Additional CPU flags + Copy host CPU configuration

[AmrnotAmr]

Hello,
I really want to thank you for this excellent piece of software with all the innovation it has.
Virt-manager now has an option to declare the extra cpu flags so as you can share the cpu capabilities with the disks.
It gets the CPU type and the extra flags from "virsh capabilities" cpu section. So the kvm process looks like this;
/usr/bin/kvm -S -M pc-0.14 -cpu core2duo,+lahf_lm,+rdtscp,+popcnt,+sse4.2,+sse4.1,+xtpr,+cx16,+tm2,+est,+vmx,+ds_cpl,+pbe,+tm,+ht,+ss,+acpi,+ds

virt-manager allows you to type the CPU info you want, or you can "Copy host CPU configuration" so it detects and adds it automatically as shown.
I hope you can implement the "Copy host CPU configuration" feature in archipel.

[JordanP]

Hi,
This sounds good. But does setting extra CPU flags improve VM performance ? It would be great if you could make a benchmark.

And this seems KVM-specific.

[AmrnotAmr]

Yes It does.
I've run some media benchmarks on Scientific Linux 6 guest running on RHEL6 host with "Intel(R) Xeon(R) CPU X3470 @ 2.93GHz" .... The guest has 4 virtual CPUs.
I ran the benchmarks with no "-cpu" at all, with "-cpu host" and with "-cpu Nehalem,+rdtscp,+xtpr,+tm2,+est,+vmx,+ds_cpl,+monitor,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme", the extra flags are generated by virt-manager "Copy Host CPU".
The time is estimated using time command.

The results is as follows;

++ nginx compilation with "make -j4" -- No performance gain/loss at all.

++ x264 media encoding - I've used the latest x264 compiled from source code with yasm - ;
Converting y4m file to flv.
1- With no "-cpu" the jop timing as follows;
real 0m25.778s
user 1m7.216s
sys 0m1.213s
2- With "-cpu host" and "-cpu Nehalem+....etc" results are almost the same as follows;
real 0m20.356s
user 0m53.403s
sys 0m1.210s

So for media processing it helps for sure.

By the way, it's recommended as of official KVM docs; http://www.linux-kvm.org/page/Tuning_KVM

[JordanP]

Thanks

@primalmotion : this can potentialy clash with the "arch" parameter in VM definition. Ie : you want an i686 VM but your (physical) server is amd64, passing "-cpu host" could be weird

[primalmotion]

so you would like to be able to pick from caps:

<cpu>
      <arch>x86_64</arch>
      <model>core2duo</model>
      <vendor>Intel</vendor>
      <topology cores='2' threads='1' sockets='1'/>
      <feature name='lahf_lm'/>
      <feature name='xtpr'/>
      <feature name='cx16'/>
      <feature name='tm2'/>
      <feature name='est'/>
      <feature name='vmx'/>
      <feature name='ds_cpl'/>
      <feature name='pbe'/>
      <feature name='tm'/>
      <feature name='ht'/>
      <feature name='ss'/>
      <feature name='acpi'/>
      <feature name='ds'/>
    </cpu>

and make definition looking like this?

  <cpu match='exact'>
    <model>core2duo</model>
    <feature policy='require' name='lahf_lm'/>
    ...
    <feature name='est'/>
  </cpu>

[nikdoof]

Also a thing to note, in Migration scenarios if you migrate a VM using the cx16 flag onto a non-cx16 box it'll die horribly in a fire. The low defaults (Pentium 2, small flag subset) is the standard to avoid these situations. It may be worth noting this in the documentation if this feature goes ahead.

[guilhem]

In libvirt 0.9.10, we can use mode=host-model :

<cpu mode='host-model'/>

http://www.redhat.com/archives/libvir-list/2012-January/msg00232.html

[yhaenggi]

What is the current status of this and/or #962 ? Will this be implemented? At least the possibility to set host-model is often necessary.

[CyrilPeponnet]

You can do it by editing the definition through the xml editor. There is no plan for now to add these features.

On Dec 27, 2014, at 10:46 AM, K1773R notifications@github.com wrote:

What is the current status of this and/or #962 ? Will this be implemented? Atleast the possibility to set host-model.

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Reply to this email directly or view it on GitHub.

[yhaenggi]

it works if i set <cpu mode='host-model'/>, tough everytime i "validate" the config, it will be removed. can i tell archipel to ignore this section, or just stick to manually doing it everytime. As its possible to set it, im happy with it already ;)