On 05/14/2014 05:44 PM, Peter Zijlstra wrote:
[snip]
>> and then:
>> echo $$ > /sys/fs/cgroup/cpu/A/tasks ; ./my_tool -l
>> echo $$ > /sys/fs/cgroup/cpu/B/tasks ; ./my_tool -l
>> echo $$ > /sys/fs/cgroup/cpu/C/tasks ; ./my_tool 50
>>
>> the results in top is around:
>>
>> A B C
>> CPU% 550 550 100
>
> top doesn't do per-cgroup accounting, so how do you get these numbers,
> per the above all instances of the prog are also called the same,
> further making it error prone and difficult to get sane numbers.
Oh, my bad to make it confusing, I myself was checking the PID of my_tool
instant inside top, like:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
24968 root 20 0 55600 720 648 S 558.1 0.0 2:08.76 my_tool
24984 root 20 0 55600 720 648 S 536.2 0.0 1:10.29 my_tool
25001 root 20 0 55600 720 648 S 88.6 0.0 0:04.39 my_tool
By 'cat /sys/fs/cgroup/cpu/C/tasks' I got the PID of './my_tool 50' is
25001, and all it's pthread's %CPU was count in, could we check like
that?
>
>
[snip]
>> void consume(int spin, int total)
>> {
>> unsigned long long begin, now;
>> begin = stamp();
>>
>> for (;;) {
>> pthread_mutex_lock(&my_mutex);
>> now = stamp();
>> if ((long long)(now - begin) > spin) {
>> pthread_mutex_unlock(&my_mutex);
>> usleep(total - spin);
>> pthread_mutex_lock(&my_mutex);
>> begin += total;
>> }
>> pthread_mutex_unlock(&my_mutex);
>> }
>> }
>
> Uh,.. that's just insane.. what's the point of having a multi-threaded
> program do busy-wait loops if you then serialize the lot on a global
> mutex such that only 1 thread can run at any one time?
>
> How can one such prog ever consume more than 100% cpu.
That's a good point... however the top show that when only './my_tool 50'
25001 running, it used around 300%, like below:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
25001 root 20 0 55600 720 648 S 284.3 0.0 5:18.00 my_tool
2376 root 20 0 950m 85m 29m S 4.4 0.2 163:47.94 python
1658 root 20 0 1013m 19m 11m S 3.0 0.1 97:06.11 libvirtd
IMHO, if pthread-mutex was similar like the kernel one's behaviour, then
it may not going to sleep when it's the only one running on CPU.
Oh, I think we got the reason here, when there are other task running,
mutex will going to sleep and the %CPU dropped to serialized case that is
around 100%.
But for the dbench, stress combination, that's not spin-wasted, dbench
throughput do dropped, how could we explain that one?
Regards,
Michael Wang
>
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