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

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Message-ID: <1338376107.2760.156.camel@edumazet-glaptop>
Date:	Wed, 30 May 2012 13:08:27 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hiroaki SHIMODA <shimoda.hiroaki@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Denys Fedoryshchenko <denys@...p.net.lb>,
	netdev@...r.kernel.org, e1000-devel@...ts.sourceforge.net,
	jeffrey.t.kirsher@...el.com, jesse.brandeburg@...el.com,
	davem@...emloft.net
Subject: Re: Strange latency spikes/TX network stalls on Sun Fire
 X4150(x86) and e1000e

On Wed, 2012-05-30 at 19:43 +0900, Hiroaki SHIMODA wrote:

> While examining ping problem, below pattern is often observed.
> 
>                                                TIME
>        dql_queued()         dql_completed()     |
>       a) initial state                          |
>                                                 |
>       b) X bytes queued                         V
> 
>       c) Y bytes queued
>                            d) X bytes completed
>       e) Z bytes queued
>                            f) Y bytes completed
> 
> a) dql->limit has already some value and there is no in-flight packet.
> b) X bytes queued.
> c) Y bytes queued and excess limit.
> d) X bytes completed and dql->prev_ovlimit is set and also
>    dql->prev_num_queued is set Y.
> e) Z bytes queued.
> f) Y bytes completed. inprogress and prev_inprogress are true.
> 
> At f), if I read the comment correctly, all_prev_completed becomes
> true and limit should be increased. But POSDIFF() ignores
> (A == B) case, so limit is decreased.

Which POSDIFF(), because there are many ;)

By the way, given complexity of this I suggest you split your ideas in
independent patches.

Mabe we should change all POSDIFF(), not selected ones.

#define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)



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