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SubjectRe: O(1) scheduler gives big boost to tbench 192
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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