netdev - Re: Strange latency spikes/TX network stalls on Sun Fire X4150(x86) and e1000e

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Date:	Wed, 06 Jun 2012 11:23:32 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	therbert@...gle.com
Cc:	shimoda.hiroaki@...il.com, jesse.brandeburg@...el.com,
	eric.dumazet@...il.com, denys@...p.net.lb, netdev@...r.kernel.org,
	e1000-devel@...ts.sourceforge.net, jeffrey.t.kirsher@...el.com
Subject: Re: Strange latency spikes/TX network stalls on Sun Fire
 X4150(x86) and e1000e

From: Tom Herbert <therbert@...gle.com>
Date: Wed, 6 Jun 2012 11:21:40 -0700

> I'm not exactly sure what the exact effect of WTHRESH is here.  Does
> the device coalesce 5 completions regardless of size?  Would the
> problem be avoided if bql limit_min were MTU, or could same issue be
> hit with larger that 64 byte packets?

The problem is that no TX completions are signalled happen until at
least WTHRESH are pending.

BQL is the least of the problems generated by this kind of behavior.

All drivers must TX complete in a small, finite, amount of time so
it is absolutely illegal to have the behavior that WRTHRESH > 1
gives.
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