Messages in this thread | | | Date | Wed, 26 Nov 2008 17:27:15 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [ltt-dev] [PATCH] Poll : introduce poll_wait_exclusive() new function |
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* Andrew McDermott (andrew.mcdermott@windriver.com) wrote: > > 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? >
Not if those threads are either
- analysing the data on-the-fly without exporting it to disk - sending the data through more than one network card - Writing data to multiple disks
There are therefore ways to improve scalability by adding more data output paths. Therefore, I don't want to limit scalability due to the inner design, so that if someone has the resources to send the information out at great speed scaleably, he can.
Mathieu
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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