netdev - Re: [net-next RFC PATCH 5/5] virtio-net: flow director support

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Date:	Tue, 6 Dec 2011 18:14:23 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sridhar Samudrala <sri@...ibm.com>
Cc:	Stefan Hajnoczi <stefanha@...il.com>,
	Jason Wang <jasowang@...hat.com>, krkumar2@...ibm.com,
	kvm@...r.kernel.org, netdev@...r.kernel.org, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org, levinsasha928@...il.com,
	bhutchings@...arflare.com
Subject: Re: [net-next RFC PATCH 5/5] virtio-net: flow director support

On Tue, Dec 06, 2011 at 07:42:54AM -0800, Sridhar Samudrala wrote:
> On 12/6/2011 5:15 AM, Stefan Hajnoczi wrote:
> >On Tue, Dec 6, 2011 at 10:21 AM, Jason Wang<jasowang@...hat.com>  wrote:
> >>On 12/06/2011 05:18 PM, Stefan Hajnoczi wrote:
> >>>On Tue, Dec 6, 2011 at 6:33 AM, Jason Wang<jasowang@...hat.com>    wrote:
> >>>>On 12/05/2011 06:55 PM, Stefan Hajnoczi wrote:
> >>>>>On Mon, Dec 5, 2011 at 8:59 AM, Jason Wang<jasowang@...hat.com>
> >>>>>  wrote:
> >>>The vcpus are just threads and may not be bound to physical CPUs, so
> >>>what is the big picture here?  Is the guest even in the position to
> >>>set the best queue mappings today?
> >>
> >>Not sure it could publish the best mapping but the idea is to make sure the
> >>packets of a flow were handled by the same guest vcpu and may be the same
> >>vhost thread in order to eliminate the packet reordering and lock
> >>contention. But this assumption does not take the bouncing of vhost or vcpu
> >>threads which would also affect the result.
> >Okay, this is why I'd like to know what the big picture here is.  What
> >solution are you proposing?  How are we going to have everything from
> >guest application, guest kernel, host threads, and host NIC driver
> >play along so we get the right steering up the entire stack.  I think
> >there needs to be an answer to that before changing virtio-net to add
> >any steering mechanism.
> >
> >
> Yes. Also the current model of  a vhost thread per VM's interface
> doesn't help with packet steering
> all the way from the guest to the host physical NIC.
> 
> I think we need to have vhost thread(s) per-CPU that can handle
> packets to/from physical NIC's
> TX/RX queues.
> Currently we have a single vhost thread for a VM's i/f
> that handles all the packets from
> various flows coming from a multi-queue physical NIC.
> 
> Thanks
> Sridhar

It's not hard to try that:
1. revert c23f3445e68e1db0e74099f264bc5ff5d55ebdeb
   this will convert our thread to a workqueue
2. convert the workqueue to a per-cpu one

It didn't work that well in the past, but YMMV

On the surface I'd say a single thread makes some sense
as long as guest uses a single queue.

-- 
MST
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