On Wed, Nov 27, 2013 at 12:55:07PM +0200, Abel Gordon wrote: > > > "Michael S. Tsirkin" <m...@redhat.com> wrote on 27/11/2013 12:29:43 PM: > > > > > On Wed, Nov 27, 2013 at 11:49:03AM +0200, Abel Gordon wrote: > > > > > > > > > "Michael S. Tsirkin" <m...@redhat.com> wrote on 27/11/2013 11:21:00 AM: > > > > > > > > > > > On Wed, Nov 27, 2013 at 11:03:57AM +0200, Abel Gordon wrote: > > > > > > > > > > > > > > > "Michael S. Tsirkin" <m...@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 <anth...@codemonkey.ws> wrote on 26/11/2013 > > > 08:05:00 > > > > > PM: > > > > > > > > > > > > > > > > > > > > > > > Razya Ladelsky <ra...@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. > > > > > > > > > > We can expose information such as the amount of I/O being > > > handled for each queue, the amount of CPU cycles consumed for > > > processing the I/O, latency and more. > > > If we start with a simple mechanism that just enables the > > > feature we can later expose more information to implement a policy > > > framework that will be responsible for taking the decisions > > > (the orchestration part). > > > > What kind of possible policies do you envision? > > If we just react to load by balancing the work done, > > and when over-committed anyway, localize work so > > we get less IPIs, then this is not policy, this is the mechanism. > > (CCing Eyal Moscovici who is actually prototyping with multiple > policies and may want to join this thread) > > Starting with basic policies: we can use a single vhost thread > and create new vhost threads if it becomes saturated and there > are enough cpu cycles available in the system > or if the latency (how long the requests in the virtio queues wait > until they are handled) is too high. > We can merge threads if the latency is already low or if the threads > are not saturated. > > There is a hidden trade-off here: when you run more vhost threads you > may actually be stealing cpu cycles from the vcpu threads and also > increasing context switches. So, from the vhost perspective it may > improve performance but from the vcpu threads perspective it may > degrade performance.
So this is a very interesting problem to solve but what does management know that suggests it can solve it better? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 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
