On Wed, Jan 16, 2013 at 07:32:49AM +0000, Alex Shi wrote: > On 01/15/2013 01:00 AM, Morten Rasmussen wrote: > >>> Why multiply rq->util by nr_running? > >>> > > > >>> > > Let's take an example where rq->util = 50, nr_running = 2, and putil = > >>> > > 10. In this case the value of putil doesn't really matter as vacancy > >>> > > would be negative anyway since FULL_UTIL - rq->util * nr_running is > >>> > > -1. > >>> > > However, with rq->util = 50 there should be plenty of spare cpu time > >>> > > to > >>> > > take another task. > >> > > >> > for this example, the util is not full maybe due to it was just wake up, > >> > it still is possible like to run full time. So, I try to give it the > >> > large guess load. > > I don't see why rq->util should be treated different depending on the > > number of tasks causing the load. rq->util = 50 means that the cpu is > > busy about 50% of the time no matter how many tasks contibute to that > > load. > > > > If nr_running = 1 instead in my example, you would consider the cpu > > vacant if putil = 6, but if nr_running > 1 you would not. Why should the > > two scenarios be treated differently? > > > >>> > > > >>> > > Also, why multiply putil by 8? rq->util must be very close to 0 for > >>> > > vacancy to be positive if putil is close to 12 (12.5%). > >> > > >> > just want to pack small util tasks, since packing is possible to hurt > >> > performance. > > I agree that packing may affect performance. But why don't you reduce > > FULL_UTIL instead of multiplying by 8? With current expression you will > > not pack a 10% task if rq->util = 20 and nr_running = 1, but you would > > pack a 6% task even if rq->util = 50 and the resulting cpu load is much > > higher. > > > > Yes, the threshold has no strong theory or experiment support. I had > tried cyclitest which Vicent used, the case's load avg is too small to > be caught. so just use half of Vicent value as 12.5%. If you has more > reasonable value, let me know. > > As to nr_running engaged as multiple mode. it's base on 2 reasons. > 1, load avg/util need 345ms to accumulate as 100%. so, if a tasks is > cost full cpu time, it still has 345ms with rq->util < 1.
I agree that load avg may not be accurate, especially for new tasks. But why use it if you don't trust its value anyway? The load avg (sum/period) of a new task will reach 100% instantly if the task is consuming all the cpu time it can get. An old task can reach 50% within 32ms. So you should fairly quickly be able to see if it is a light task or not. You may under-estimate its load in the beginning, but only for a very short time. > 2, if there are more tasks, like 2 tasks running on one cpu, it's > possible to has capacity to burn 200% cpu time, while the biggest > rq->util is still 100%. If you want to have a better metric for how much cpu time the task on the runqueue could potentially use, I would suggest using cfs_rq->runnable_load_avg which is the load_avg_contrib sum of all tasks on the runqueue. It would give you 200% in your example above. On the other hand, I think rq->util is fine for this purpose. If rq->util < 100% you know for sure that cpu is not fully utilized no matter how many tasks you have on the runqueue. So as long as rq->util is well below 100% (like < 50%) it should be safe to pack more small tasks on that cpu even if it has multiple tasks running already. > > Consider to figure out precise utils is complicate and cost much. I do > this simple calculation. It is not very precise, but it is efficient and > more bias toward performance. It is indeed very biased towards performance. I would prefer more focus on saving power in a power scheduling policy :) Morten > > -- > Thanks Alex > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
