Messages in this thread | | | Date | Mon, 24 Sep 2012 17:22:40 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler |
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On 09/24/2012 05:04 PM, Peter Zijlstra wrote: > On Fri, 2012-09-21 at 17:29 +0530, Raghavendra K T wrote: >> In some special scenarios like #vcpu<= #pcpu, PLE handler may >> prove very costly, because there is no need to iterate over vcpus >> and do unsuccessful yield_to burning CPU. > > What's the costly thing? The vm-exit, the yield (which should be a nop > if its the only task there) or something else entirely? > Both vmexit and yield_to() actually,
because unsuccessful yield_to() overall is costly in PLE handler.
This is because when we have large guests, say 32/16 vcpus, and one vcpu is holding lock, rest of the vcpus waiting for the lock, when they do PL-exit, each of the vcpu try to iterate over rest of vcpu list in the VM and try to do directed yield (unsuccessful). (O(n^2) tries).
this results is fairly high amount of cpu burning and double run queue lock contention.
(if they were spinning probably lock progress would have been faster). As Avi/Chegu Vinod had felt it is better to avoid vmexit itself, which seems little complex to achieve currently.
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