netdev - Re: [RFC] kvm tools: Implement multiple VQ for virtio-net

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Date:	Tue, 15 Nov 2011 17:30:29 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	Asias He <asias.hejun@...il.com>, gorcunov@...il.com,
	kvm@...r.kernel.org, mingo@...e.hu,
	"Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
	penberg@...nel.org, Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC] kvm tools: Implement multiple VQ for virtio-net

On Tue, 2011-11-15 at 10:14 +0530, Krishna Kumar2 wrote:
> Sasha Levin <levinsasha928@...il.com> wrote on 11/14/2011 03:45:40 PM:
> 
> > > Why both the bandwidth and latency performance are dropping so
> > > dramatically with multiple VQ?
> >
> > It looks like theres no hash sync between host and guest, which makes
> > the RX VQ change for every packet. This is my guess.
> 
> Yes, I confirmed this happens for macvtap. I am
> using ixgbe - it calls skb_record_rx_queue when
> a skb is allocated, but sets rxhash when a packet
> arrives. Macvtap is relying on record_rx_queue
> first ahead of rxhash (as part of my patch making
> macvtap multiqueue), hence different skbs result
> in macvtap selecting different vq's.

I'm seeing this behavior in non-macvtep related setup as well (simple
tap <-> virtio-net).

-- 

Sasha.

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