netdev - Re: TCP reaching to maximum throughput after a long time

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Date:	Tue, 12 Apr 2016 13:23:32 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Machani, Yaniv" <yanivma@...com>, netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>,
	open list <linux-kernel@...r.kernel.org>,
	"Kama, Meirav" <meiravk@...com>
Subject: Re: TCP reaching to maximum throughput after a long time

On 04/12/2016 01:17 PM, Eric Dumazet wrote:
> On Tue, 2016-04-12 at 13:11 -0700, Ben Greear wrote:
>> On 04/12/2016 12:31 PM, Machani, Yaniv wrote:
>>> On Tue, Apr 12, 2016 at 18:04:52, Ben Greear wrote:
>>>> On 04/12/2016 07:52 AM, Eric Dumazet wrote:
>>>>> On Tue, 2016-04-12 at 12:17 +0000, Machani, Yaniv wrote:
>>>>>>
>>>>
>>>> If you are using 'Cubic' TCP congestion control, then please try
>>>> something different.
>>>> It was broken last I checked, at least when used with the ath10k driver.
>>>>
>>>
>>> Thanks Ben, this indeed seems to be the issue !
>>> Switching to reno got me to max throughput instantly.
>>>
>>> I'm still looking through the thread you have shared, but from what I understand there is no planned fix for it ?
>>
>> I think at the time it was blamed on ath10k and no one cared to try to fix it.
>>
>> Or, maybe no one really uses CUBIC anymore?
>>
>> Either way, I have no plans to try to fix CUBIC, but maybe someone who knows
>> this code better could give it a try.
>
> Well, cubic seems to work in many cases, assuming they are not too many
> drops.
>
> Assuming one flow can get nominal speed in few RTT is kind a dream, and
> so far nobody claimed a CC was able to do that, while still being fair
> and resilient.
>
> TCP CC are full of heuristics, and by definition heuristics that were
> working 6 years ago might need to be refreshed.
>
> We are still maintaining Cubic for sure.

It worked well enough for years that I didn't even know other algorithms were
available.  It was broken around 4.0 time, and I reported it to the list,
and no one seemed to really care enough to do anything about it.  I changed
to reno and ignored the problem as well.

It is trivially easy to see the regression when using ath10k NIC, and from this email
thread, I guess other NICs have similar issues.

Thanks,
Ben


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