Date:	Thu, 05 Mar 2009 17:24:04 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Jens Låås <jelaas@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v1] hand off skb list to other cpu to submit to upper
	layer

On Thu, 2009-03-05 at 08:32 +0100, Jens Låås wrote:
> 2009/3/5, Zhang, Yanmin <yanmin_zhang@...ux.intel.com>:
> > On Thu, 2009-03-05 at 09:04 +0800, Zhang, Yanmin wrote:
> >  > On Wed, 2009-03-04 at 01:39 -0800, David Miller wrote:
> >  > > From: "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
> >  > > Date: Wed, 04 Mar 2009 17:27:48 +0800
> >  > >
> >  > > > Both the new skb_record_rx_queue and current kernel have an
> >  > > > assumption on multi-queue. The assumption is it's best to send out
> >  > > > packets from the TX of the same number of queue like the one of RX
> >  > > > if the receved packets are related to the out packets. Or more
> >  > > > direct speaking is we need send packets on the same cpu on which we
> >  > > > receive them. The start point is that could reduce skb and data
> >  > > > cache miss.
> >  > >
> >  > > We have to use the same TX queue for all packets for the same
> >  > > connection flow (same src/dst IP address and ports) otherwise
> >  > > we introduce reordering.
> >  > > Herbert brought this up, now I have explicitly brought this up,
> >  > > and you cannot ignore this issue.
> >  > Thanks. Stephen Hemminger brought it up and explained what reorder
> >  > is. I answered in a reply (sorry for not clear) that mostly we need spread
> >  > packets among RX/TX in a 1:1 mapping or N:1 mapping. For example, all packets
> >  > received from RX 8 will be spreaded to TX 0 always.
> >
> > To make it clearer, I used 1:1 mapping binding when running testing
> >  on bensley (4*2 cores) and Nehalem (2*4*2 logical cpu). So there is no reorder
> >  issue. I also worked out a new patch on the failover path to just drop
> >  packets when qlen is bigger than netdev_max_backlog, so the failover path wouldn't
> >  cause reorder.
> >
> 

> We have not seen this problem in our 
Thanks for your valuable input. We need more data on high-speed NIC.

> We do keep the skb processing with the same CPU from RX to TX.
That's a normal point. I did so when I began to investigate why forward
speed is far slower than sending speed with 10G NIC.

> This is done via setting affinity for queues and using custom select_queue.
> 
> +static u16 select_queue(struct net_device *dev, struct sk_buff *skb)
> +{
> +       if( dev->real_num_tx_queues && skb_rx_queue_recorded(skb) )
> +               return  skb_get_rx_queue(skb) % dev->real_num_tx_queues;
> +
> +       return  smp_processor_id() %  dev->real_num_tx_queues;
> +}
> +
Yes, with the function and every NIC has CPU_NUM queues, skb is processed
with the same cpu from RX to TX.

> 
> The hash based default for selecting TX-queue generates an uneven
> spread that is hard to follow with correct affinity.
> 
> We have not been able to generate quite as much traffic from the sender.
pktgen of the latest kernel supports multi-thread on the same device. If you
just starts one thread, the speed is limited. Could you try 4 or 8 threads? Perhaps
speed could double then.

> 
> Sender: (64 byte pkts)
> eth5            4.5 k bit/s        3   pps   1233.9 M bit/s    2.632 M pps
I'm a little confused with the data. Do the first 2 mean IN and last 2 mean OUT?

What kind of NIC and machines are they? How big is the last level cache of the cpu?

> 
> Router:
> eth0         1077.2 M bit/s    2.298 M pps      1.7 k bit/s        1   pps
> eth1            744   bit/s        1   pps   1076.3 M bit/s    2.296 M pps
The forward speed is quite close to the sending speed of the Sender. It seems
your machine needn't my patch.

My original case is the sending speed is 1.4M pps with careful cpu binding considering
cpu cache sharing. With my patch, the result becomes 2M pps and the sending speed is
2.36M pps. The NICs I am using are not latest.

> 
> Im not sure I like the proposed concept since it decouples RX
> processing from receiving.
> There is no point collecting lots of packets just to drop them later
> in the qdisc.
> Infact this is bad for performance, we just consume cpu for nothing.
Yes, if the skb processing cpu is very busy, and we choose to drop skb there instead of
by driver or NIC hardware, performance might be worse.

A small change on my patch and driver could reduce that possibility. Checking qlen before
collecting the 64 packets (assume driver collects 64 packets per NAPI loop). If qlen is
larger than netdev_max_backlog, driver could just return without real collection.

We need data to distinguish good or bad.

> It is important to have as strong correlation as possible between RX
> and TX so we dont receive more pkts than we can handle. Better to drop
> on the interface.
With my above small change, interface would drop packets.

> 
> We might start thinking of a way for userland to set the policy for
> multiq mapping.
I also think so.

I did more testing with different slab allocator as slab has big impact on
performance. SLQB has very different behavior from SLUB. It seems SLQB (try2) need
improve NUMA allocation/free. At least I use slub_min_objects=64 and slub_max_order=6
to get the best result on my machine.

Thanks for your comments.

> >  > >
> >  > > You must not knowingly reorder packets, and using different TX
> >  > > queues for packets within the same flow does that.
> >  > Thanks for you rexplanation which is really consistent with Stephen's speaking.
> >


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