On 01/13/2011 07:21 AM, Rik van Riel wrote:
When running SMP virtual machines, it is possible for one VCPU to be
spinning on a spinlock, while the VCPU that holds the spinlock is not
currently running, because the host scheduler preempted it to run
something else.
Both Intel and AMD CPUs have a feature that detects when a virtual
CPU is spinning on a lock and will trap to the host.
The current KVM code sleeps for a bit whenever that happens, which
results in eg. a 64 VCPU Windows guest taking forever and a bit to
boot up. This is because the VCPU holding the lock is actually
running and not sleeping, so the pause is counter-productive.
In other workloads a pause can also be counter-productive, with
spinlock detection resulting in one guest giving up its CPU time
to the others. Instead of spinning, it ends up simply not running
much at all.
This patch series aims to fix that, by having a VCPU that spins
give the remainder of its timeslice to another VCPU in the same
guest before yielding the CPU - one that is runnable but got
preempted, hopefully the lock holder.
Can you share some benchmark results?
I'm mostly interested in moderately sized guests (4-8 vcpus) under
conditions of no overcommit, and high overcommit (2x).
For no overcommit, I'd like to see comparisons against mainline with PLE
disabled, to be sure there aren't significant regressions. For
overcommit, comparisons against the no overcommit case. Comparisons
against mainline, with or without PLE disabled, are uninteresting since
we know it sucks both ways.
--
error compiling committee.c: too many arguments to function
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