netdev - Re: Why is LRO off by default on ixgbe?

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Date:	Wed, 23 Sep 2009 10:16:01 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	NetDev <netdev@...r.kernel.org>
Subject: Re: Why is LRO off by default on ixgbe?

On 09/23/2009 10:07 AM, Stephen Hemminger wrote:
> On Wed, 23 Sep 2009 09:57:51 -0700
> Ben Greear<greearb@...delatech.com>  wrote:
>
>> On 09/23/2009 09:53 AM, Stephen Hemminger wrote:
>>> On Wed, 23 Sep 2009 09:29:59 -0700
>>> Ben Greear<greearb@...delatech.com>   wrote:
>>>
>>>> I just noticed that enabling LRO on ixgbe lets me reach about 9Gbps receive on two
>>>> NICs concurrently in an NFS test, where I was only getting about 6Gbps w/out it (1500 MTU).
>>>>
>>>> Why is LRO disabled by default?
>>>>
>>>> Thanks,
>>>> Ben
>>>
>>> LRO is turned off if bridging or routing because of End to End requirements.
>>
>> That makes sense.
>>
>> If I know that all interfaces in question can handle TSO and LRO,
>> I could manually enable LRO w/out risk, right?
>>
>
> The problem is that LRO merges TCP packets, this breaks the end-to-end
> ack clocking and checksumming, and therefore is not enabled.
> That is why GRO is the replacement solution (preserves packet boundaries)

Ok.  It seems GRO was enabled the whole time, but LRO is what gave me the
extra performance boost.

In this particular case, I'm not actually routing, though I do have ip-forward
enabled, so I guess LRO will be OK as long as I'm careful...

Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com

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