Re: [Qemu-devel] [PATCH 7/7] target/i386: Enable kvm_pv_unhalt by default

[Alexander Graf]

On 09.10.17 16:40, Paolo Bonzini wrote:
> On 06/10/2017 23:52, Eduardo Habkost wrote:
>>   Commit f010bc643a (target-i386: add feature kvm_pv_unhalt) introduced the
>>   kvm_pv_unhalt feature but didn't enable it by default.
>>
>>   Without kvm_pv_unhalt we see a measurable degradation in scheduling
>>   performance, so enabling it by default does make sense IMHO. This patch
>>   just flips it to default to on by default.
>>
>>     [With kvm_pv_unhalt disabled]
>>     $ perf bench sched messaging -l 10000
>>       Total time: 8.573 [sec]
>>
>>     [With kvm_pv_unhalt enabled]
>>     $ perf bench sched messaging -l 10000
>>       Total time: 4.416 [sec]
> 
> I cannot reproduce this:
> 
> Host CPU model: Haswell-EP (Xeon E5-2697 v3 @ 2.60 GHz)
> Host physical CPUs: 56 (2 sockets 14 cores/sockets, 2 thread/core)
> Host Linux kernel: 4.14 (more or less :))
> Host memory: 64 GB
> Guest Linux kernel: 4.10.13
> 
> QEMU command line:
> 
> /usr/libexec/qemu-kvm -cpu host,+kvm_pv_unhalt -M q35 \

-cpu host already enables kvm_pv_unhalt. It's actually the only CPU type
that does :).

The problem arises when you use libvirt with automatic cpu type
detection, as it then generates something that doesn't do -cpu host.


Alex