netdev - Re: Difficulties to get 1Gbps on be2net ethernet card

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Date:	Wed, 30 May 2012 09:06:27 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jean-Michel Hautbois <jhautbois@...il.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: Difficulties to get 1Gbps on be2net ethernet card

On Wed, 2012-05-30 at 08:51 +0200, Jean-Michel Hautbois wrote:
> 2012/5/30 Eric Dumazet <eric.dumazet@...il.com>:
> > On Wed, 2012-05-30 at 08:28 +0200, Jean-Michel Hautbois wrote:
> >
> >> If this can help, setting tx queue length to 5000 seems to make the
> >> problem disappear.
> >
> > Then you should have drops at Qdisc layer (before your change to 5000)
> >
> > tc -s -d qdisc
> >
> >> I didn't specified it : MTU is 4096, UDP packets are 4000 bytes.
> >
> 
> Yes :
> qdisc mq 0: dev eth1 root
>  Sent 5710049154383 bytes 1413544639 pkt (dropped 73078, overlimits 0
> requeues 281540)
>  backlog 0b 0p requeues 281540
> 
> Why ? With a 2.6.26 kernel it works well with a tx queue length of 1000.

If you send big bursts of packets, then you need a large enough queue.

Maybe your kernel is now faster than before and queue fills faster, or
TX ring is smaller ?

ethtool -g eth0

Note that everybody try to reduce dumb queue sizes because of latencies.



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