On Sun, 30 Jun 2024, Tejun Heo wrote:
> Hello,
>
> On Sat, Jun 29, 2024 at 08:15:56PM +0200, Mikulas Patocka wrote:
>
> > With 6.5, we get 3600MiB/s; with 6.6 we get 1400MiB/s.
> >
> > The reason is that virt-manager by default sets up a topology where we
> > have 16 sockets, 1 core per socket, 1 thread per core. And that workqueue
> > patch avoids moving work items across sockets, so it processes all
> > encryption work only on one virtual CPU.
> >
> > The performance degradation may be fixed with "echo 'system'
> > >/sys/module/workqueue/parameters/default_affinity_scope" - but it is
> > regression anyway, as many users don't know about this option.
> >
> > How should we fix it? There are several options:
> > 1. revert back to 'numa' affinity
> > 2. revert to 'numa' affinity only if we are in a virtual machine
> > 3. hack dm-crypt to set the 'numa' affinity for the affected workqueues
> > 4. any other solution?
>
> Do you happen to know why libvirt is doing that? There are many other
> implications to configuring the system that way and I don't think we want to
> design kernel behaviors to suit topology information fed to VMs which can be
> arbitrary.
>
> Thanks.
I don't know why. I added [email protected] to the CC.
How should libvirt properly advertise "we have 16 threads that are
dynamically scheduled by the host kernel, so the latencies between them
are changing and unpredictable"?
Mikulas
