[MPICH] An idle communication process use the same CPU as computation process on multi-core chips
Bob Soliday
soliday at aps.anl.gov
Tue Sep 18 12:45:13 CDT 2007
Well I reported the bug and it turns out they already have a patch for
it that will be included in a future release so that it will be possible
to emulate the old scheduler.
https://bugzilla.redhat.com/show_bug.cgi?id=295071
http://lkml.org/lkml/2007/9/14/157
--Bob
Bob Soliday wrote:
> It turns out the problem is not related to the number of cores. Only the
> newest versions of the Fedora 7 kernel show the problem. I think it is
> related to the CFS scheduler in these kernels.
>
> When I run one slave and one master on the same core with
> kernel-2.6.21-1.3194 using Darius's slave code I see the slave task use
> 100% of the CPU and see the same timing values as when I run the slave
> on a different core.
>
> When I do the same test with kernel-2.6.22.4-65 or kernel-2.6.22.5.76
> the timing values double as the slave can only get 50% of the CPU time
> when on the same core.
>
> --Bob
>
> Darius Buntinas wrote:
>
>>
>> I can verify that I saw the same problem Yusong did when starting the
>> master first on a dual quadcore machine. But assigning each slave to
>> its own core (using taskset) fixed that.
>>
>> Interestingly, when there are less than 8 slaves, top shows that the
>> master has 100% usage (when top is in "irix mode", and 12.5% (1/8)
>> when not in irix mode). When I have 8 slaves, the usage of the master
>> process goes to 0.
>>
>> Yusong, I'm betting that if you set the cpu affinity for the slaves,
>> you'll see no impact of the master on the slaves. Can you try that?
>>
>> e.g.,:
>> ./master &
>> for i in `seq 0 3` ; do taskset -c $i ./slave & done
>>
>> -d
>>
>> On 09/17/2007 02:31 AM, Sylvain Jeaugey wrote:
>>
>>> This seems to be the key of the problem. When the master is launched
>>> before others, it takes one CPU and this won't change until for any
>>> scheduling reason he comes to share its CPU (with a slave). It then
>>> falls to 0% and we're saved.
>>>
>>> So, to conduct you experiment, you definetely need to taskset your
>>> slaves. Just launch them with
>>> taskset -c <cpu> ./slave (1 process per cpu)
>>> or use the -p option of taskset to do it after launch and ensure that
>>> each slave _will_ take one CPU. Thus, the master will be obliged to
>>> share the cpu with others and sched_yield() will be effective.
>>>
>>> Sylvain
>>>
>>> On Sun, 16 Sep 2007, Yusong Wang wrote:
>>>
>>>> I did the experiments on four types of muti-core chips (2
>>>> dual-core, 1 quad-core and 1 eight-core). All of my tests shows the
>>>> idle master process has a big impact on the other slave processes
>>>> except for the test of the quad-core, in which I found the order
>>>> does matter: when the master was launched after the slave processes
>>>> were launched, there is no affect, while if the master started
>>>> first, two slaves processes would go to the same core and cause the
>>>> two processes to slow down significantly than others.
>>>>
>>>> Yusong
>>>>
>>>> ----- Original Message -----
>>>> From: Darius Buntinas <buntinas at mcs.anl.gov>
>>>> Date: Friday, September 14, 2007 12:55 pm
>>>> Subject: Re: [MPICH] An idle communication process use the same CPU
>>>> as computation process on multi-core chips
>>>>
>>>>>
>>>>> It's possible that different versions of the kernel/os/top compute
>>>>> %cpu
>>>>> differently. "CPU utilization" is really a nebulous term. What
>>>>> you
>>>>> really want to know is whether the master is stealing significant
>>>>> cycles
>>>>> from the slaves. A test of this would be to replace Sylvain's
>>>>> slave
>>>>> code with this:
>>>>>
>>>>> #include <sys/time.h>
>>>>> int main() {
>>>>> while (1) {
>>>>> int i;
>>>>> struct timeval t0,t1;
>>>>> double usec;
>>>>>
>>>>> gettimeofday(&t0, 0);
>>>>> for (i = 0; i < 100000000; ++i)
>>>>> ;
>>>>> gettimeofday(&t1, 0);
>>>>>
>>>>> usec = (t1.tv_sec * 1e6 + t1.tv_usec) - (t0.tv_sec * 1e6 +
>>>>> t0.tv_usec);
>>>>> printf ("%8.0f\n", usec);
>>>>> }
>>>>> return 0;
>>>>> }
>>>>>
>>>>> This will repeatedly time the inner loop. On an N core system, run
>>>>> N of
>>>>> these, and look at the times reported. Then start the master and
>>>>> see if
>>>>> the timings change. If the master does steal significant cycles
>>>>> from
>>>>> the slaves, then you'll see the timings reported by the slaves
>>>>> increase.
>>>>> On my single processor laptop (fc6, 2.6.20), running one slave, I
>>>>> see
>>>>> no impact from the master.
>>>>>
>>>>> Please let me know what you find.
>>>>>
>>>>> As far as slave processes hopping around on processors, you can set
>>>>> processor affinity ( http://www.linuxjournal.com/article/6799 has a
>>>>> good
>>>>> description) on the slaves.
>>>>>
>>>>> -d
>>>>>
>>>>> On 09/14/2007 12:11 PM, Bob Soliday wrote:
>>>>>
>>>>>> Sylvain Jeaugey wrote:
>>>>>>
>>>>>>> That's unfortunate.
>>>>>>>
>>>>>>> Still, I did two programs. A master :
>>>>>>> ----------------------
>>>>>>> int main() {
>>>>>>> while (1) {
>>>>>>> sched_yield();
>>>>>>> }
>>>>>>> return 0;
>>>>>>> }
>>>>>>> ----------------------
>>>>>>> and a slave :
>>>>>>> ----------------------
>>>>>>> int main() {
>>>>>>> while (1);
>>>>>>> return 0;
>>>>>>> }
>>>>>>> ----------------------
>>>>>>>
>>>>>>> I launch 4 slaves and 1 master on a bi dual-core machine. Here
>>>>>
>>>>>
>>>>> is the
>>>>>
>>>>>>> result in top :
>>>>>>>
>>>>>>> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
>>>>>
>>>>>
>>>>> COMMAND>> 12361 sylvain 25 0 2376 244 188 R 100 0.0
>>>>> 0:18.26 slave
>>>>>
>>>>>>> 12362 sylvain 25 0 2376 244 188 R 100 0.0 0:18.12 slave
>>>>>>> 12360 sylvain 25 0 2376 244 188 R 100 0.0 0:18.23 slave
>>>>>>> 12363 sylvain 25 0 2376 244 188 R 100 0.0 0:18.15 slave
>>>>>>> 12364 sylvain 20 0 2376 248 192 R 0 0.0 0:00.00 master
>>>>>>> 12365 sylvain 16 0 6280 1120 772 R 0 0.0 0:00.08 top
>>>>>>>
>>>>>>> If you are seeing 66% each, I guess that your master is not
>>>>>>> sched_yield'ing as much as expected. Maybe you should look at
>>>>>>> environment variables to force yield when no message is
>>>>>
>>>>>
>>>>> available, and
>>>>>
>>>>>>> maybe your master isn't so idle after all and has message to
>>>>>
>>>>>
>>>>> send
>>>>>
>>>>>>> continuously, thus not yield'ing.
>>>>>>>
>>>>>>
>>>>>> On our FC5 nodes with 4 cores we get similar results. But on our
>>>>>
>>>>>
>>>>> FC7
>>>>>
>>>>>> nodes with 8 cores we don't. The kernel seems to think that all 9
>>>>>
>>>>>
>>>>> jobs
>>>>>
>>>>>> require 100% and they end up jumping from one core to another.
>>>>>
>>>>>
>>>>> Often the
>>>>>
>>>>>> master job is left on it's own core while two slaves run on another.
>>>>>>
>>>>>> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ P
>>>>>
>>>>>
>>>>> COMMAND> 20127 ywang25 20 0 106m 22m 4168 R 68 0.5
>>>>> 0:06.84 0 slave
>>>>>
>>>>>> 20131 ywang25 20 0 106m 22m 4184 R 73 0.5 0:07.26 1 slave
>>>>>> 20133 ywang25 20 0 106m 22m 4196 R 75 0.5 0:07.49 2 slave
>>>>>> 20129 ywang25 20 0 106m 22m 4176 R 84 0.5 0:08.44 3 slave
>>>>>> 20135 ywang25 20 0 106m 22m 4176 R 73 0.5 0:07.29 4 slave
>>>>>> 20132 ywang25 20 0 106m 22m 4188 R 70 0.5 0:07.04 4 slave
>>>>>> 20128 ywang25 20 0 106m 22m 4180 R 78 0.5 0:07.79 5 slave
>>>>>> 20130 ywang25 20 0 106m 22m 4180 R 74 0.5 0:07.45 6 slave
>>>>>> 20134 ywang25 20 0 106m 24m 6708 R 80 0.6 0:07.98 7
>>>>>
>>>>>
>>>>> master>
>>>>>
>>>>>> 20135 ywang25 20 0 106m 22m 4176 R 75 0.5 0:14.75 0 slave
>>>>>> 20132 ywang25 20 0 106m 22m 4188 R 79 0.5 0:14.96 1 slave
>>>>>> 20130 ywang25 20 0 106m 22m 4180 R 99 0.5 0:17.32 2 slave
>>>>>> 20129 ywang25 20 0 106m 22m 4176 R 100 0.5 0:18.44 3 slave
>>>>>> 20127 ywang25 20 0 106m 22m 4168 R 75 0.5 0:14.36 4 slave
>>>>>> 20133 ywang25 20 0 106m 22m 4196 R 96 0.5 0:17.09 5 slave
>>>>>> 20131 ywang25 20 0 106m 22m 4184 R 78 0.5 0:15.02 6 slave
>>>>>> 20128 ywang25 20 0 106m 22m 4180 R 99 0.5 0:17.70 6 slave
>>>>>> 20134 ywang25 20 0 106m 24m 6708 R 100 0.6 0:17.97 7
>>>>>
>>>>>
>>>>> master>
>>>>>
>>>>>> 20130 ywang25 20 0 106m 22m 4180 R 87 0.5 0:25.99 0 slave
>>>>>> 20132 ywang25 20 0 106m 22m 4188 R 79 0.5 0:22.83 0 slave
>>>>>> 20127 ywang25 20 0 106m 22m 4168 R 75 0.5 0:21.89 1 slave
>>>>>> 20133 ywang25 20 0 106m 22m 4196 R 98 0.5 0:26.94 2 slave
>>>>>> 20129 ywang25 20 0 106m 22m 4176 R 100 0.5 0:28.45 3 slave
>>>>>> 20135 ywang25 20 0 106m 22m 4176 R 74 0.5 0:22.12 4 slave
>>>>>> 20134 ywang25 20 0 106m 24m 6708 R 98 0.6 0:27.73 5
>>>>>
>>>>>
>>>>> master> 20128 ywang25 20 0 106m 22m 4180 R 90 0.5
>>>>> 0:26.72 6 slave
>>>>>
>>>>>> 20131 ywang25 20 0 106m 22m 4184 R 99 0.5 0:24.96 7 slave
>>>>>>
>>>>>> 20133 ywang25 20 0 91440 5756 4852 R 87 0.1 0:44.20 0 slave
>>>>>> 20132 ywang25 20 0 91436 5764 4860 R 80 0.1 0:39.32 0
>>>>>
>>>>>
>>>>> slave
>>>>>
>>>>>> 20134
>>>>>> ywang25 20 0 112m 36m 11m R 96 0.9 0:47.35 5 master
>>>>>> 20129 ywang25 20 0 91440 5736 4832 R 91 0.1 0:46.84 1 slave
>>>>>> 20130 ywang25 20 0 91440 5748 4844 R 83 0.1 0:43.07 3 slave
>>>>>> 20131 ywang25 20 0 91432 5744 4840 R 84 0.1 0:41.20 4 slave
>>>>>> 20134 ywang25 20 0 112m 36m 11m R 96 0.9 0:47.35 5
>>>>>
>>>>>
>>>>> master> 20128 ywang25 20 0 91432 5752 4844 R 93 0.1
>>>>> 0:45.36 5 slave
>>>>>
>>>>>> 20127 ywang25 20 0 91440 5724 4824 R 94 0.1 0:40.56 6 slave
>>>>>> 20135 ywang25 20 0 91440 5736 4832 R 92 0.1 0:39.75 7 slave
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>
>
More information about the mpich-discuss
mailing list