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Re: [PATCH 1/1] virtio-blk-ccw: tweak the default for num_queues

From: Michael Mueller
Subject: Re: [PATCH 1/1] virtio-blk-ccw: tweak the default for num_queues
Date: Tue, 10 Nov 2020 15:16:49 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.12.1

On 09.11.20 19:53, Halil Pasic wrote:
On Mon, 9 Nov 2020 17:06:16 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

@@ -20,6 +21,11 @@ static void virtio_ccw_blk_realize(VirtioCcwDevice *ccw_dev, 
Error **errp)
      VirtIOBlkCcw *dev = VIRTIO_BLK_CCW(ccw_dev);
      DeviceState *vdev = DEVICE(&dev->vdev);
+    VirtIOBlkConf *conf = &dev->vdev.conf;
+    if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
+        conf->num_queues = MIN(4, current_machine->smp.cpus);
+    }

I would like to have a comment explaining the numbers here, however.

virtio-pci has a pretty good explanation (use 1:1 for vqs:vcpus if
possible, apply some other capping). 4 seems to be a bit arbitrary
without explanation, although I'm sure you did some measurements :)

Frankly, I don't have any measurements yet. For the secure case,
I think Mimu has assessed the impact of multiqueue, hence adding Mimu to
the cc list. @Mimu can you help us out.

Regarding the normal non-protected VMs I'm in a middle of producing some
measurement data. This was admittedly a bit rushed because of where we
are in the cycle. Sorry to disappoint you.

I'm talking with the perf team tomorrow. They have done some measurements with multiqueue for PV guests and I asked for a comparison to non PV guests as well.


The number 4 was suggested by Christian, maybe Christian does have some
readily available measurement data for the normal VM case. @Christian:
can you help me out?


Mit freundlichen Grüßen / Kind regards
Michael Müller

IBM Deutschland Research & Development GmbH
Vorsitzender des Aufsichtsrats: Gregor Pillen
Geschäftsführung: Dirk Wittkopp
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