Messages in this thread | | | From | Andrew McDermott <> | Subject | Re: [ltt-dev] [PATCH] Poll : introduce poll_wait_exclusive() new function | Date | Wed, 26 Nov 2008 11:20:24 +0000 |
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Mathieu Desnoyers <compudj@krystal.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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