Messages in this thread Patch in this message | | | Date | Thu, 21 Feb 2008 08:07:33 +0100 | From | Ingo Molnar <> | Subject | Re: Make yield_task_fair more efficient |
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* 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; } 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;
> [...] 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).
> [...] 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.
Ingo
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