netdev - Re: [PATCH] macvtap: Fix macvtap_get

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Date:	Fri, 25 Nov 2011 08:37:16 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	arnd@...db.de, davem@...emloft.net, jasowang <jasowang@...hat.com>,
	levinsasha928@...il.com, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] macvtap: Fix macvtap_get_queue to use rxhash first

"Michael S. Tsirkin" <mst@...hat.com> wrote on 11/24/2011 09:44:31 PM:

> > As far as I can see, ixgbe binds queues to physical cpu, so let
consider:
> >
> > vhost thread transmits packets of flow A on processor M
> > during packet transmission, ixgbe driver programs the card to
> > deliver the packet of flow A to queue/cpu M through flow director
> > (see ixgbe_atr())
> > vhost thread then receives packet of flow A with from M
> > ...
> > vhost thread transmits packets of flow A on processor N
> > ixgbe driver programs the flow director to change the delivery of
> > flow A to queue N ( cpu N )
> > vhost thread then receives packet of flow A with from N
> > ...
> >
> > So, for a single flow A, we may get different queue mappings. Using
> > rxhash instead may solve this issue.
>
> Or better, transmit a single flow from a single vhost thread.
>
> If packets of a single flow get spread over different CPUs,
> they will get reordered and things are not going to work well.

My testing so far shows that guest sends on (e.g.) TXQ#2
only, which is handled by vhost#2; and this doesn't change
for the entire duration of the test. Incoming keeps
changing for different packets but become same with
this patch. To iterate, I have not seen the following:

"
vhost thread transmits packets of flow A on processor M
...
vhost thread transmits packets of flow A on processor N
"

- KK

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