cvs commit: src/sys/kern sched_ule.c
kris at FreeBSD.org
Tue Mar 4 02:03:52 PST 2008
Jeff Roberson wrote:
> On Tue, 4 Mar 2008, Garrett Wollman wrote:
>> <<On Sat, 1 Mar 2008 22:29:50 -1000 (HST), Jeff Roberson
>> <jroberson at chesapeake.net> said:
>>> Kris has done some excellent benchmarking as usual. Here you can see
>>> improvement in postgres depending on various scheduler debug settings:
>> Can you comment on the area under the knee in the 8-cpu topologies? I
>> seems surprising that 16 cores performs worse than 8 cores in this
> Depending on the flags you can see different scaling properties of
> different cpu selection algorithms. That's what the userret=x,
> tryself=y parameters are changing. Certain parameters can cause less
> concurrency which works better when workloads are heavily contended.
> See the light blue line, tryself=0, userret=0. This scales up more
> poorly because there is less concurrency when there is no lock
> contention but behaves better when there is contention because we're
> less likely to distribute load that would preempt a lock holder.
> The default settings scale the best when there is little or no
> contention. That's userret=1, tryself=1. There are other parameters
> that are important but these were the ones we were most recently
> experimenting with. This drops off harshly when there is significant
> contention because most of the threads end up blocked against the same
> lock and may be preempted then rely on priority propagation to kick in.
> The default settings should encourage further refinements to subsystem
> locking to yield the best performance.
I didnt run the 8-core configuration with the ULE topology patch, so
part of the reason why it has a kink at 5 threads is probably due to
poor scheduling. This system is very sensitive to scheduling decisions,
as you can see from the previous CVS curve.
I think there is also something else going on at high loads (>15) on
this test, so it should be viewed as a WIP. Specifically, contention
doesnt seem to be high enough to account for a 30% performance drop, and
I see similar drops on other 8-core tests where contention is eliminated.
What you should focus on is the large difference between the green curve
showing previous CVS performance, with the brown curve showing current
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