netdev - Re: [PATCH] macvtap: Fix macvtap_get

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Date:	Fri, 25 Nov 2011 08:28:16 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	jasowang <jasowang@...hat.com>
Cc:	arnd@...db.de, davem@...emloft.net, jeffrey.t.kirsher@...el.com,
	levinsasha928@...il.com, "Michael S. Tsirkin" <mst@...hat.com>,
	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] macvtap: Fix macvtap_get_queue to use rxhash first

jasowang <jasowang@...hat.com> wrote on 11/24/2011 06:30:52 PM:

>
> >> On Thu, Nov 24, 2011 at 01:47:14PM +0530, Krishna Kumar wrote:
> >>> It was reported that the macvtap device selects a
> >>> different vhost (when used with multiqueue feature)
> >>> for incoming packets of a single connection. Use
> >>> packet hash first. Patch tested on MQ virtio_net.
> >> So this is sure to address the problem, why exactly does this happen?
> >> Does your device spread a single flow across multiple RX queues? Would
> >> not that cause trouble in the TCP layer?
> >> It would seem that using the recorded queue should be faster with
> >> less cache misses. Before we give up on that, I'd
> >> like to understand why it's wrong. Do you know?
> > I am using ixgbe. From what I briefly saw, ixgbe_alloc_rx_buffers
> > calls skb_record_rx_queue when a skb is allocated. When a packet
> > is received (ixgbe_alloc_rx_buffers), it sets rxhash. The
> > recorded value is different for most skbs when I ran a single
> > stream TCP stream test (does skbs move between the rx_rings?).
>
> Yes, it moves. It depends on last processor or tx queue who transmits
> the packets of this stream. Because ixgbe select tx queue based on the
> processor id, so if vhost thread transmits skbs on different processors,
> the skb of a single stream may comes from different rx ring.

But I don't see transmit going on different queues,
only incoming.

- KK

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