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SubjectRe: [PATCH v5] sched: Avoid unnecessary work in reweight_entity
On 02/20/2012 09:08 PM, Peter Zijlstra wrote:

Hi, Peter

Sorry for reply so late, I was blocked by some issue army while setup the
testing environment.

> On Sat, 2012-02-18 at 09:43 +0800, Michael Wang wrote:
>> And as reight_entity is invoked very often, I think this patch can do some help to the
>> performance, although there are no numbers, we can prove it logically :)
>
> Well, you're probably right (although I think you completely ignored the
> optimizing compiler), but still it would be good to get some numbers to
> confirm reality :-)


That's right, if consider the compiler's optimization, the logic improvements
I listed may not be true...

>

> Just pick your favourite cgroup workload/benchmark and run a pre/post
> comparison, possible using perf record.
>
> If all squares up you should see an improvement in your benchmark score
> as well as see a reduction in time spend in the function you're
> patching.


So I created a cpuset cgroup 'rg1' and his sub memory group 'sub',
attached current shell to 'sub', then use 'time make kernel' as benchmark.

Below is the test result:

'time make':
old
real: 87m53.804s user: 66m41.886s sys: 11m51.792s
new
real: 86m8.485s user: 65m49.211s sys: 11m47.264s

'time make -j14':
old:
real: 42m43.825s user: 124m13.726s sys: 17m57.183s
new
real: 39m0.072s user: 114m33.258s sys: 15m30.662s

I also try to use 'perf sched record', but I can only record few seconds time,
otherwise it will be too big and report some error, as the record time is too
short, results are very different from each other, I failed to use them to prove
the patch:(

I also have try to use some other method, I moved 'reweight_entity' and related
functions to user space, and invoke it 10000000 times in 'main', I have append
part of the code (really raw...) in the end.

Three times output is:

old:
real 0m0.715s 0m0.710s 0m0.739s
user 0m0.716s 0m0.708s 0m0.736s
sys 0m0.000s 0m0.000s 0m0.000s

new:
real 0m0.318s 0m0.308s 0m0.317s
user 0m0.316s 0m0.304s 0m0.316s
sys 0m0.000s 0m0.000s 0m0.000s

It seems like that new code can save more then half execution time, but also we
can see, after calculation, what I have done can only save 0.04ns(too small...).

The user space test result is not accurate but at least we can know new code is
faster then old.

Please tell me if we need to do some thing else, and thanks for your suggestion :)

Regards,
Michael Wang



User space code:

void
account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
update_load_add(&cfs_rq->load, se->load.weight);
if (1)
update_load_add(&cfs_rq->load, se->load.weight);
if (1) {
add_cfs_task_weight(cfs_rq, se->load.weight);
list_add(&se->group_node, &cfs_rq->tasks);
}
cfs_rq->nr_running++;
}

void
account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
update_load_sub(&cfs_rq->load, se->load.weight);
if (1)
update_load_sub(&cfs_rq->load, se->load.weight);
if (1) {
add_cfs_task_weight(cfs_rq, -se->load.weight);
list_del_init(&se->group_node);
}
cfs_rq->nr_running--;
}

void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
unsigned long weight)
{
if (1) {
account_entity_dequeue(cfs_rq, se);
}

update_load_set(&se->load, weight);

if (1)
account_entity_enqueue(cfs_rq, se);
}

void reweight_entity_new(struct cfs_rq *cfs_rq, struct sched_entity *se,
unsigned long weight)
{
if (1) {
update_load_add(&cfs_rq->load, weight - se->load.weight);
if(1)
update_load_add(&cfs_rq->load, weight -
se->load.weight);
if(1)
add_cfs_task_weight(cfs_rq, weight
-se->load.weight);
}
update_load_set(&se->load, weight);
}

int main()
{
struct cfs_rq cfsrq;
struct sched_entity se;
memset(&cfsrq, 0, sizeof(struct cfs_rq));
memset(&se, 0, sizeof(struct sched_entity));
INIT_LIST_HEAD(&se.group_node);
INIT_LIST_HEAD(&cfsrq.tasks);
int i = 10000000;
while(i) {
i--;
reweight_entity_new(&cfsrq, &se, 10);
//reweight_entity(&cfsrq, &se, 10);
}
}

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