linux-kernel - Re: [ltt-dev] [PATCH] Poll : introduce poll_wait

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Date:	Wed, 26 Nov 2008 16:08:11 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>, ltt-dev@...ts.casi.polymtl.ca,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	William Lee Irwin III <wli@...omorphy.com>
Subject: Re: [ltt-dev] [PATCH] Poll : introduce poll_wait_exclusive() new
 function

On Wed, 26 Nov 2008, Mathieu Desnoyers wrote:

> One of the key design rule of LTTng is to do not depend on such
> system-wide data structures, or entity (e.g. single manager thread).
> Everything is per-cpu, and it does scale very well.
> 
> I wonder how badly the approach you propose can scale on large NUMA
> systems, where having to synchronize everything through a single thread
> might become an important point of contention, just due to the cacheline
> bouncing and extra scheduler activity involved.

I dunno the LTT architecture, so I'm staying out of that discussion.
But, if the patch you're trying to push is to avoid thundering herd of so 
many threads waiting on the single file*, you've got the same problem 
right there. You've got at least the spinlock protecting the queue 
where these threads are focusing, whose cacheline is bounced gets bounced 
all over the CPUs.
Do you have any measure of the improvements that such poll_wait_exclusive()
will eventually lead to?

- Davide

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