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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