linux-kernel - Re: [PATCH] loop: Limit the number of requests in the bio list

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Message-ID: <x49txubvd9d.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 03 Oct 2012 10:30:54 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Dave Chinner <dchinner@...hat.com>
Cc:	Lukáš Czerner <lczerner@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] loop: Limit the number of requests in the bio list

Dave Chinner <dchinner@...hat.com> writes:

> On Tue, Oct 02, 2012 at 10:52:05AM +0200, Lukáš Czerner wrote:
>> On Mon, 1 Oct 2012, Jeff Moyer wrote:
>> > Date: Mon, 01 Oct 2012 12:52:19 -0400
>> > From: Jeff Moyer <jmoyer@...hat.com>
>> > To: Lukas Czerner <lczerner@...hat.com>
>> > Cc: Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
>> >     Dave Chinner <dchinner@...hat.com>
>> > Subject: Re: [PATCH] loop: Limit the number of requests in the bio list
>> > 
>> > Lukas Czerner <lczerner@...hat.com> writes:
>> > 
>> > > Currently there is not limitation of number of requests in the loop bio
>> > > list. This can lead into some nasty situations when the caller spawns
>> > > tons of bio requests taking huge amount of memory. This is even more
>> > > obvious with discard where blkdev_issue_discard() will submit all bios
>> > > for the range and wait for them to finish afterwards. On really big loop
>> > > devices this can lead to OOM situation as reported by Dave Chinner.
>> > >
>> > > With this patch we will wait in loop_make_request() if the number of
>> > > bios in the loop bio list would exceed 'nr_requests' number of requests.
>> > > We'll wake up the process as we process the bios form the list.
>> > 
>> > I think you might want to do something similar to what is done for
>> > request_queues by implementing a congestion on and off threshold.  As
>> > Jens writes in this commit (predating the conversion to git):
>> 
>> Right, I've had the same idea. However my first proof-of-concept
>> worked quite well without this and my simple performance testing did
>> not show any regression.

Did you look at system time?

-Jeff
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