On 2016/5/23 22:18, Paolo Bonzini wrote:
On 23/05/2016 03:34, Yang Zhang wrote:
On 2016/5/20 14:38, Denis V. Lunev wrote:
On 05/20/2016 04:15 AM, Yang Zhang wrote:
On 2016/5/19 13:40, Denis V. Lunev wrote:
On 05/19/2016 05:01 AM, Yang Zhang wrote:
On 2016/5/18 22:48, Roman Kagan wrote:
The function to update APICv on/off state (in particular, to
deactivate
it when enabling Hyper-V SynIC), used to be incomplete: it didn't
adjust
APICv-related fields among secondary processor-based VM-execution
controls.
Roman,
I have question about the performance between APICv and Hyper-V SynIC.
As we known APICv is a hardware feature which including three
features: APIC register virtualization, virtual interrupt delivery and
Posted Interrupt. My gut feeling is that the average performance that
improved by APICv should greater than Hyper-v SynIC. Am i right? If
yes, current policy that disable the whole APICv seems too aggressive.
Argh.. We have faced this situation in Parallels Desktop may be
3 years ago. Unfortunately, there is no data at the moment.
It was toooo old and made by other team. As far as I remember
(for that time), interrupt delivery becomes faster, but operations
with on of CR registers becomes much slower and general
performance score becomes lower.
I guess the data may be collected on KVM w/o APICv supporting.
Normally, APICv provides the faster way to delivery interrupt than
software solution.
this seems useless.
Once again, interrupt delivery with APICv will be faster,
this is out of question. Though integral tests can show
performance degradation. I know, this sounds silly
and this is test-dependent.
We are going to make this testing after implementing
of HyperV TSC page avoid extra VM exit on context
switch. This seems more beneficial at the moment.
For this reason APICv is disabled in Parallels Desktop
and Parallels Server v6, which are not KVM based.
The problem with SynIC is that it is mandatory prerequisite
to enable HyperV bus in the guest, which is our final goal.
Thus there is no other way for us.
I see. Actually, we saw the performance improvement with SynIC timer
but we don't want to turn off APICv since we think it may hurt the
performance. Is it possible to turn on SynIC timer without disable
APICv?
unfortunately no. The problem is AutoEOI feature. At
least we have no idea how to do that.
Ok. Thanks for your explanation.
You can search the KVM mailing list archives; there are some discussions
between me, Andrey and Roman regarding APICv---when I tried their unit
tests on a Haswell-EP machine I found that they broke due to AutoEOI,
and we came up with the solution of disabling APICv.
Thanks. I will check it.
For what it's worth, we've also seen only very small performance
improvements from APICv, with the exception of nested virtualization
where APICv's impact is large.
I think it depends on the workload. For some micro benchmarks, we have
see more than 10% improvement, like ping latency testing. But for normal
workload, you may only seen less than 2% improvement.
The reason i raise this concern is that VT-d PI also depends on APICv.
This means all windows guests with SynIC enabled cannot benefit from
VT-d PI.
Paolo
--
best regards
yang
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