Abel Gordon <ABELG@xxxxxxxxxx> writes: > "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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" <mst@xxxxxxxxxx> wrote on 26/11/2013 11:11:57 PM: >> > >> > > From: "Michael S. Tsirkin" <mst@xxxxxxxxxx> >> > > To: Abel Gordon/Haifa/IBM@IBMIL, >> > > Cc: Anthony Liguori <anthony@xxxxxxxxxxxxx>, abel.gordon@xxxxxxxxx, >> > > asias@xxxxxxxxxx, digitaleric@xxxxxxxxxx, Eran Raichstein/Haifa/ >> > > IBM@IBMIL, gleb@xxxxxxxxxx, jasowang@xxxxxxxxxx, Joel Nider/Haifa/ >> > > IBM@IBMIL, kvm@xxxxxxxxxxxxxxx, pbonzini@xxxxxxxxxx, 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 <anthony@xxxxxxxxxxxxx> wrote on 26/11/2013 > 08:05:00 >> > PM: >> > > > >> > > > > >> > > > > Razya Ladelsky <RAZYA@xxxxxxxxxx> 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". Paul Turner has a proposal for exactly this: http://www.linuxplumbersconf.org/2013/ocw/sessions/1653 The video is up on Youtube I think. It definitely is a general problem that is not at all virtual I/O specific. Regards, Anthony Liguori > >> >> > > >> > > >> > > >> > Phone: 972-4-829-6326 | Mobile: 972-54-3155635 (Embedded >> image moved to file: >> > E-mail: JOELN@xxxxxxxxxx >> 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 majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
