On Fri, Sep 14, 2012 at 04:34:24PM -0400, Konrad Rzeszutek Wilk wrote: > > The concern I have is that even though we have gone through changes to > > help reduce the candidate vcpus we yield to, we still have a very poor > > idea of which vcpu really needs to run. The result is high cpu usage in > > the get_pid_task and still some contention in the double runqueue lock. > > To make this scalable, we either need to significantly reduce the > > occurrence of the lock-holder preemption, or do a much better job of > > knowing which vcpu needs to run (and not unnecessarily yielding to vcpus > > which do not need to run). > > The patches that Raghavendra has been posting do accomplish that. > > > > On reducing the occurrence: The worst case for lock-holder preemption > > is having vcpus of same VM on the same runqueue. This guarantees the > > situation of 1 vcpu running while another [of the same VM] is not. To > > prove the point, I ran the same test, but with vcpus restricted to a > > range of host cpus, such that any single VM's vcpus can never be on the > > same runqueue. In this case, all 10 VMs' vcpu-0's are on host cpus 0-4, > > vcpu-1's are on host cpus 5-9, and so on. Here is the result: > > > > kvm_cpu_spin, and all > > yield_to changes, plus > > restricted vcpu placement: 8823 +/- 3.20% much, much better > > > > On picking a better vcpu to yield to: I really hesitate to rely on > > paravirt hint [telling us which vcpu is holding a lock], but I am not > > sure how else to reduce the candidate vcpus to yield to. I suspect we > > are yielding to way more vcpus than are prempted lock-holders, and that > > IMO is just work accomplishing nothing. Trying to think of way to > > further reduce candidate vcpus.... > > ... the patches are posted - you could try them out? Radim and I have done some testing with the pvticketlock series. While we saw a gain over PLE alone, it wasn't huge, and without PLE also enabled it could hardly support 2.0x overcommit. spinlocks aren't the only place where cpu_relax() is called within a relatively tight loop, so it's likely that PLE yielding just generally helps by getting schedule() called more frequently. Drew -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
