"Michael S. Tsirkin" <m...@redhat.com> wrote on 27/11/2013 12:27:19 PM:
> > On Wed, Nov 27, 2013 at 09:43:33AM +0200, Joel Nider wrote: > > Hi, > > > > Razya is out for a few days, so I will try to answer the questions as well > > as I can: > > > > "Michael S. Tsirkin" <m...@redhat.com> wrote on 26/11/2013 11:11:57 PM: > > > > > From: "Michael S. Tsirkin" <m...@redhat.com> > > > To: Abel Gordon/Haifa/IBM@IBMIL, > > > Cc: Anthony Liguori <anth...@codemonkey.ws>, abel.gor...@gmail.com, > > > as...@redhat.com, digitale...@google.com, Eran Raichstein/Haifa/ > > > IBM@IBMIL, g...@redhat.com, jasow...@redhat.com, Joel Nider/Haifa/ > > > IBM@IBMIL, kvm@vger.kernel.org, pbonz...@redhat.com, Razya Ladelsky/ > > > Haifa/IBM@IBMIL > > > Date: 27/11/2013 01:08 AM > > > Subject: Re: Elvis upstreaming plan > > > > > > On Tue, Nov 26, 2013 at 08:53:47PM +0200, Abel Gordon wrote: > > > > > > > > > > > > Anthony Liguori <anth...@codemonkey.ws> wrote on 26/11/2013 08:05:00 > > PM: > > > > > > > > > > > > > > Razya Ladelsky <ra...@il.ibm.com> writes: > > > > > > > <edit> > > > > > > > > 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? > > > > If you mean "why serve multiple devices from a single thread" the answer is > > that we cannot rely on the Linux scheduler which has no knowledge of I/O > > queues to do a decent job of scheduling I/O. The idea is to take over the > > I/O scheduling responsibilities from the kernel's thread scheduler with a > > more efficient I/O scheduler inside each vhost thread. So by combining all > > of the I/O devices from the same guest (disks, network cards, etc) in a > > single I/O thread, it allows us to provide better scheduling by giving us > > more knowledge of the nature of the work. So now instead of relying on the > > linux scheduler to perform context switches between multiple vhost threads, > > we have a single thread context in which we can do the I/O scheduling more > > efficiently. We can closely monitor the performance needs of each queue of > > each device inside the vhost thread which gives us much more information > > than relying on the kernel's thread scheduler. > > This does not expose any additional opportunities for attacks (DoS or > > other) than are already available since all of the I/O traffic belongs to a > > single guest. > > You can make the argument that with low I/O loads this mechanism may not > > make much difference. However when you try to maximize the utilization of > > your hardware (such as in a commercial scenario) this technique can gain > > you a large benefit. > > > > Regards, > > > > Joel Nider > > Virtualization Research > > IBM Research and Development > > Haifa Research Lab > > So all this would sound more convincing if we had sharing between VMs. > When it's only a single VM it's somehow less convincing, isn't it? > Of course if we would bypass a scheduler like this it becomes harder to > enforce cgroup limits. True, but here the issue becomes isolation/cgroups. We can start to show the value for VMs that have multiple devices / queues and then we could re-consider extending the mechanism for multiple VMs (at least as a experimental feature). > But it might be easier to give scheduler the info it needs to do what we > need. Would an API that basically says "run this kthread right now" > do the trick? ...do you really believe it would be possible to push this kind of change to the Linux scheduler ? In addition, we need more than "run this kthread right now" because you need to monitor the virtio ring activity to specify "when" you will like to run a "specific kthread" and for "how long". > > > > > > > > > Phone: 972-4-829-6326 | Mobile: 972-54-3155635 (Embedded > image moved to file: > > E-mail: jo...@il.ibm.com > pic39571.gif)IBM > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > 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 > > > > > > > > > > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
