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

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Date:	Tue, 25 Nov 2008 13:19:54 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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: [PATCH] Poll : introduce poll_wait_exclusive() new function

On Tue, 25 Nov 2008, KOSAKI Motohiro wrote:

> 
> patch againt: tip/tracing/marker
> 
> ==========
> Currently, wake_up() function behavior depend on the way of
> wait queue adding function.
> 
> 
>                               wake_up()          wake_up_all()
> ---------------------------------------------------------------
> add_wait_queue()              wake up all        wake up all
> add_wait_queue_exclusive()    wake up one task   wake up all
> 
> 
> Unforunately, poll_wait() always use add_wait_queue().
> it means there is no way that wake up only one process in polled processes.
> wake_up() also wake up all sleeping processes, not 1 process.
> 
> 
> 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.



- Davide


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