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

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Message-ID: <x49zj5pft68.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 30 Apr 2015 12:59:59 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ming Lei <ming.lei@...onical.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

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.

Cheers,
Jeff
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