On 2013-10-16 02:14, Christoffer Dall wrote:
On Tue, Oct 08, 2013 at 06:38:13PM +0100, Marc Zyngier wrote:
On an (even slightly) oversubscribed system, spinlocks are quickly
becoming a bottleneck, as some vcpus are spinning, waiting for a
lock to be released, while the vcpu holding the lock may not be
running at all.
This creates contention, and the observed slowdown is 40x for
hackbench. No, this isn't a typo.
The solution is to trap blocking WFEs and tell KVM that we're
now spinning. This ensures that other vpus will get a scheduling
boost, allowing the lock to be released more quickly. Also, using
CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT slightly improves the
performance
when the VM is severely overcommited.
Quick test to estimate the performance: hackbench 1 process 1000
2xA15 host (baseline): 1.843s
2xA15 guest w/o patch: 2.083s
4xA15 guest w/o patch: 80.212s
8xA15 guest w/o patch: Could not be bothered to find out
2xA15 guest w/ patch: 2.102s
4xA15 guest w/ patch: 3.205s
8xA15 guest w/ patch: 6.887s
So we go from a 40x degradation to 1.5x in the 2x overcommit case,
which is vaguely more acceptable.
Patch looks good, I can just apply it and add the other one I just
send
as a reply if there are no objections.
Yeah, I missed the updated comments on this one, thanks for taking care
of it.
Sorry for the long turn-around on this one.
No worries. As long as it goes in, I'm happy. It makes such a
difference on my box, it is absolutely mind boggling.
Thanks,
M.
--
Fast, cheap, reliable. Pick two.
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