linux-kernel - Re: [patch] aio: remove aio-max-nr and instead use the memlock rlimit to limit the number of pages pinned for the aio completion ring

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Message-ID: <x49sklmd1fo.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 09 Mar 2009 13:57:15 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	linux-aio <linux-aio@...ck.org>, zach.brown@...cle.com,
	bcrl@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] aio: remove aio-max-nr and instead use the memlock rlimit to limit the number of pages pinned for the aio completion ring

Avi Kivity <avi@...hat.com> writes:

> Jeff Moyer wrote:
>> Hi,
>>
>> Believe it or not, I get numerous questions from customers about the
>> suggested tuning value of aio-max-nr.  aio-max-nr limits the total
>> number of io events that can be reserved, system wide, for aio
>> completions.  Each time io_setup is called, a ring buffer is allocated
>> that can hold nr_events I/O completions.  That ring buffer is then
>> mapped into the process' address space, and the pages are pinned in
>> memory.  So, the reason for this upper limit (I believe) is to keep a
>> malicious user from pinning all of kernel memory.  Now, this sounds like
>> a much better job for the memlock rlimit to me, hence the following
>> patch.
>>   
>
> Is it not possible to get rid of the pinning entirely?  Pinning
> interferes with page migration which is important for NUMA, among
> other issues.

aio_complete is called from interrupt handlers, so can't block faulting
in a page.  Zach mentions there is a possibility of handing completions
off to a kernel thread, with all of the performance worries and extra
bookkeeping that go along with such a scheme (to help frame my concerns,
I often get lambasted over .5% performance regressions).

I'm happy to look into such a scheme, should anyone show me data that
points to this NUMA issue as an actual performance problem today.  In
the absence of such data, I simply can't justify the work at the moment.

Thanks for taking a look!

-Jeff
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