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

[Paolo Bonzini <pbonzini@redhat.com>]

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 \
   -m XXX -smp YYY \
   /path/to/vm.qcow2

(XXX = MiB of guest memory, YYY = number of guest processors)

"perf bench sched messaging -l 50000" has the following result for me:

Guest vCPUs    Guest memory     without PV unhalt      with PV unhalt
1*96           32 GB            24.6 s                 24.2 s
2*96           24 GB            47.9 s (both VMs)      46.8 s (both VMs)
2*48           16 GB            50.4 s (both VMs)      49.3 s (both VMs)
4*24           12 GB            82.1 - 89.0 s          82.3 - 88.8 s
4*4            12 GB            87.2 - 91.3 s          90.3 - 94.9 s

All but the first line are running the benchmark in multiple guests,
concurrently.  The improvement seems to be about 2-3% for guests larger
than 1 NUMA node, and zero or negative for smaller guests (especially as
the host is no longer overcommitted).

The difference for large NUMA guests is small but I ran the benchmark
multiple times and it is statistically significant---but not as large as
what Alex reported.

Paolo