[pve-devel] KVM with same cores as host is as slow as one core

Alexandre DERUMIER aderumier at odiso.com
Fri Jul 19 09:22:41 CEST 2013


What are the physical processors ? (Is it an numa architecture ? if yes, could be a memory bandwith problem)

Do you have tried to launch unixbench on the host to compare ?


Maybe can you try to add +x2apic cpu flag ? It should reduce interrupts. (It's already in current qemu-server git)


----- Mail original ----- 

De: "Stefan Priebe - Profihost AG" <s.priebe at profihost.ag> 
À: pve-devel at pve.proxmox.com 
Envoyé: Vendredi 19 Juillet 2013 09:07:27 
Objet: [pve-devel] KVM with same cores as host is as slow as one core 

Hi, 

while benchmarking my kvm machines using unixbench i see the following 
reproducable behaviour. 

Host has 24 cores. 

KVM Machine set to 2 cores: Unixbench Value 2514 
KVM Machine set to 16 cores: Unixbench Value 6714 
KVM Machine set to 24 cores: Unixbench Value 1742 

Also i see a max of 105% CPU Usage of the KVM process when setting the 
cores to 24. 

While i'm seeing more than 1000% setting it to 16 cores. 

Stefan 
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