[PVE-User] high cpu load on 100mbits/sec download with virtio nic

[Maxime AUGER]

Hello,

I can t enable PCID/Spectre-ctrl, host does not support it
Same results using CPU type host
I notice that GUEST %CPU decrease by allocating a single vcpu
In that situation I have same 7% CPU on PROXMOX and VMWARE GUESTS
But on host side it s drastically different 
On VMWARE HOST  : 8%  CPU
On PROXMOX HOST : 50% CPU (40% qemu process and 10% vhost kernel thread)

I also notice that with single vcpu ksoftirq CPU time consuming on GUEST is close to zero
With dual vcpu (or more) ksoftirq increase even when GUEST is idling

It appears to be an improper use of the CPU whether with the HOST CPU type or kvm64
I started with last stable proxmox ISO (5.2), I dont know how to improve CPU utilisation

Do you notice the same behavior, GUEST/HOST CPU usage difference ?

Thank you

-----Message d'origine-----
De : pve-user [mailto:pve-user-bounces at pve.proxmox.com] De la part de Alwin Antreich
Envoyé : samedi 9 juin 2018 19:48
À : PVE User List <pve-user at pve.proxmox.com>
Objet : ###SPAM### Re: [PVE-User] high cpu load on 100mbits/sec download with virtio nic

On Fri, Jun 08, 2018 at 07:39:17AM +0000, Maxime AUGER wrote:
> Hello,
>
> Let me clarify my statement.
> GUEST CPU load is acceptable (25% of a single CPU) It is the 
> cumulative load of the kvm process and the vhost thread that is high, 
> on the HOST side kvm-thread-1=30% kvm-thred-2=30% vhost(net)=10% 70% 
> CPU without i/o disk (-O /dev/null)
>
> it is x10 the load observed with the same conditions under old Vmware 
> ESXi (same GUEST system and wget process) I think it is a kvm issue, 
> but I'm curious about proxmox's positioning in relation to this 
> performance level
>
>
> -----Message d'origine-----
> De : pve-user [mailto:pve-user-bounces at pve.proxmox.com] De la part de 
> Josh Knight Envoyé : jeudi 7 juin 2018 20:47 À : PVE User List 
> <pve-user at pve.proxmox.com> Objet : Re: [PVE-User] high cpu load on 
> 100mbits/sec download with virtio nic
>
> I'm not convinced this is a proxmox issue, or even an issue to begin with.
>
> I'm running proxmox 5.1-49, in my Linux 4.1 guest when I run wget -O /dev/null <https to ~1.2GB iso> I'm seeing ~30% according to top as well.
>
>   PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
> 12788 root      20   0   65512   7616   5988 S  31.3  0.1   0:03.00 wget
>
>
> Even on a physical box running Ubuntu I'm getting around 20-30% or more.
>
>   PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
> 36070 root      20   0   32888   4028   3576 S  37.6  0.0   0:03.15 wget
>
>
> This could be an issue/quirk with the way top is calculating cpu usage, or it's just a process using available cpu as normal. I couldn't reproduce high load using ping -f from a remote host to my VM, and iotop confirmed that -O /dev/null wasn't somehow writing to disk.  I was able to lower the CPU usage by running wget with --limit-rate=.
>
> Related, I would not recommend macvtap if you're running routers.  At 
> least running on an Ubuntu 16.04 host, under load of ~1Gb we were 
> seeing anywhere from 0.01% to 0.16% packet loss before it reached the 
> guest's virtio interface.  We switched to linux bridge and then 
> finally to openvswitch with the default switching config (no custom 
> openflow rules.)
>
>
> Josh Knight
>
I guess, this could be well related to the Meltdown/Spectre mitigation.
At least it would fit to the "old" ESXi showing a different performance hit.

Try to set the CPU type to host or activate PCID/Spectre-ctrl if applicable.
https://pve.proxmox.com/pve-docs/chapter-qm.html#qm_cpu

--
Cheers,
Alwin
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