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SubjectRe: [PATCH 2.6.20-rc4 1/4] futex priority based wakeup
Ulrich Drepper a écrit :
>
> I have never seen performance numbers for this. If it is punishing
> existing code in a measurable way I think it's not anacceptable default
> behavior.
>

Here are some numbers. My test program measures the latency of pthread_broadcast
with 1000 pthreads (all threads are blocked on pthread_cond_wait, the time is
measured between the broadcast call and the last woken pthread).

Here are the average latencies after 5000 measures.

[only this patch is used, not the following.
The system is a dual Xeon 2.80GHz with HT enable]

First case: all threads are SCHED_OTHER
* with simple list:
Iterations=5000
Latency (us) min max avg stddev
3869 7400 6656.73 539.35

* with plist:
Iterations=5000
Latency (us) min max avg stddev
3684 7629 6787.97 479.41

Second case: all threads are SCHED_FIFO with priority equally distributed from
priomin to priomax
* with simple list:
Iterations=5000
Latency (us) min max avg stddev
4548 7197 6656.85 463.30

* with plist:
Iterations=5000
Latency (us) min max avg stddev
8289 11752 9720.12 426.45


So, yes it (logically) has a cost, depending of the number of different
priorities used, so it's specially measurable with real-time threads.
With SCHED_OTHER, I suppose that the priorities are not be very distributed.

May be, supposing it makes sense to respect the priority order only for
real-time pthreads, I can register all SCHED_OTHER threads to the same
MAX_RT_PRIO priotity ?
Or do you think this must be set behind a CONFIG* option ?
(Or finally not interesting enough for mainline ?)

--
Pierre
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