Re: balance storm

Libo Chen
2014-05-26T12:17:12Z

On 2014/5/26 13:11, Mike Galbraith wrote:
> On Mon, 2014-05-26 at 11:04 +0800, Libo Chen wrote:
>> hi,
>> my box has 16 cpu (E5-2658,8 core, 2 thread per core), i did a test on
>> 3.4.24stable, startup 50 same process, every process is sample:
>>
>> #include <unistd.h>
>>
>> int main()
>> {
>> for (;;)
>> {
>> unsigned int i = 0;
>> while (i< 100){
>> i++;
>> }
>> usleep(100);
>> }
>>
>> return 0;
>> }
>>
>> the result is process uses 15% cpu time, perf tool shows 70w migrations in 5 second.
>
> See e0a79f52 sched: Fix select_idle_sibling() bouncing cow syndrome
>
> That commit will fix expensive as hell bouncing for most real loads, but
> it won't fix your test. Doing nothing but wake, select_idle_sibling()
> will be traversing all cores/siblings mightily, taking L2 misses as it
> traverses, bouncing wakees that do _nothing_ when an idle CPU is found.
>
> Your synthetic test is the absolute worst case scenario. There has to
> be work between wakeups for select_idle_sibling() to have any chance
> whatsoever of turning in a win. At 0 work, it becomes 100% overhead.

not synthetic, it is a real problem in our product. under no load, waste
much cpu time.

>
>> I guess task migration takes up a lot of cpu, so i did another test. use taskset tool to bind
>> a task to a fixed cpu. Results in line with expectations, cpu usage is down to 5%.
>>
>> other test:
>> - 3.15upstream has the same problem with 3.4.24.
>> - suse sp2 has low cpu usage about 5%.
>
> SLE11-SP2 has a patch which fixes that behavior, but of course at the
> expense of other load types. A trade. It also throttles nohz, which
> can have substantial cost when cross CPU scheduling.

which patch ?

>
>> so I think 15% cpu usage and migration event are too high, how to fixed?
>
> You can't for free, low latency wakeup can be worth one hell of a lot.
>
> You could do a decayed hit/miss or such to shut the thing off when the
> price is just too high. Restricting migrations per unit time per task
> also helps cut the cost, but hurts tasks that could have gotten to the
> CPU quicker, and started your next bit of work. Anything you do there
> is going to be a rob Peter to pay Paul thing.
>

I had tried to change sched_migration_cost and sched_nr_migrate in /proc,
but no use. any other suggestion?

I still think this is a problem to schedular. it is better to directly solve
this issue instead of a workaroud

thanks,
Libo

> -Mike
>
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