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Date: Sun, 9 Aug 2015 15:45:47 +0300 From: "Michael S. Tsirkin" <mst@...hat.com> To: Bandan Das <bsd@...hat.com> Cc: kvm@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Eyal Moscovici <EYALMO@...ibm.com>, Razya Ladelsky <RAZYA@...ibm.com>, cgroups@...r.kernel.org, jasowang@...hat.com Subject: Re: [RFC PATCH 0/4] Shared vhost design On Sat, Aug 08, 2015 at 07:06:38PM -0400, Bandan Das wrote: > Hi Michael, > > "Michael S. Tsirkin" <mst@...hat.com> 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. Sorry, I wasn't clear. If VM1's memory is in swap, attempts to access it might block the service thread, so it won't complete VM2's job. > > >> > >> #* 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 linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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