netdev - Re: [net-next RFC V5 4/5] virtio

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Date:	Mon, 30 Jul 2012 11:29:43 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	"Michael S. Tsirkin" <mst@...hat.com>, mashirle@...ibm.com,
	krkumar2@...ibm.com, habanero@...ux.vnet.ibm.com,
	rusty@...tcorp.com.au, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, edumazet@...gle.com,
	tahm@...ux.vnet.ibm.com, jwhan@...ewood.snu.ac.kr,
	davem@...emloft.net, akong@...hat.com, kvm@...r.kernel.org,
	sri@...ibm.com
Subject: Re: [net-next RFC V5 4/5] virtio_net: multiqueue support

On 07/23/2012 05:28 PM, Sasha Levin wrote:
> On 07/23/2012 07:54 AM, Jason Wang wrote:
>> On 07/21/2012 08:02 PM, Sasha Levin wrote:
>>> On 07/20/2012 03:40 PM, Michael S. Tsirkin wrote:
>>>>> -    err = init_vqs(vi);
>>>>>> +    if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
>>>>>> +        vi->has_cvq = true;
>>>>>> +
>>>> How about we disable multiqueue if there's no cvq?
>>>> Will make logic a bit simpler, won't it?
>>> multiqueues don't really depend on cvq. Does this added complexity really justifies adding an artificial limit?
>>>
>> Yes, it does not depends on cvq. Cvq were just used to negotiate the number of queues a guest wishes to use which is really useful (at least for now). Since multiqueue can not out-perform for single queue in every kinds of workloads or benchmark, so we want to let guest driver use single queue by default even when multiqueue were enabled by management software and let use to enalbe it through ethtool. So user could not feel regression when it switch to use a multiqueue capable driver and backend.
> Why would you limit it to a single vq if the user has specified a different number of vqs (>1) in the virtio-net device config?
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The only reason is to prevent the user from seeing the regression. The 
performance of small packet sending is wrose than single queue, it tends 
to send more but small packets when multiqueue is enabled. If we make 
multiqueue bahave as good as single queue, we can remove this limit.

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