Re: [PATCH v4 0/7] Move memory listener register to vhost_vdpa_init

[Jason Wang <jasowang@redhat.com>]

On Thu, May 15, 2025 at 8:17 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
>
> Hi Eugenio,
>
> On 5/14/2025 8:49 AM, Eugenio Perez Martin wrote:
> > On Wed, May 7, 2025 at 8:47 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
> >> Current memory operations like pinning may take a lot of time at the
> >> destination.  Currently they are done after the source of the migration is
> >> stopped, and before the workload is resumed at the destination.  This is a
> >> period where neigher traffic can flow, nor the VM workload can continue
> >> (downtime).
> >>
> >> We can do better as we know the memory layout of the guest RAM at the
> >> destination from the moment that all devices are initializaed.  So
> >> moving that operation allows QEMU to communicate the kernel the maps
> >> while the workload is still running in the source, so Linux can start
> >> mapping them.
> >>
> >> As a small drawback, there is a time in the initialization where QEMU
> >> cannot respond to QMP etc.  By some testing, this time is about
> >> 0.2seconds.  This may be further reduced (or increased) depending on the
> >> vdpa driver and the platform hardware, and it is dominated by the cost
> >> of memory pinning.
> >>
> >> This matches the time that we move out of the called downtime window.
> >> The downtime is measured as checking the trace timestamp from the moment
> >> the source suspend the device to the moment the destination starts the
> >> eight and last virtqueue pair.  For a 39G guest, it goes from ~2.2526
> >> secs to 2.0949.
> >>
> > Hi Jonah,
> >
> > Could you update this benchmark? I don't think it changed a lot but
> > just to be as updated as possible.
> Jonah is off this week and will be back until next Tuesday, but I recall
> he indeed did some downtime test with VM with 128GB memory before taking
> off, which shows obvious improvement from around 10 seconds to 5.8
> seconds after applying this series. Since this is related to update on
> the cover letter, would it be okay for you and Jason to ack now and then
> proceed to Michael for upcoming merge?

I will go through the series.

Thanks