netdev - Re: Make TCP work better with re-ordered frames?

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Date:	Wed, 18 May 2016 08:07:51 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Make TCP work better with re-ordered frames?

On 05/18/2016 07:29 AM, Eric Dumazet wrote:
> On Wed, 2016-05-18 at 07:00 -0700, Ben Greear wrote:
>> We are investigating a system that has fairly poor TCP throughput
>> with the 3.17 and 4.0 kernels, but evidently it worked pretty well
>> with 3.14 (I should be able to verify 3.14 later today).
>>
>> One thing I notice is that a UDP download test shows lots of reordered
>> frames, so I am thinking maybe TCP is running slow because of this.
>>
>> (We see about 800Mbps UDP download, but only 500Mbps TCP, even when
>>    using 100 concurrent TCP streams.)
>>
>> Is there some way to tune the TCP stack to better handle reordered frames?
>
> Nothing yet. Are you the sender or the receiver ?
>
> You really want to avoid reorders as much as possible.
>
> Are you telling us something broke in networking layers between 3.14 and
> 3.17 leadings to reorders ?

I am both sender and receiver, through an access-controller and wifi AP as DUT.
The sender is Intel 1G NIC, so I suspect it is not causing reordering, which
indicates most likely DUT is to blame.

Using several off-the-shelf APs in our lab we do not see this problem.

I am not certain yet what is the difference, but customer reports 600+Mbps
with their older code, and best I can get is around 500Mbps with newer stuff.

Lots of stuff changed though (ath10k firmware, user-space at least slightly,
kernel, etc), so possibly the regression is elsewhere.

Thanks,
Ben

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