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

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Date:	Wed, 30 May 2012 09:25:46 +0200
From:	Jean-Michel Hautbois <jhautbois@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: Difficulties to get 1Gbps on be2net ethernet card

2012/5/30 Eric Dumazet <eric.dumazet@...il.com>:
> 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.
>

TX ring is not the same :
On 3.2 :
$> ethtool -g eth1
Ring parameters for eth1:
Pre-set maximums:
RX:             1024
RX Mini:        0
RX Jumbo:       0
TX:             2048
Current hardware settings:
RX:             1024
RX Mini:        0
RX Jumbo:       0
TX:             2048


On 2.6.26 :
$>ethtool -g eth1
Ring parameters for eth1:
Pre-set maximums:
RX:             1024
RX Mini:        0
RX Jumbo:       0
TX:             2048
Current hardware settings:
RX:             1003
RX Mini:        0
RX Jumbo:       0
TX:             0

I can't set TX ring using ethtool -G eth1 tx N : operation not supported
I am not really impacted by latency, but the lower the better.

JM
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