Re: [RFC PATCH 0/4] Shared vhost design

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Aug 09, 2015 at 05:57:53PM +0300, Eyal Moscovici wrote:
> Eyal Moscovici
> HL-Cloud Infrastructure Solutions
> IBM Haifa Research Lab
> 
> "Michael S. Tsirkin" <mst@redhat.com> wrote on 08/09/2015 03:45:47 PM:
> 
> > From: "Michael S. Tsirkin" <mst@redhat.com>
> > To: Bandan Das <bsd@redhat.com>
> > Cc: kvm@vger.kernel.org, netdev@vger.kernel.org, linux-
> > kernel@vger.kernel.org, Eyal Moscovici/Haifa/IBM@IBMIL, Razya
> > Ladelsky/Haifa/IBM@IBMIL, cgroups@vger.kernel.org, jasowang@redhat.com
> > Date: 08/09/2015 03:46 PM
> > 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@redhat.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 twoidentical
> > > >> 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.
> >
> 
> We are not talking about correctness only about performance issues. In this
> case, if
> the VM is swapped out you are most likely in a state of memory pressure.
> Aren't the effects on performance of swapping in only the specific pages of the
> vrings is negligible as compared to the performance effects in a state of
> memory pressure?

VM1 is under pressure, but VM2 might not be.

> >
> >
> > >
> > > >>
> > > >> #* 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 slightlyhacked
> 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
> >