On Wed, Jun 01, 2022 at 05:57:14PM -0400, Laurent Dumont wrote:

Hey folks,

I am curious to understand a bit more the core use of the emulatorpin CPUs with libvirt.

For example :

<vcpupin vcpu='0' cpuset='34'/> <vcpupin vcpu='1' cpuset='14'/> <vcpupin vcpu='2' cpuset='10'/> <vcpupin vcpu='3' cpuset='30'/> <emulatorpin cpuset='10,14,30,34'/>

qemu launched like this is one process with (at least, but let's omit that) 5 threads. There is one thread for each vCPU and an extra thread for all the extra work not running in the guest. That can be I/O, serving the monitor etc. If you pin it onto the same host CPUs as the vCPUs then you cannot expect to have enough resources allocated for serving 100% utilisation in the guest. But it is of course configurable because overcommitting is sometimes wanted.

In this case :

- The VM has 4 vCPUs. - Each of the core is pinned to a physical/thread core - 34,14,10,30 - The emulatorpin is also attached to the same physical CPUs.

In our case :

- Same setup for cores. - Running DPDK inside the virtual machine. - The compute/libvirt is using OVS but without DPDK.

What we saw is the following :

- DPDK is polling two cores at 100% to process packets. - From the compute metrics, we see USER %usage at 100% when the application starts. - From the compute metrics, as traffic increases, we see %SYSTEM going up. - As %SYSTEM goes up, %USER goes down.

We are not quite sure, but we are wondering if this could cause a direct impact to the ability of DPDK to process traffic at the same X speed?

- We can "live" move the emulator CPU to other cores. - This seems to be the recommended default setup for VNF/NFV deployments.

Feels like I am missing a piece of the puzzle but gotta start somewhere!

Thanks!