Re: RFC: KVM: x86/mmu: Eager Page Splitting

[David Matlack <dmatlack@google.com>]

On Fri, Nov 05, 2021 at 06:17:11PM +0100, Janis Schoetterl-Glausch wrote:
> On 11/4/21 23:45, David Matlack wrote:
> 
> [...]
> > 
> > The last alternative is to perform dirty tracking at a 2M granularity.
> > This would reduce the amount of splitting work required by 512x,
> > making the current approach of splitting on fault less impactful to
> > customer performance. We are in the early stages of investigating 2M
> > dirty tracking internally but it will be a while before it is proven
> > and ready for production. Furthermore there may be scenarios where
> > dirty tracking at 4K would be preferable to reduce the amount of
> > memory that needs to be demand-faulted during precopy.

Oops I meant to say "demand-faulted during post-copy" here.

> I'm curious how you're going about evaluating this, as I've experimented with
> 2M dirty tracking in the past, in a continuous checkpointing context however.
> I suspect it's very sensitive to the workload. If the coarser granularity
> leads to more memory being considered dirty, the length of pre-copy rounds
> increases, giving the workload more time to dirty even more memory.
> Ideally large pages would be used only for regions that won't be dirty or
> regions that would also be pretty much completely dirty when tracking at 4K.
> But deciding the granularity adaptively is hard, doing 2M tracking instead
> of 4K robs you of the very information you'd need to judge that.

We're planning to look at how 2M tracking affects the amount of memory
that needs to be demand-faulted during the post-copy phase for different
workloads.