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

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Date:	Thu, 07 Feb 2008 10:30:08 -0800
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	"Kok, Auke" <auke-jan.h.kok@...el.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



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. 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.

Max
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