linux-kernel - Re: regression introduced by - timers: fix itimer/many thread hang

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Message-Id: <1226328152.7685.192.camel@twins>
Date:	Mon, 10 Nov 2008 15:42:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Frank Mayhar <fmayhar@...gle.com>,
	Doug Chapman <doug.chapman@...com>, mingo@...e.hu,
	roland@...hat.com, adobriyan@...il.com, akpm@...ux-foundation.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: regression introduced by - timers: fix itimer/many thread hang

On Mon, 2008-11-10 at 08:38 -0600, Christoph Lameter wrote:
> On Fri, 7 Nov 2008, Peter Zijlstra wrote:
> 
> > The advantage is that the memory foot-print scales with nr_tasks and the
> > runtime cost is min(nr_tasks, nr_cpus) where nr_cpus is limited to the
> > cpus the process actually runs on, so this takes full advantage of
> > things like cpusets.
> 
> Typically you want threads of a process to spread out as far as possible.
> The point of having multiple threads is concurrency after all. So this
> will deteriorate in the common cases where you want the full aggregate
> processing power of a machine to work on something. Timer processing is
> already a latency problem (isnt there some option to switch that off?) and
> a solution like this is going to make things worse.
> 
> Can we at least somehow make sure that nothing significantly happens in a
> timer interrupt on a processor if the thread has not scheduled any events
> or not odone any system calls?

Do threads actually scale that far? I thought mmap_sem contention and
other shared state would render threads basically useless on these very
large machines.

But afaiu this stuff, the per-cpu loop is only done when an itimer is
actually active.

The detail I've not looked at is, if when this itimer is indeed active
and we are running 256 threads of the same application on all cpus do we
then do the per-cpu loop for each tick on each cpu?

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