netdev - Re: bnx2/BCM5709: why 5 interrupts on a 4 core system (2.6.33.3)

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Date:	Sun, 16 May 2010 23:26:20 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Krzysztof Olędzki <ole@....pl>
Cc:	Michael Chan <mchan@...adcom.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: bnx2/BCM5709: why 5 interrupts on a 4 core system (2.6.33.3)

Le dimanche 16 mai 2010 à 23:12 +0200, Krzysztof Olędzki a écrit :
> On 2010-05-16 22:47, Eric Dumazet wrote:
> > Le dimanche 16 mai 2010 à 22:34 +0200, Krzysztof Olędzki a écrit :
> >> On 2010-05-16 22:15, Eric Dumazet wrote:
> >
> >>> All tx packets through bonding will use txqueue 0, since bnx2 doesnt
> >>> provide a ndo_select_queue() function.
> >>
> >> OK, that explains everything. Thank you Eric. I assume it may take some
> >> time for bonding to become multiqueue aware and/or bnx2x to provide
> >> ndo_select_queue?
> >>
> >
> > bonding might become multiqueue aware, there are several patches
> > floating around.
> >
> > But with your ping tests, it wont change the selected txqueue anyway (it
> > will be the same for any targets, because skb_tx_hash() wont hash the
> > destination address, only the skb->protocol.
> 
> What do you mean by "wont hash the destination address, only the 
> skb->protocol"? It won't hash the destination address for ICMP or for 
> all IP protocols?

locally generated ICMP packets all use same tx queue, because
sk->sk_hash is not set :

        if (skb->sk && skb->sk->sk_hash)
                hash = skb->sk->sk_hash;
        else
                hash = (__force u16) skb->protocol;

        hash = jhash_1word(hash, hashrnd);

        return (u16) (((u64) hash * dev->real_num_tx_queues) >> 32);
 



However, replies will spread four queues, if hardware is capable to
perform hashing of ICMP packets, using IP addresses (source/destination)

> 
> My normal workload is TCP and UDP based so if it is only ICMP then there 
> is no problem. Actually I have noticeably more UDP traffic than an 
> average network, mainly because of LWAPP/CAPWAP, so I'm interested in 
> good performance for both TCP and UDP.
> 
> During my initial tests ICMP ping showed the same behavior like UDP/TCP 
> with iperf, so I sticked with it. I'll redo everyting with UDP and TCP 
> of course. :)
> 
> >> BTW: With a normal router workload, should I expect big performance drop
> >> when receiving and forwarding the same packet using different CPUs?
> >> Bonding provides very important functionality, I'm not able to drop it. :(
> >>
> >
> > Not sure what you mean by forwarding same packet using different CPUs.
> > You probably meant different queues, because in normal case, only one
> > cpu is involved (the one receiving the packet is also the one
> > transmitting it, unless you have congestion or trafic shaping)
> 
> I mean to receive it on a one CPU and to send it on a different one. I 
> would like to assing different vectors (eth1-0 .. eth1-4) to different 
> CPUs, but with bnx2x+bonding packets are received on queues 1-4 (eth1-1 
> .. eth1-4) and sent from queue 0 (eth1-0). So, for a one packet, two 
> different CPUs will be involved (RX on q1-q4, TX on q0).

As I said, (unless you use RPS), one forwarded packet only uses one CPU.
How tx queue is selected is another story. We try to do a 1-1 mapping.

> 
> > If you have 4 cpus, you can use following patch and have a transparent
> > bonding against multiqueue.
> 
> Thanks! If I get it right: with the patch, packets should be sent using 
> the same CPU (queue?) that was used when receiving?

Yes, for forwarding loads.

(You might use 5 or 8 instead of 4, because its not clear to me if bnx2
has 5 txqueues or 4 in your case)

> 
> > Still bonding xmit path hits a global
> > rwlock, so performance is not what you can get without bonding.
> 
> It may not be perfect, but it should be much better than nothing, right?
> 

Sure.


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