Re: [RFC PATCH 0/4] Shared vhost design
From: Bandan Das
Date: Mon Aug 10 2015 - 16:00:51 EST
"Michael S. Tsirkin" <mst@xxxxxxxxxx> writes:
> On Sat, Aug 08, 2015 at 07:06:38PM -0400, Bandan Das wrote:
>> Hi Michael,
>> > - 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.
Ah ok, I understand now. I am pretty sure the current RFC doesn't
take care of this :) I will add this to my todo list for v2.
>> >> #* 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
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