Re: [PATCH 1/2] credit: Limit load balancing to once per millisecond

[Marcus Granado <marcus.granado@xxxxxxxxx>]

On Fri, 30 Jun 2023 at 12:38, George Dunlap <george.dunlap@xxxxxxxxx> wrote:
> ...
> Add a parameter, load_balance_ratelimit, to limit the frequency of
> load balance operations on a given pcpu.  Default this to 1
> millisecond.
> ...
> Without this patch, on a system with a vcpu:pcpu ratio of 2:1, running
> Windows guests (which will end up calling YIELD during spinlock
> contention), this patch increased performance significantly.
> ---

I tested this patch with the following:
- the vcpu:pcpu ratio of circa 2:1 in the host,
- host with 72 pcpus
- and 9 Windows guests with 16 vcpus each,
- with background load inside the guests, where a few Windows apps
were starting and stopping in parallel in a loop in order to hit 100%
vcpu usage in each guest, so that the host pcpu usage was constantly
at 100%.
and observed a significant improvement in the responsiveness
experienced by a user moving the mouse and using the keyboard inside
the Windows guests, compared to when this patch is not present.

In addition, when launching a trivial executable like 'ver' remotely
in the Windows guests during the background load above, I noticed
that:
- when vcpu:pcpu is 1:1 and host has 100% pcpu usage -> with and
without this patch, launching 'ver' remotely took 1 second or so
- when vcpu:pcpu is 2:1 and host has 100% pcpu usage -> without this
patch, launching 'ver' remotely took several seconds
- when vcpu:pcpu is 2:1 and host has 100% pcpu usage -> with this
patch, launching 'ver' remotely took a couple of second only -- so
with this patch the host seems to be handling the extreme load at 100%
pcpu usage more gracefully and the latency inside the guest is more
linearly correlated to vcpu:pcpu..