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. 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%. 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. -- 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/