netdev - Re: [PATCH] tcp_cubic: limit delayed_ack ratio to prevent divide error

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Message-ID: <4DC43269.9040503@techboom.com>
Date:	Fri, 06 May 2011 13:39:53 -0400
From:	TB <lkml@...hboom.com>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
	David Miller <davem@...emloft.net>,
	Sangtae Ha <sangtae.ha@...il.com>,
	Injong Rhee <injongrhee@...il.com>,
	"Valdis.Kletnieks@...edu" <Valdis.Kletnieks@...edu>,
	"rdunlap@...otime.net" <rdunlap@...otime.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tcp_cubic: limit delayed_ack ratio to prevent divide
 error

On 11-05-06 12:53 PM, Stephen Hemminger wrote:
> On Fri, 06 May 2011 12:15:46 -0400
> TB <lkml@...hboom.com> wrote:
> 
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>>
>> On 11-05-04 04:53 PM, Brandeburg, Jesse wrote:
>>>
>>>
>>> On Wed, 4 May 2011, Stephen Hemminger wrote:
>>>
>>>> TCP Cubic keeps a metric that estimates the amount of delayed
>>>> acknowledgements to use in adjusting the window. If an abnormally
>>>> large number of packets are acknowledged at once, then the update
>>>> could wrap and reach zero. This kind of ACK could only
>>>> happen when there was a large window and huge number of
>>>> ACK's were lost.
>>>>
>>>> This patch limits the value of delayed ack ratio. The choice of 32
>>>> is just a conservative value since normally it should be range of 
>>>> 1 to 4 packets.
>>>>
>>>> Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
>>>
>>> patch seems fine, but please credit the reporter (lkml@...hboom.com) with 
>>> reporting the issue with logs, maybe even with Reported-by: and some kind 
>>> of reference to the panic message or the email thread in the text or 
>>> header?
>>
>> We're currently testing the patch on 6 production servers
> 
> Thank you, is there some regularity to the failures previously?

Not really, there was more chance of it happening after a reboot and
during the night (when there is less traffic) for some weird reason.

As a workaround we switched most of the servers to reno
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