netdev - Re: [PATCH net] e1000e: Change wthresh to 1 to avoid possible Tx stalls.

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Date:	Tue, 09 Oct 2012 23:02:07 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:	Frank Reppin <frank@...ermydesk.org>, netdev@...r.kernel.org,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	e1000-devel@...ts.sourceforge.net
Subject: Re: [PATCH net] e1000e: Change wthresh to 1 to avoid possible Tx
 stalls.

On Tue, 2012-10-09 at 10:36 -0700, Jesse Brandeburg wrote:
> > > > Jesse did not share any performance numbers with me, I am sure he can
> > > > give some background tomorrow when he is back online.
> > > > 
> > > > I am working on an alternative patch now and should have something to
> > > > share tomorrow.
> > > Please allow me to ask if there's any progess here?
> > > 
> > > I've tried 3.5.4 a couple of days ago on a SuperMicro X8SIE-LN4 (82574L)
> > > and could still observe severe latency (up to 3000ms) spikes.
> > > 
> > > Applying Hiroakis suggested patch did fix this for me as well.
> > > [please note as well that I didn't had this issue in any 3.4.x kernel
> > > before - so +1 for fixing the regression]
> 
> I'm not sure what went wrong internally here that this hasn't been
> fixed, and I'm personally embarrassed.  I am working on it until I have
> a patch/solution.
> 
> currently am trying to reproduce the issue, am in some weird how to
> use BQL limbo, the lack of documentation on user usage of BQL is slowing
> me down.
> 

BQL is blocking qdisc from delivering additional packet to TX as long as
previous packets were not completed. Not sure what you want to know
about BQL.

> Hints or clues (I'm trying to follow the repro steps mentioned in
> some related threads) are appreciated.


Problem is BQL depends on TX completion being done in a reasonable time.

It seems e1000e can hold an skb in TX ring for up to 3000ms (not
reasonable it seems)

Aggregation / coalescing is fine, as long as we dont hold a packet too
long, in case no other packet follows.



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