[MPICH] An idle communication process use the same CPU as computation process on multi-core chips
Darius Buntinas
buntinas at mcs.anl.gov
Fri Sep 14 12:55:27 CDT 2007
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
>
>
>
>
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