Re: [RFC PATCH 0/4] Shared vhost design
From: Bandan Das
Date: Sat Aug 08 2015 - 19:07:17 EST
"Michael S. Tsirkin" <mst@xxxxxxxxxx> writes:
> On Mon, Jul 13, 2015 at 12:07:31AM -0400, Bandan Das wrote:
>> 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.
>> Later, I posted a cmwq based approach for performance comparisions
>> More recently was the Elvis work that was presented in KVM Forum 2013
>> 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.
>> # 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)
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
>> 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.
>> 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(-)
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