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

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Message-Id: <20081126100749.3CAC.KOSAKI.MOTOHIRO@jp.fujitsu.com>
Date:	Wed, 26 Nov 2008 10:09:29 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	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>,
	Davide Libenzi <davidel@...ilserver.org>
Subject: Re: [PATCH] Poll : introduce poll_wait_exclusive() new function

> > Mathieu Desnoyers explained it cause following problem to LTTng.
> > 
> >    In LTTng, all lttd readers are polling all the available debugfs files
> >    for data. This is principally because the number of reader threads is
> >    user-defined and there are typical workloads where a single CPU is
> >    producing most of the tracing data and all other CPUs are idle,
> >    available to consume data. It therefore makes sense not to tie those
> >    threads to specific buffers. However, when the number of threads grows,
> >    we face a "thundering herd" problem where many threads can be woken up
> >    and put back to sleep, leaving only a single thread doing useful work.
> 
> Why do you need to have so many threads banging a single device/file?
> Have one (or any other very little number) puller thread(s), that 
> activates with chucks of pulled data the other processing threads. That 
> way there's no need for a new wakeup abstraction.

Mathieu, I don't hope I unstrictly explain to LTTng.
Could you please explain LTTng design?



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