Re: [PATCH v5 00/13] WIP: Use Intel DSA accelerator to offload zero page checking in multifd live migration.

[Michael S. Tsirkin]

On Mon, Jul 15, 2024 at 03:57:42PM +0000, Liu, Yuan1 wrote:
> > > > > > > > that is 23% total CPU usage savings.
> > > > > > >
> > > > > > >
> > > > > > > Here the DSA was mostly idle.
> > > > > > >
> > > > > > > Sounds good but a question: what if several qemu instances are
> > > > > > > migrated in parallel?
> > > > > > >
> > > > > > > Some accelerators tend to basically stall if several tasks
> > > > > > > are trying to use them at the same time.
> > > > > > >
> > > > > > > Where is the boundary here?
> 
> If I understand correctly, you are concerned that in some scenarios the
> accelerator itself is the migration bottleneck, causing the migration 
> performance
> to be degraded.
> 
> My understanding is to make full use of the accelerator bandwidth, and once
> the accelerator is the bottleneck, it will fall back to zero-page detection
> by the CPU.
> 
> For example, when the enqcmd command returns an error which means the work 
> queue
> is full, then we can add some retry mechanisms or directly use CPU detection.


How is it handled in your patch? If you just abort migration unless
enqcmd succeeds then would that not be a bug, where loading the system
leads to migraton failures?


-- 
MST