Messages in this thread | | | Date | Wed, 8 May 2002 10:02:24 -0700 | From | Mike Kravetz <> | Subject | Re: O(1) scheduler gives big boost to tbench 192 |
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On Wed, May 08, 2002 at 09:31:39AM -0700, Robert Love wrote: > On Wed, 2002-05-08 at 08:34, Jussi Laako wrote: > > > > Maybe this is the reason why O(1) scheduler has big latencies with > > pthread_cond_*() functions which original scheduler doesn't have? > > I think I tracked the problem down to try_to_wake_up(), but I was unable to > > fix it. > > Ah this could be the same case. I just looked into the definition of > the conditional variable pthread stuff and it looks like it _could_ be > implemented using pipes but I do not see why it would per se. If it > does not use pipes, then this sync issue is not at hand (only the pipe > code passed 1 for the sync flag). > > If it does not use pipes, we could have another problem - but I doubt > it. Maybe the benchmark is just another case where it shows worse > performance due to some attribute of the scheduler or load balancer? >
In some cases, the O(1) scheduler will produce higher latencies than the old scheduler. On 'some' workloads/benchmarks the old scheduler was better because it had a greater tendency to schedule tasks on the same CPU. This is certainly the case with the lat_ctx and lat_pipe components of LMbench. Note that this has nothing to do with the wake_up sync behavior. Rather, it is the difference between scheduling a new task on the current CPU as opposed to a 'remote' CPU. You can schedule the task on the current CPU quicker, but this is not good for optimal cache usage. I believe the O(1) scheduler makes the correct trade off in this area.
Is there anything simple I can do to check the latencies of the pthread_cond_*() functions? I'd like to do some analysis of scheduler behavior, but am unfamiliar with the user level code.
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