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

From: jason wang <[email-masked]>
Date: Wed, 16 Nov 2011 14:10:45 +0800

On 11/15/2011 12:44 PM, Krishna Kumar2 wrote:
> Sasha Levin <[email-masked]> 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.
>
> Reordering macvtap to use rxhash first results in
> all packets going to the same VQ. The code snippet
> is:
>
> {
> 	...
> 	if (!numvtaps)
>                 goto out;
>
> 	rxq = skb_get_rxhash(skb);
> 	if (rxq) {
> 		tap = rcu_dereference(vlan->taps[rxq % numvtaps]);
> 		if (tap)
> 			goto out;
> 	}
>
> 	if (likely(skb_rx_queue_recorded(skb))) {
> 		rxq = skb_get_rx_queue(skb);
>
> 		while (unlikely(rxq >= numvtaps))
> 			rxq -= numvtaps;
> 			tap = rcu_dereference(vlan->taps[rxq]);
> 			if (tap)
> 				goto out;
> 	}
> }
>
> I will submit a patch for macvtap separately. I am working
> towards the other issue pointed out - different vhost
> threads handling rx/tx of a single flow.
Hello Krishna:

Have any thought in mind to solve the issue of flow handling?

Maybe some performance numbers first is better, it would let us know
where we are. During the test of my patchset, I find big regression of
small packet transmission, and more retransmissions were noticed. This
maybe also the issue of flow affinity. One interesting things is to see
whether this happens in your patches :)

I've played with a basic flow director implementation based on my series
which want to make sure the packets of a flow was handled by the same
vhost thread/guest vcpu. This is done by:

- bind virtqueue to guest cpu
- record the hash to queue mapping when guest sending packets and use
this mapping to choose the virtqueue when forwarding packets to guest

Test shows some help during for receiving packets from external host and
packet sending to local host. But it would hurt the performance of
sending packets to remote host. This is not the perfect solution as it
can not handle guest moving processes among vcpus, I plan to try
accelerate RFS and sharing the mapping between host and guest.

Anyway this is just for receiving, the small packet sending need more
thoughts.

Thanks

>
> thanks,
>
> - KK
>
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