Re: [LSF/MM/BPF TOPIC] VM Memory Overcommit

["T.J. Alumbaugh" <talumbau@google.com>]

Hi Pankaj,

On Mon, Feb 27, 2023 at 11:02 PM Gupta, Pankaj <pankaj.gupta@amd.com> wrote:
>
>
> - What kind/frequency of data we expect WSS would relay to the host in
>    good and worst case?
>

This is a good question. Right now, we communicate a histogram of page
counts (not very much data) during periods of memory pressure, with
user configurable rate limiting. The goal is to notify when it's
important to take action, but we also allow the host to request a
report.

> - Thoughts already considered about the overhead VM will have based on
>    WSS relay data to the host and host taking action on the data?
>
>    -- One thing iirc from the free page hinting design experiments is
>       preventing guest and host acting simultaneously (e.g for free) on
>       same page (pfn) and also reducing the overhead of sending data from
>       guest to host by sending the highest possible page order.
>

I think you are correct that the best way to reduce overhead is by
sending the highest possible page order. So far we have not directly
addressed this problem. The goal here is to solve the "when" and "how
much" problems for balloon adjustment, which will go far in minimizing
fragmentation caused by frequent balloon adjustments. But it seems
clear that sending a higher page order is important too.

> Maybe some of these or other similar techniques are already considered.
>

David's idea of pairing this capability with free page reporting (in
the next reply on this thread) seems like an intriguing next step.

-T.J.