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

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Date:	Fri, 23 Jan 2009 11:28:38 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
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 Fri, 23 Jan 2009, Bron Gondwana wrote:

> On Thu, 22 Jan 2009 21:16 -0800, "Greg KH" <gregkh@...e.de> wrote:
> > On Fri, Jan 23, 2009 at 03:51:01PM +1100, Bron Gondwana wrote:
> > > On Wed, 03 Dec 2008 11:48 -0800, "Greg KH" <gregkh@...e.de> wrote:
> > > > The default value for "max_user_instances" is set to 128, that should be enough too.
> > > 
> > > Our fairly heavily loaded postfix backup mx (lots of spams rejected per day) hit this
> > > limit running kernel 2.6.27.8.  Any particular reason for it being as low as 128
> > > by default?
> > 
> > Something had to be picked :)
> 
> Fair enough :)
> 
> > > This is a kvm virtual machine running on a reasonably beefy external box, but
> > > with 2Gb RAM allocated to the mx instance because that's all kvm would let me
> > > use last time I checked.  We're using KVM so the local copy of the database is
> > > a little further away from the "internet facing side" and so we can build each
> > > machine with our standard FAI setup.
> > 
> > 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.
> 
> Yes - I've pushed it up to 4096 now.  Should be plenty!
> 
> 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.
> 
> 2) if we're going to stick with 128, is there any way to query the kernel as to how
>    close to the limit it's getting?  As an example, our system checks poll
>    /proc/sys/fs/file-max every 2 minutes, and warn us if its getting "full".

Why? If you know you have a loaded, non multi-user server, just bump the 
value up and forget about it. An higher value is not going to cost you 
anything in terms of resource allocation. Adding more /proc code to 
monitor a silly value, probably is.



- Davide


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