[j-nsp] vMX questions - vCPU math

[James Bensley]

On 30 December 2018 21:54:17 CET, Robert Hass <robhass at gmail.com> wrote:
...
>My confusion is related to HT setting, as you wrote to disable it.
>
>But vMX Getting Started Guide for KVM says:
>
>"CPU pinning with flow caching enabled (performance mode) is different
>than
>with flow
>caching disabled (lite mode). For both modes, you must enable
>hyperthreading"

Hmm strange. I've not done any performance testing with vMX, I don't use it outside of the lab, so I'm not up to date with vMX best practices but for DPDK powered applications the recommendation is always to disable HT and vMX is DPDK powered so I made the same recommendation here.

I'm curious to know Juniper's reason for advising HT be turned on. The case for disabling it seems clear to me; DPDK will lock the tx/rx cores at 99%, any hyper threading on those cores would then cause a high number of context switches which degrades performance (latency more then throughput). To what extent depends on various factors but, if you're tuning for performance (isolating cores, CPU pinning, CPU power management, NUMA affinity, hugepages etc.) it seems reasonable to me you wouldn't want to use something like HT that can degrade performance.

I'm all ears on this one.

Cheers,
James.