On Thu, Dec 19, 2013 at 12:13 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > On Thu, Dec 19, 2013 at 08:40:44AM +0200, Abel Gordon wrote: >> On Wed, Dec 18, 2013 at 12:43 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: >> > On Tue, Dec 17, 2013 at 12:04:42PM +0200, Razya Ladelsky wrote: >> >> Hi, >> >> >> >> Thank you all for your comments. >> >> I'm sorry for taking this long to reply, I was away on vacation.. >> >> >> >> It was a good, long discussion, many issues were raised, which we'd like >> >> to address with the following proposed roadmap for Elvis patches. >> >> In general, we believe it would be best to start with patches that are >> >> as simple as possible, providing the basic Elvis functionality, >> >> and attend to the more complicated issues in subsequent patches. >> >> >> >> Here's the road map for Elvis patches: >> > >> > Thanks for the follow up. Some suggestions below. >> > Please note they suggestions below merely represent >> > thoughts on merging upstream. >> > If as the first step you are content with keeping this >> > work as out of tree patches, in order to have >> > the freedom to experiment with interfaces and >> > performance, please feel free to ignore them. >> > >> >> 1. Shared vhost thread for multiple devices. >> >> >> >> The way to go here, we believe, is to start with a patch having a shared >> >> vhost thread for multiple devices of the SAME vm. >> >> The next step/patch may be handling vms belonging to the same cgroup. >> >> >> >> Finally, we need to extend the functionality so that the shared vhost >> >> thread >> >> serves multiple vms (not necessarily belonging to the same cgroup). >> >> >> >> There was a lot of discussion about the way to address the enforcement >> >> of cgroup policies, and we will consider the various solutions with a >> >> future >> >> patch. >> > >> > With respect to the upstream kernel, >> > I'm not sure a bunch of changes just for the sake of guests with >> > multiple virtual NIC cards makes sense. >> > And I wonder how this step, in isolation, will affect e.g. >> > multiqueue workloads. >> > But I guess if the numbers are convincing, this can be mergeable. >> >> Even if you have a single multiqueue device this change allows >> to create one vhost thread for all the queues, one vhost thread per >> queue or any other combination. I guess that depending on the workload >> and depending on the system utilization (free cycles/cores, density) >> you would prefer >> to use one or more vhost threads. > > That is already controllable from the guest though, which likely has a better > idea about the workload. but the guest has no idea about what's going on in the host system (e.g. other VMs I/O, cpu utilization of the host cores...) > >> > >> >> >> >> 2. Creation of vhost threads >> >> >> >> We suggested two ways of controlling the creation and removal of vhost >> >> threads: >> >> - statically determining the maximum number of virtio devices per worker >> >> via a kernel module parameter >> >> - dynamically: Sysfs mechanism to add and remove vhost threads >> >> >> >> It seems that it would be simplest to take the static approach as >> >> a first stage. At a second stage (next patch), we'll advance to >> >> dynamically >> >> changing the number of vhost threads, using the static module parameter >> >> only as a default value. >> > >> > I'm not sure how independent this is from 1. >> > With respect to the upstream kernel, >> > Introducing interfaces (which we'll have to maintain >> > forever) just for the sake of guests with >> > multiple virtual NIC cards does not look like a good tradeoff. >> > >> > So I'm unlikely to merge this upstream without making it useful cross-VM, >> > and yes this means isolation and accounting with cgroups need to >> > work properly. >> >> Agree, but even if you use a single multiqueue device having the >> ability to use 1 thread to serve all the queues or multiple threads to >> serve all the queues looks like a useful feature. > > Could be. At the moment, multiqueue is off by default because it causes > regressions for some workloads as compared to a single queue. > If we have heuristics in vhost that fix this by auto-tuning threading, that > would be nice. But if you need to tune it manually anyway, > then from upstream perspective it does not seem to be worth it - you can just > turn multiqueue on/off in the guest. I see. But we are mixing again between the policy and the mechanism. We first need a mechanism to control the system and then we need to implement the policy to orchestrate it (whenever it will be implemented in the kernel as part of vhost or outside in user-space). I don't see why to wait to have a policy to upstream the mechanism. If we upstream the mechanism in a manner that the defaults do not affect today's vhost behavior, then it will be possible to play with the policies and upstream them later. > > >> > >> >> Regarding cwmq, it is an interesting mechanism, which we need to explore >> >> further. >> >> At the moment we prefer not to change the vhost model to use cwmq, as some >> >> of the issues that were discussed, such as cgroups, are not supported by >> >> cwmq, and this is adding more complexity. >> >> However, we'll look further into it, and consider it at a later stage. >> > >> > Hmm that's still assuming some smart management tool configuring >> > this correctly. Can't this be determined automatically depending >> > on the workload? >> > This is what the cwmq suggestion was really about: detect >> > that we need more threads and spawn them. >> > It's less about sharing the implementation with workqueues - >> > would be very nice but not a must. >> >> But how cwmq can consider cgroup accounting ? > > I think cwmq is just a replacement for our own thread pool. > It doesn't make cgroup accounting easier or harder. cwmq doesn't solve the cgroup issue, that's the problem. IMHO, it will be much simpler consider cgrouups with our own threading model. > >> In any case, IMHO, the kernel should first provide the "mechanism" so >> later on a user-space management application (the "policy") can >> orchestrate it. > > I think policy would be something coarse-grained, like setting priority. > Making detailed scheduling decisions in userspace seems wrong somehow: > what does management application know that kernel doesn't? The code of the management application can be changed/modified/customized... also the management application can consider additional inputs (like per VM SLAs) that are not available to the kernel. Think about MOM (memory over-commit manager) but for vhost (vhost I/O manager ?VHIOM ?) > > >> > >> > >> > >> >> 3. Adding polling mode to vhost >> >> >> >> It is a good idea making polling adaptive based on various factors such as >> >> the I/O rate, the guest kick overhead(which is the tradeoff of polling), >> >> or the amount of wasted cycles (cycles we kept polling but no new work was >> >> added). >> >> However, as a beginning polling patch, we would prefer having a naive >> >> polling approach, which could be tuned with later patches. >> >> >> > >> > While any polling approach would still need a lot of testing to prove we >> > don't for example steal CPU from guest which could be doing other useful >> > work, given that an exit is at least 1.5K cycles at least in theory it >> > seems like something that can improve performance. I'm not sure how >> > naive we can be without introducing regressions for some workloads. >> > For example, if we are on the same host CPU, there's no >> > chance busy waiting will help us make progress. >> > How about detecting that the VCPU thread that kicked us >> > is currently running on another CPU, and only polling in >> > this case? >> > >> >> 4. vhost statistics >> >> >> >> The issue that was raised for the vhost statistics was using ftrace >> >> instead of the debugfs mechanism. >> >> However, looking further into the kvm stat mechanism, we learned that >> >> ftrace didn't replace the plain debugfs mechanism, but was used in >> >> addition to it. >> >> >> >> We propose to continue using debugfs for statistics, in a manner similar >> >> to kvm, >> >> and at some point in the future ftrace can be added to vhost as well. >> > >> > IMHO which kvm stat is a useful script, the best tool >> > for perf stats is still perf. So I would try to integrate with that. >> > How it works internally is IMHO less important. >> > >> >> Does this plan look o.k.? >> >> If there are no further comments, I'll start preparing the patches >> >> according to what we've agreed on thus far. >> >> Thank you, >> >> Razya >> > >> > I think a good place to try to start merging upstream would be 3 and 4. >> > So if you want to make it easier to merge things upstream, try to keep 3 >> > and 4 independent from 1 and 2. >> >> Note -1- and -3- are strongly related. If you have a thread that >> serves multiple queues (whenever they belong to the same device/vm or >> not) then this thread will be polling multiple queues at the same >> time. This increases the chances you will find pending work to do in >> some queue. In other words, you reduce the cycles wasted for polling. >> In the other hand, if you run multiple threads and these threads do >> polling simultaneously then the threads may starve each other and >> reduce performance (if they are scheduled to run in the same core). In >> addition, a shared thread can decide when it should stop processing a >> given queue and switch to other queue because by polling the thread >> knows when new requests were added to a queue (this is what we called >> fine-grained I/O scheduled heuristics) >> >> So, seems like polling makes more sense when you serve multiple queues >> with the same thread. >> >> Abel. > > A combination might bring gains in more workloads, but it should work on its > own too. It's quite possible that only a single VM is active, others are > idle. So either polling should handle that well or be smart enough to turn > itself off in this case. Agree, but we are discussing again about the policy. We first need the mechanism :) Abel. > >> > >> > Thanks again, >> > >> > -- >> > MST -- 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
