Re: Elvis upstreaming plan

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 26, 2013 at 08:53:47PM +0200, Abel Gordon wrote:
> 
> 
> Anthony Liguori <anthony@codemonkey.ws> wrote on 26/11/2013 08:05:00 PM:
> 
> >
> > Razya Ladelsky <RAZYA@il.ibm.com> writes:
> >
> > > Hi all,
> > >
> > > I am Razya Ladelsky, I work at IBM Haifa virtualization team, which
> > > developed Elvis, presented by Abel Gordon at the last KVM forum:
> > > ELVIS video:  https://www.youtube.com/watch?v=9EyweibHfEs
> > > ELVIS slides:
> https://drive.google.com/file/d/0BzyAwvVlQckeQmpnOHM5SnB5UVE
> > >
> > >
> > > According to the discussions that took place at the forum, upstreaming
> > > some of the Elvis approaches seems to be a good idea, which we would
> like
> > > to pursue.
> > >
> > > Our plan for the first patches is the following:
> > >
> > > 1.Shared vhost thread between mutiple devices
> > > This patch creates a worker thread and worker queue shared across
> multiple
> > > virtio devices
> > > We would like to modify the patch posted in
> > > https://github.com/abelg/virtual_io_acceleration/commit/
> > 3dc6a3ce7bcbe87363c2df8a6b6fee0c14615766
> > > to limit a vhost thread to serve multiple devices only if they belong
> to
> > > the same VM as Paolo suggested to avoid isolation or cgroups concerns.
> > >
> > > Another modification is related to the creation and removal of vhost
> > > threads, which will be discussed next.
> >
> > I think this is an exceptionally bad idea.
> >
> > We shouldn't throw away isolation without exhausting every other
> > possibility.
> 
> Seems you have missed the important details here.
> Anthony, we are aware you are concerned about isolation
> and you believe we should not share a single vhost thread across
> multiple VMs.  That's why Razya proposed to change the patch
> so we will serve multiple virtio devices using a single vhost thread
> "only if the devices belong to the same VM". This series of patches
> will not allow two different VMs to share the same vhost thread.
> So, I don't see why this will be throwing away isolation and why
> this could be a "exceptionally bad idea".
> 
> By the way, I remember that during the KVM forum a similar
> approach of having a single data plane thread for many devices
> was discussed....
> > We've seen very positive results from adding threads.  We should also
> > look at scheduling.
> 
> ...and we have also seen exceptionally negative results from
> adding threads, both for vhost and data-plane. If you have lot of idle
> time/cores
> then it makes sense to run multiple threads. But IMHO in many scenarios you
> don't have lot of idle time/cores.. and if you have them you would probably
> prefer to run more VMs/VCPUs....hosting a single SMP VM when you have
> enough physical cores to run all the VCPU threads and the I/O threads is
> not a
> realistic scenario.
> 
> That's why we are proposing to implement a mechanism that will enable
> the management stack to configure 1 thread per I/O device (as it is today)
> or 1 thread for many I/O devices (belonging to the same VM).
> 
> > Once you are scheduling multiple guests in a single vhost device, you
> > now create a whole new class of DoS attacks in the best case scenario.
> 
> Again, we are NOT proposing to schedule multiple guests in a single
> vhost thread. We are proposing to schedule multiple devices belonging
> to the same guest in a single (or multiple) vhost thread/s.
> 

I guess a question then becomes why have multiple devices?


> >
> > > 2. Sysfs mechanism to add and remove vhost threads
> > > This patch allows us to add and remove vhost threads dynamically.
> > >
> > > A simpler way to control the creation of vhost threads is statically
> > > determining the maximum number of virtio devices per worker via a
> kernel
> > > module parameter (which is the way the previously mentioned patch is
> > > currently implemented)
> > >
> > > I'd like to ask for advice here about the more preferable way to go:
> > > Although having the sysfs mechanism provides more flexibility, it may
> be a
> > > good idea to start with a simple static parameter, and have the first
> > > patches as simple as possible. What do you think?
> > >
> > > 3.Add virtqueue polling mode to vhost
> > > Have the vhost thread poll the virtqueues with high I/O rate for new
> > > buffers , and avoid asking the guest to kick us.
> > > https://github.com/abelg/virtual_io_acceleration/commit/
> > 26616133fafb7855cc80fac070b0572fd1aaf5d0
> >
> > Ack on this.
> 
> :)
> 
> Regards,
> Abel.
> 
> >
> > Regards,
> >
> > Anthony Liguori
> >
> > > 4. vhost statistics
> > > This patch introduces a set of statistics to monitor different
> performance
> > > metrics of vhost and our polling and I/O scheduling mechanisms. The
> > > statistics are exposed using debugfs and can be easily displayed with a
> 
> > > Python script (vhost_stat, based on the old kvm_stats)
> > > https://github.com/abelg/virtual_io_acceleration/commit/
> > ac14206ea56939ecc3608dc5f978b86fa322e7b0
> > >
> > >
> > > 5. Add heuristics to improve I/O scheduling
> > > This patch enhances the round-robin mechanism with a set of heuristics
> to
> > > decide when to leave a virtqueue and proceed to the next.
> > > https://github.com/abelg/virtual_io_acceleration/commit/
> > f6a4f1a5d6b82dc754e8af8af327b8d0f043dc4d
> > >
> > > This patch improves the handling of the requests by the vhost thread,
> but
> > > could perhaps be delayed to a
> > > later time , and not submitted as one of the first Elvis patches.
> > > I'd love to hear some comments about whether this patch needs to be
> part
> > > of the first submission.
> > >
> > > Any other feedback on this plan will be appreciated,
> > > Thank you,
> > > Razya
> >