Scheduler weirdness
O. Hartmann
ohartman at zedat.fu-berlin.de
Mon Oct 12 07:44:38 UTC 2009
Steve Kargl wrote:
> On Mon, Oct 12, 2009 at 03:35:15PM +1100, Alex R wrote:
>> Steve Kargl wrote:
>>> On Mon, Oct 12, 2009 at 01:49:27PM +1100, Alex R wrote:
>>>
>>>> Steve Kargl wrote:
>>>>
>>>>> So, you have 4 cpus and 4 folding-at-home processes and you're
>>>>> trying to use the system with other apps? Switch to 4BSD.
>>>>>
>>>>>
>>>>>
>>>> I thought SCHED_ULE was meant to be a much better choice under an SMP
>>>> environment. Why are you suggesting he rebuild his kernel and use the
>>>> legacy scheduler?
>>>>
>>>>
>>> If you have N cpus and N+1 numerical intensitive applications,
>>> ULE may have poor performance compared to 4BSD. In OP's case,
>>> he has 4 cpus and 4 numerical intensity (?) applications. He,
>>> however, also is trying to use the system in some interactive
>>> way.
>>>
>>>
>> Ah ok. Is this just an accepted thing by the freebsd dev's or are they
>> trying to fix it?
>>
>
> Jeff appears to be extremely busy with other projects. He is aware of
> the problem, and I have set up my system to give him access when/if it
> is so desired.
>
> Here's the text of my last set of tests that I sent to him
>
> OK, I've manage to recreate the problem. User kargl launches a mpi
> job on node10 that creates two images on node20. This is command z
> in the top(1) info. 30 seconds later, user sgk lauches a mpi process
> on node10 that creates 8 images on node20. This is command rivmp in
> top(1) info. With 8 available cpus, this is a (slightly) oversubscribed
> node.
>
> For 4BSD, I see
>
> last pid: 1432; load averages: 8.68, 5.65, 2.82 up 0+01:52:14 17:07:22
> 40 processes: 11 running, 29 sleeping
> CPU: 100% user, 0.0% nice, 0.0% system, 0.0% interrupt, 0.0% idle
> Mem: 32M Active, 12M Inact, 203M Wired, 424K Cache, 29M Buf, 31G Free
> Swap: 4096M Total, 4096M Free
>
> PID USERNAME THR PRI NICE SIZE RES STATE C TIME CPU COMMAND
> 1428 sgk 1 124 0 81788K 5848K CPU3 6 1:13 78.81% rivmp
> 1431 sgk 1 124 0 81788K 5652K RUN 1 1:13 78.52% rivmp
> 1415 kargl 1 124 0 78780K 4668K CPU7 1 1:38 78.42% z
> 1414 kargl 1 124 0 78780K 4664K CPU0 0 1:37 77.25% z
> 1427 sgk 1 124 0 81788K 5852K CPU4 3 1:13 78.42% rivmp
> 1432 sgk 1 124 0 81788K 5652K CPU2 4 1:13 78.27% rivmp
> 1425 sgk 1 124 0 81788K 6004K CPU5 5 1:12 78.17% rivmp
> 1426 sgk 1 124 0 81788K 5832K RUN 6 1:13 78.03% rivmp
> 1429 sgk 1 124 0 81788K 5788K CPU6 7 1:12 77.98% rivmp
> 1430 sgk 1 124 0 81788K 5764K RUN 2 1:13 77.93% rivmp
>
>
> Notice, the accumulated times appear reasonable. At this point in the
> computations, rivmp is doing no communication between processes. z is
> the netpipe benchmark and is essentially sending messages between the
> two processes over the memory bus.
>
>
> For ULE, I see
>
> last pid: 1169; load averages: 7.56, 2.61, 1.02 up 0+00:03:15 17:13:01
> 40 processes: 11 running, 29 sleeping
> CPU: 100% user, 0.0% nice, 0.0% system, 0.0% interrupt, 0.0% idle
> Mem: 31M Active, 9392K Inact, 197M Wired, 248K Cache, 26M Buf, 31G Free
> Swap: 4096M Total, 4096M Free
>
> PID USERNAME THR PRI NICE SIZE RES STATE C TIME CPU COMMAND
> 1168 sgk 1 118 0 81788K 5472K CPU6 6 1:18 100.00% rivmp
> 1169 sgk 1 118 0 81788K 5416K CPU7 7 1:18 100.00% rivmp
> 1167 sgk 1 118 0 81788K 5496K CPU5 5 1:18 100.00% rivmp
> 1166 sgk 1 118 0 81788K 5564K RUN 4 1:18 100.00% rivmp
> 1151 kargl 1 118 0 78780K 4464K CPU3 3 1:48 99.27% z
> 1152 kargl 1 110 0 78780K 4464K CPU0 0 1:18 62.89% z
> 1164 sgk 1 113 0 81788K 5592K CPU1 1 0:55 80.76% rivmp
> 1165 sgk 1 110 0 81788K 5544K RUN 0 0:52 62.16% rivmp
> 1163 sgk 1 107 0 81788K 5624K RUN 2 0:40 50.68% rivmp
> 1162 sgk 1 107 0 81788K 5824K CPU2 2 0:39 50.49% rivmp
>
>
> In the above, processes 1162-1165 are clearly not receiving sufficient time
> slices to keep up with the other 4 rivmp images. From watching top at a
> 1 second interval, once the 4 rivmp hit 100% CPU, they stayed pinned to
> their cpu and stay at 100% CPU. It is also seen that processes 1152, 1165
> and 1162, 1163 are stuck on cpus 0 and 2, respectively.
>
This isn't only bound to floating-point intense applications, even the
operating system itselfs seems to suffer from SCHED_ULE. I saw, see and
reported several performance issue under heavy load and for seconds (if
not minutes!) 4+ CPU boxes get as stuck as a UP box does. Those sticky
sitiuations are painful in cases where the box needs to be accessed via
X11.
The remaining four FreeBSD 8.0-boxes used for numerical applications in
our lab (others switched to Linux a long time ago) all uses SCHED_ULE,
as this scheduler was introduced to be the superior scheduler over the
legacy 4BSD. Well, I'll give 4BSD a chance again.
At the moment, even our 8-core DELL Poweredge box is in production use,
but if there is something I can do, menas: benchmarking, I'll give it a try.
Regards,
Oliver
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