Messages in this thread | | | Date | Thu, 21 Feb 2008 13:09:05 +0530 | From | Balbir Singh <> | Subject | Re: Make yield_task_fair more efficient |
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Ingo Molnar wrote: > * Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > >> I disagree. The cost is only adding a field to cfs_rq [...] > > wrong. The cost is "only" of adding a field to cfs_rq and _updating it_, > in the hottest paths of the scheduler: > > @@ -256,6 +257,7 @@ static void __enqueue_entity(struct cfs_ > */ > if (key < entity_key(cfs_rq, entry)) { > link = &parent->rb_left; > + rightmost = 0;
That's an update when we move leftwards.
> } else { > link = &parent->rb_right; > leftmost = 0; > @@ -268,6 +270,8 @@ static void __enqueue_entity(struct cfs_ > */ > if (leftmost) > cfs_rq->rb_leftmost = &se->run_node; > + if (rightmost) > + cfs_rq->rb_rightmost = &se->run_node; >
&se->run_node is already in the cache, we are assigning cfs_rq->rb_rightmost to it.
>> [...] For a large number of tasks - say 10000, we need to walk 14 >> levels before we reach the node (each time). [...] > > 10,000 yield-ing tasks is not a common workload we care about. It's not > even a rare workload we care about. _Especially_ we dont care about it > if it slows down every other workload (a tiny bit). >
sched_yield() is supported API and also look at http://lkml.org/lkml/2007/9/19/351. I am trying to make sched_yield() efficient when compat_sched_yield is turned on (which is most likely), since people will want that behaviour (Hint, please read the man-page for sched_yield).There are already several applications using sched_yield(), so they all suffer.
>> [...] Doesn't matter if the data is cached, we are still spending CPU >> time looking through pointers and walking to the right node. [...] > > have you actually measured how much it takes to walk the tree that deep > on recent hardware? I have.
I have measured how much time can be saved by not doing that and it's quite a lot.
-- Warm Regards, Balbir Singh Linux Technology Center IBM, ISTL
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