Re: [Libvirt][emulator-cores][dpdk] Emulatorpin cpuset best practices

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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
  - 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!


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