Greetings,
This all started as I'm looking to find a replacement for one of my VMs to an upgraded OS for newer software versions. I've been running Centos 7 practically exclusively which has been great. Now that Centos 8 is here, I fired it up.
I came to realize that even with a Cent 8 minimal install and it sitting idle, the Proxmox console is reporting about 5% CPU usage. I thought that was a little high so began investigating. All my Centos 7 minimal VMs are low at about .5% or range <1. Running top though, all the idle VMs report <1%, including the new Centos 8. So that means kvm host and guest on Centos7 are congruent whereas host and guest on Centos 8 has a discrepancy.
After some researching, I found that it might be polling related but this doesn't make much sense since my Centos 7 VMs are unaffected. Just to be sure, I fired up a new 7 one after the 8 and it's still the same low reporting from host and guest as the others. I thought c/p states might be coming into play but after playing around with that and moving to tsc clocksource, didn't have any effect. IRQs didn't seem to be related.
I moved to perf on the host. Tracing the syscalls shows a large variance in context switching between centos 7 and centos 8 kvm processes. Attached is some data on that. It does appear to be polling related but I'm not sure why that would exist for certain VM types and not others.
I also tested Debian 9 and Ubuntu 18 LTS which have the same behavior as Centos 8. I asked a friend who also runs Proxmox and he reports 1% usage on Ubuntu 18 idle.
Questions are:
1. Is this polling or whatever this overhead is expected?
2. Will it adversely affect performance of the overall system? I don't have the most amount of horse power so I'd like to save as many cycles as possible.
3. If this is abnormal, are there any ideas of what I could try to address it?
This all started as I'm looking to find a replacement for one of my VMs to an upgraded OS for newer software versions. I've been running Centos 7 practically exclusively which has been great. Now that Centos 8 is here, I fired it up.
I came to realize that even with a Cent 8 minimal install and it sitting idle, the Proxmox console is reporting about 5% CPU usage. I thought that was a little high so began investigating. All my Centos 7 minimal VMs are low at about .5% or range <1. Running top though, all the idle VMs report <1%, including the new Centos 8. So that means kvm host and guest on Centos7 are congruent whereas host and guest on Centos 8 has a discrepancy.
After some researching, I found that it might be polling related but this doesn't make much sense since my Centos 7 VMs are unaffected. Just to be sure, I fired up a new 7 one after the 8 and it's still the same low reporting from host and guest as the others. I thought c/p states might be coming into play but after playing around with that and moving to tsc clocksource, didn't have any effect. IRQs didn't seem to be related.
I moved to perf on the host. Tracing the syscalls shows a large variance in context switching between centos 7 and centos 8 kvm processes. Attached is some data on that. It does appear to be polling related but I'm not sure why that would exist for certain VM types and not others.
I also tested Debian 9 and Ubuntu 18 LTS which have the same behavior as Centos 8. I asked a friend who also runs Proxmox and he reports 1% usage on Ubuntu 18 idle.
Questions are:
1. Is this polling or whatever this overhead is expected?
2. Will it adversely affect performance of the overall system? I don't have the most amount of horse power so I'd like to save as many cycles as possible.
3. If this is abnormal, are there any ideas of what I could try to address it?
