Hi Michael, "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > On Mon, Jul 13, 2015 at 12:07:31AM -0400, Bandan Das wrote: >> Hello, >> >> There have been discussions on improving the current vhost design. The first >> attempt, to my knowledge was Shirley Ma's patch to create a dedicated vhost >> worker per cgroup. >> >> http://comments.gmane.org/gmane.linux.network/224730 >> >> Later, I posted a cmwq based approach for performance comparisions >> http://comments.gmane.org/gmane.linux.network/286858 >> >> More recently was the Elvis work that was presented in KVM Forum 2013 >> http://www.linux-kvm.org/images/a/a3/Kvm-forum-2013-elvis.pdf >> >> The Elvis patches rely on common vhost thread design for scalability >> along with polling for performance. Since there are two major changes >> being proposed, we decided to split up the work. The first (this RFC), >> proposing a re-design of the vhost threading model and the second part >> (not posted yet) to focus more on improving performance. >> >> I am posting this with the hope that we can have a meaningful discussion >> on the proposed new architecture. We have run some tests to show that the new >> design is scalable and in terms of performance, is comparable to the current >> stable design. >> >> Test Setup: >> The testing is based on the setup described in the Elvis proposal. >> The initial tests are just an aggregate of Netperf STREAM and MAERTS but >> as we progress, I am happy to run more tests. The hosts are two identical >> 16 core Haswell systems with point to point network links. For the first 10 runs, >> with n=1 upto n=10 guests running in parallel, I booted the target system with nr_cpus=8 >> and mem=12G. The purpose was to do a comparision of resource utilization >> and how it affects performance. Finally, with the number of guests set at 14, >> I didn't limit the number of CPUs booted on the host or limit memory seen by >> the kernel but boot the kernel with isolcpus=14,15 that will be used to run >> the vhost threads. The guests are pinned to cpus 0-13 and based on which >> cpu the guest is running on, the corresponding I/O thread is either pinned >> to cpu 14 or 15. >> Results >> # X axis is number of guests >> # Y axis is netperf number >> # nr_cpus=8 and mem=12G >> #Number of Guests #Baseline #ELVIS >> 1 1119.3 1111.0 >> 2 1135.6 1130.2 >> 3 1135.5 1131.6 >> 4 1136.0 1127.1 >> 5 1118.6 1129.3 >> 6 1123.4 1129.8 >> 7 1128.7 1135.4 >> 8 1129.9 1137.5 >> 9 1130.6 1135.1 >> 10 1129.3 1138.9 >> 14* 1173.8 1216.9 > > I'm a bit too busy now, with 2.4 and related stuff, will review once we > finish 2.4. But I'd like to ask two things: > - did you actually test a config where cgroups were used? Here are some numbers with a simple cgroup setup. Three cgroups with cpusets cpu=0,2,4 for cgroup1, cpu=1,3,5 for cgroup2 and cpu=6,7 for cgroup3 (even though 6,7 have different numa nodes) I run netperf for 1 to 9 guests starting with assigning the first guest to cgroup1, second to cgroup2, third to cgroup3 and repeat this sequence upto 9 guests. The numbers - (TCP_STREAM + TCP_MAERTS)/2 #Number of Guests #ELVIS (Mbps) 1 1056.9 2 1122.5 3 1122.8 4 1123.2 5 1122.6 6 1110.3 7 1116.3 8 1121.8 9 1118.5 Maybe, my cgroup setup was too simple but these numbers are comparable to the no cgroups results above. I wrote some tracing code to trace cgroup_match_groups() and find cgroup search overhead but it seemed unnecessary for this particular test. > - does the design address the issue of VM 1 being blocked > (e.g. because it hits swap) and blocking VM 2? Good question. I haven't thought of this yet. But IIUC, the worker thread will complete VM1's job and then move on to executing VM2's scheduled work. It doesn't matter if VM1 is blocked currently. I think it would be a problem though if/when polling is introduced. >> >> #* Last run with the vCPU and I/O thread(s) pinned, no CPU/memory limit imposed. >> # I/O thread runs on CPU 14 or 15 depending on which guest it's serving >> >> There's a simple graph at >> http://people.redhat.com/~bdas/elvis/data/results.png >> that shows how task affinity results in a jump and even without it, >> as the number of guests increase, the shared vhost design performs >> slightly better. >> >> Observations: >> 1. In terms of "stock" performance, the results are comparable. >> 2. However, with a tuned setup, even without polling, we see an improvement >> with the new design. >> 3. Making the new design simulate old behavior would be a matter of setting >> the number of guests per vhost threads to 1. >> 4. Maybe, setting a per guest limit on the work being done by a specific vhost >> thread is needed for it to be fair. >> 5. cgroup associations needs to be figured out. I just slightly hacked the >> current cgroup association mechanism to work with the new model. Ccing cgroups >> for input/comments. >> >> Many thanks to Razya Ladelsky and Eyal Moscovici, IBM for the initial >> patches, the helpful testing suggestions and discussions. >> >> Bandan Das (4): >> vhost: Introduce a universal thread to serve all users >> vhost: Limit the number of devices served by a single worker thread >> cgroup: Introduce a function to compare cgroups >> vhost: Add cgroup-aware creation of worker threads >> >> drivers/vhost/net.c | 6 +- >> drivers/vhost/scsi.c | 18 ++-- >> drivers/vhost/vhost.c | 272 +++++++++++++++++++++++++++++++++++-------------- >> drivers/vhost/vhost.h | 32 +++++- >> include/linux/cgroup.h | 1 + >> kernel/cgroup.c | 40 ++++++++ >> 6 files changed, 275 insertions(+), 94 deletions(-) >> >> -- >> 2.4.3 -- 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
