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

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Date:	Wed, 26 Nov 2008 11:20:24 +0000
From:	Andrew McDermott <andrew.mcdermott@...driver.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	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


Mathieu Desnoyers <compudj@...stal.dyndns.org> writes:

[...]

>> > 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
>
> 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.

But at the end of the day these threads end up writing to the (possibly)
single spindle.  Isn't that the biggest bottlneck here?

-- 
andy
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