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SubjectRe: High priority threads causing severe CPU load imbalances
On Tue, 06 Apr 2010 16:08:10 +0200, Peter Zijlstra wrote:

> On Tue, 2010-04-06 at 18:42 +0530, Suresh Jayaraman wrote:
> > I have a simple test program that accepts number of threads(pthreads) to
> > be created as a input. Each of these threads that gets created invokes a
> > function which is just a infinite while loop. The main function after
> > creating those threads goes in a infinite loop itself
> >
> > My test machine is a Dual Core AMD Opteron(tm) 860 with 8
> > sockets(non-HT), I run this test program with number of threads ==
> > number of CPUs:
> >
> > ./loadcpu -t 16
> >
> > I see 100% CPU utilization on almost all CPUs (via mpstat/htop/vmstat).
> >
> > When the above threads are running, if I introduce a few high priority
> > threads by doing:
> >
> > nice -n -13 ./loadcpu -t 3
> >
> > After a short while, I see a few CPUs becoming idle at ~0% utilization
> > (the number of CPUs becoming idle equals roughly the number of high
> > priority threads i.e. 3). When I stop the high priority threads, the CPU
> > utilization comes back to normal i.e. ~100%.
> >
> > This is reproducible on 2.6.32.10 stable kernel with all the recent all
> > SMT fixes (I hope) and I think it would be reproducible in current
> > upstream as well.
>
> Why bother using -stable for reporting bugs?
>
> > sched_mc_power_savings has been always set to 0.
> >
> > I spent a while staring at the load balancing and the thread migration
> > code, but could not figure out why this is happening. Would appreciate
> > any pointers.
>
> Right, except its not a severe imbalance as the subject suggests. For
> some reason it seems to end up in a semi-stable state that is actually
> quite balanced.
>
> for ((i=0; i<8; i++)) do while :; do :; done & done
> for ((i=0; i<3; i++)) do while :; do :; done & renice -n -15 -p $! ;
> done
>
> gets me:
>
> Cpu0 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
> Cpu1 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
> Cpu2 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
> Cpu3 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
> Cpu4 : 99.0%us, 1.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
> Cpu5 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
> Cpu6 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
> Cpu7 : 0.0%us, 0.0%sy, 0.0%ni,100.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
> Mem: 16440840k total, 1073672k used, 15367168k free, 105844k buffers
> Swap: 16777212k total, 0k used, 16777212k free, 296504k cached
>
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 4370 root 5 -15 105m 804 304 R 100.1 0.0 0:45.02 bash
> 4374 root 5 -15 105m 804 304 R 100.1 0.0 0:44.95 bash
> 4372 root 5 -15 105m 804 304 R 99.1 0.0 0:45.00 bash
> 4364 root 20 0 105m 804 304 R 51.0 0.0 0:33.06 bash
> 4362 root 20 0 105m 800 300 R 50.0 0.0 0:33.17 bash
> 4365 root 20 0 105m 804 304 R 50.0 0.0 0:33.75 bash
> 4368 root 20 0 105m 804 304 R 50.0 0.0 0:33.32 bash
> 4369 root 20 0 105m 804 304 R 50.0 0.0 0:33.38 bash
> 4363 root 20 0 105m 804 304 R 49.1 0.0 0:33.65 bash
> 4366 root 20 0 105m 804 304 R 49.1 0.0 0:33.29 bash
> 4367 root 20 0 105m 804 304 R 49.1 0.0 0:33.54 bash
>
> So we have the 3 -15 loops on a cpu each, and the 8 0 loops on 2 cpus
> each, and 1 cpu idle. That is actually quite balanced, 'better' would be
> if those 0 loops would rotate over the 5 available cpus, but that would
> also trash more caches I guess.
>
> I'm not quite sure what makes the load-balancer end up in this situation
> though, but I suspect the various imbalance_pct things might have
> something to do with it.
>
> It doesn't always end up in this state either, if you only start 2 -15
> loops its a roll of the dice on what happens, sometimes it ends up with
> the 6 cpus cycling the 2 extra tasks around, sometimes its 1 cpu idle
> with cycling 1 task.
>
> Unexpected, maybe, severe imbalance, no. Would be nice to get it to be a
> little more stable behaviour though.


I found a similar(maybe same) problem by using the cgroup cpu-subsystem like following:

My test machine has Xeon(Quad Core) with 2 sockets(non-HT).
# mount -t cgroup -o cpu none /dev/cgroup-cpu/
# mkdir -p /dev/cgroup-cpu/204800 /dev/cgroup-cpu/1024
# echo 204800 > /dev/cgroup-cpu/204800/cpu.shares
# for ((i=0; i<3; i++)) do while :; do :; done & echo $! > /dev/cgroup-cpu/204800/tasks ; done
# for ((i=0; i<5; i++)) do while :; do :; done & echo $! > /dev/cgroup-cpu/1024/tasks ; done

gets me:

Tasks: 190 total, 9 running, 181 sleeping, 0 stopped, 0 zombie
Cpu0 : 1.0%us, 0.0%sy, 0.0%ni, 99.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu1 : 0.0%us, 0.3%sy, 0.0%ni, 99.3%id, 0.0%wa, 0.0%hi, 0.3%si, 0.0%st
Cpu2 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu3 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu4 : 0.0%us, 0.0%sy, 0.0%ni,100.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu5 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu6 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu7 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 8180292k total, 2430940k used, 5749352k free, 204988k buffers
Swap: 0k total, 0k used, 0k free, 1931820k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ P COMMAND
30923 root 20 0 5808 540 264 R 100 0.0 2:30.64 3 bash
30922 root 20 0 5808 540 264 R 100 0.0 2:30.64 2 bash
30924 root 20 0 5808 540 264 R 100 0.0 2:30.63 6 bash
30925 root 20 0 5808 540 264 R 42 0.0 1:00.19 7 bash
30928 root 20 0 5808 540 264 R 41 0.0 0:57.26 5 bash
30929 root 20 0 5808 540 264 R 40 0.0 0:57.03 7 bash
30926 root 20 0 5808 540 264 R 39 0.0 0:58.37 7 bash
30927 root 20 0 5808 540 264 R 39 0.0 0:58.57 5 bash

I don't expect this behavior.
(I expect that all 8 processes use 100%CPU.)
So I'm investigating this problem.
And I suspect that the cause is find_busiest_group() returns the sched_group
(as the busiest sched_group) with a high priority process
although this sched_group has a 100% idle cpu.

IIUC, This problem was caused by changing the load calculation way by this patch,
---
commit 2dd73a4f09beacadde827a032cf15fd8b1fa3d48
Author: Peter Williams <pwil3058@bigpond.net.au>
Date: Tue Jun 27 02:54:34 2006 -0700
[PATCH] sched: implement smpnice
---
This patch changed the load calculation way from nr_running to weighted_load.
So the scheduler looks on the high priority process as many processes in the load calculation.
I don't find the solution of this problem yet.
I'll dig down more to find the solution.

Thanks.
--
Masayuki Igawa



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