Re: Big TCG slowdown when using zstd with aarch64

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Jun 02, 2023 at 10:22:28AM +0100, Peter Maydell wrote:
> On Fri, 2 Jun 2023 at 10:10, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > I suspect that the zstd logic takes a little bit longer in setup,
> > which allows often allows the guest dirty workload to get ahead of
> > it, resulting in a huge amount of data to transfer. Every now and
> > then the compression code gets ahead of the workload and thus most
> > data is zeros and skipped.
> >
> > IMHO this feels like just another example of compression being largely
> > useless. The CPU overhead of compression can't keep up with the guest
> > dirty workload, making the supposedly network bandwidth saving irrelevant.
> 
> It seems a bit surprising if compression can't keep up with
> a TCG guest workload, though...

The multifd code seems to be getting slower and slower through the
migration. It peaks at 39 mbps, but degrades down to 4 mbps when i
test it.

I doubt that the aarch64 is specifically a problem, rather it is just
affecting timing that exposes some migration issue.

With regards,
Daniel
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