Re: Elvis upstreaming plan

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

On Wed, Nov 27, 2013 at 11:03:57AM +0200, Abel Gordon wrote:
> 
> 
> "Michael S. Tsirkin" <mst@redhat.com> wrote on 26/11/2013 11:11:57 PM:
> 
> > 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?
> 
> I assume that there are guests that have multiple vhost devices
> (net or scsi/tcm).

These are kind of uncommon though.  In fact a kernel thread is not a
unit of isolation - cgroups supply isolation.
If we had use_cgroups kind of like use_mm, we could thinkably
do work for multiple VMs on the same thread.


> We can also extend the approach to consider
> multiqueue devices, so we can create 1 vhost thread shared for all the
> queues,
> 1 vhost thread for each queue or a few threads for multiple queues. We
> could also share a thread across multiple queues even if they do not belong
> to the same device.
> 
> Remember the experiments Shirley Ma did with the split
> tx/rx ? If we have a control interface we could support both
> approaches: different threads or a single thread.


I'm a bit concerned about interface managing specific
threads being so low level.
What exactly is it that management knows that makes it
efficient to group threads together?
That host is over-committed so we should use less CPU?
I'd like the interface to express that knowledge.


> >
> >
> > > >
> > > > > 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
> > > >
> >