linux-kernel - Re: [patch 016/104] epoll: introduce resource usage limits

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Date:	Sat, 24 Jan 2009 09:36:22 +0100
From:	Vegard Nossum <vegard.nossum@...il.com>
To:	Bron Gondwana <brong@...tmail.fm>
Cc:	Greg KH <gregkh@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable@...nel.org, Justin Forbes <jmforbes@...uxtx.org>,
	Zwane Mwaikambo <zwane@....linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>,
	Randy Dunlap <rdunlap@...otime.net>,
	Dave Jones <davej@...hat.com>,
	Chuck Wolber <chuckw@...ntumlinux.com>
Subject: Re: [patch 016/104] epoll: introduce resource usage limits

On Sat, Jan 24, 2009 at 4:50 AM, Bron Gondwana <brong@...tmail.fm> wrote:
> On Fri, Jan 23, 2009 at 09:06:31AM -0800, Greg KH wrote:
>> On Fri, Jan 23, 2009 at 08:47:45PM +1100, Bron Gondwana wrote:
>> > On Thu, 22 Jan 2009 21:16 -0800, "Greg KH" <gregkh@...e.de> wrote:
>> > >
>> > > I would suggest just changing this default value then, it's a simple
>> > > userspace configuration item, and for your boxes, it sounds like a
>> > > larger value would be more suitable.
>
> If everyone, or every distribution at least, has to change it then the
> default is probably wrong.  The error message in the postfix logs didn't
> immediately point me at the issue, especially since I tried debugging on
> one of our "production" mxes, only to discover that the epoll limit
> didn't exist there.  They're slightly behind in kernel versions.
>
>> > I guess Postfix is a bit of an odd case here.   It runs lots of
>> > processes, yet uses epoll within many of them as well - sort of
>> > a historical design in some ways, but also to enforce maximum
>> > privilege separation with many of the daemons able to
>> > be run under chroot with limited capabilities.
>> >
>> > So I guess I have a few questions left:
>> >
>> > 1) is this value ever supposed to be hit in practice by
>> >    non-malicious software? If not, it appears 128 is too low.
>>
>> It does appear a bit low.  What looks to you like a good value to use as
>> a default?
>
> This thread suggests that it's not just postfix having the issue, and
> offers 1024 as a saner default:
>
> http://www.mail-archive.com/fedora-kernel-list@redhat.com/msg01618.html
>
> There's also a Russian thread that pointed me at this patch in the first
> place, and another place that suggested 1024 as well.  Seems "the
> cloud"[tm] is converging on 1024.

With the default limit of 128 (max_user_instances) and 274274
(max_user_watches) on my machine, the maximum amount of memory
consumed by one user's epoll instances is barely noticable (around
1.5M).

Raising the max_user_instances to 512 brings us up to a maximum memory
usage of 43M already. However, from here on, we are already getting
limited by the number of user watches.


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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