linux-kernel - Re: [E1000-devel] e1000 1sec latency problem

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Date:	Thu, 07 Feb 2008 10:40:27 -0800
From:	"Kok, Auke" <auke-jan.h.kok@...el.com>
To:	Max Krasnyansky <maxk@...lcomm.com>
CC:	Pavel Machek <pavel@....cz>, e1000-devel@...ts.sourceforge.net,
	jesse.brandeburg@...el.com,
	kernel list <linux-kernel@...r.kernel.org>,
	john.ronciak@...el.com, jeffrey.t.kirsher@...el.com
Subject: Re: [E1000-devel] e1000 1sec latency problem

Max Krasnyansky wrote:
> 
> Kok, Auke wrote:
>> Max Krasnyansky wrote:
>>> Kok, Auke wrote:
>>>> Max Krasnyansky wrote:
>>>>> So you don't think it's related to the interrupt coalescing by any chance ?
>>>>> I'd suggest to try and disable the coalescing and see if it makes any difference.
>>>>> We've had lots of issues with coalescing misbehavior. Not this bad (ie 1 second) though.
>>>>>
>>>>> Add this to modprobe.conf and reload e1000 module
>>>>>
>>>>> options e1000 RxIntDelay=0,0 RxAbsIntDelay=0,0 InterruptThrottleRate=0,0 TxIntDelay=0,0 TxAbsIntDelay=0,0
>>>> that can't be the problem. irq moderation would only account for 2-3ms variance
>>>> maximum.
>>> Oh, I've definitely seen worse than that. Not as bad as a 1second though. Plus you're talking
>>> about the case when coalescing logic is working as designed ;-). What if there is some kind of 
>>> bug where timer did not expire or something.
>> we don't use a software timer in e1000 irq coalescing/moderation, it's all in
>> hardware, so we don't have that problem at all. And I certainly have never seen
>> anything you are referring to with e1000 hardware, and I do not know of any bug
>> related to this.
>>
>> are you maybe confused with other hardware ?
>>
>> feel free to demonstrate an example...
> 
> Just to give you a background. I wrote and maintain http://libe1000.sf.net
> So I know E1000 HW and SW in and out.

wow, even I do not dare to say that!

> And no I'm not confused with other HW and I know that we're
> not using SW timers for the coalescing. HW can be buggy as well. Note that I'm not saying that I
> know for sure that the problem is coalescing, I'm just suggesting to take it out of the equation
> while Pavel is investigating.
> 
> Unfortunately I cannot demonstrate an example but I've seen unexplained packet delays in the range 
> of 1-20 milliseconds on E1000 HW (and boy ... I do have a lot of it in my labs). Once coalescing 
> was disabled those problems have gone away.

this sounds like you have some sort of PCI POST-ing problem and those can indeed
be worse if you use any form of interrupt coalescing. In any case that is largely
irrelevant to the in-kernel drivers, and as I said we definately have no open
issues on that right now, and I really do not recollect any as well either (other
than the issue of interference when both ends are irq coalescing)

Cheers,

Auke
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