ID: <CACVXFVOYKF7b6g+JKn8J0bU2nEAWXX6Jw-H=aq_oikwA=M1-UQ@mail.gmail.com>
Date:	Fri, 1 May 2015 09:20:58 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Justin M. Forbes" <jforbes@...oraproject.org>,
	"v4.0" <stable@...r.kernel.org>
Subject: Re: [PATCH] block: loop: avoiding too many pending per work I/O

On Fri, May 1, 2015 at 12:59 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Ming Lei <ming.lei@...onical.com> writes:
>
>> On Wed, Apr 29, 2015 at 12:36 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
>>> Ming Lei <ming.lei@...onical.com> writes:
>>>
>>>> If there are too many pending per work I/O, too many
>>>> high priority work thread can be generated so that
>>>> system performance can be effected.
>>>>
>>>> This patch limits the max pending per work I/O as 16,
>>>> and will fackback to single queue mode when the max
>>>> number is reached.
>>>
>>> Actually, it limits it to 32.  Also, there is no discussion on what
>>> variables might affect this number.  Will that magic number change
>>> depending on the number of cpus on the system, for example?
>>
>> My fault, it should have been 16.
>>
>> It is just used to keep more IOs in flight, but can't cause obvious
>> costs like the case of Fedora live booting.
>>
>> IMO, it shouldn't depend much on number of CPUs, and more
>> related with I/O performance of the backing file, and the number
>> is like 'iodepth' of fio.
>
> OK, that makes more sense.  I'm still not a huge fan of hard-coding
> numbers that are storage-specific, but I don't have a better suggestion
> at the moment, either.

OK, thanks for your review.
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