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SubjectRe: [PATCH net-next] virtio-net: enable multiqueue by default
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From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Fri, 25 Nov 2016 06:43:08 +0200

> On Fri, Nov 25, 2016 at 12:37:26PM +0800, Jason Wang wrote:
>> We use single queue even if multiqueue is enabled and let admin to
>> enable it through ethtool later. This is used to avoid possible
>> regression (small packet TCP stream transmission). But looks like an
>> overkill since:
>>
>> - single queue user can disable multiqueue when launching qemu
>> - brings extra troubles for the management since it needs extra admin
>> tool in guest to enable multiqueue
>> - multiqueue performs much better than single queue in most of the
>> cases
>>
>> So this patch enables multiqueue by default: if #queues is less than or
>> equal to #vcpu, enable as much as queue pairs; if #queues is greater
>> than #vcpu, enable #vcpu queue pairs.
>>
>> Cc: Hannes Frederic Sowa <hannes@redhat.com>
>> Cc: Michael S. Tsirkin <mst@redhat.com>
>> Cc: Neil Horman <nhorman@redhat.com>
>> Cc: Jeremy Eder <jeder@redhat.com>
>> Cc: Marko Myllynen <myllynen@redhat.com>
>> Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> OK at some level but all uses of num_online_cpus()
> like this are racy versus hotplug.
> I know we already have this bug but shouldn't we fix it
> before we add more?

This is more being used like a heuristic in this scenerio, and in
fact I would say one would keep the code this way even once proper
hotplug handlers are installed to adjust the queued dynamically if
there is a desired (which is also not necessarily the case).

I really don't think this change should be held on up on this issue.
So can we please make some forward progress here?

Thanks.

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