netdev - Re: [PATCH net-next] net_sched: sfq: extend limits

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Message-ID: <1325787722.4759.25.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Thu, 05 Jan 2012 19:22:02 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Laurent Chavey <chavey@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Dave Taht <dave.taht@...il.com>
Subject: Re: [PATCH net-next] net_sched: sfq: extend limits

Le jeudi 05 janvier 2012 à 10:08 -0800, Laurent Chavey a écrit :

> >  /*
> > @@ -131,9 +143,9 @@ struct sfq_sched_data {
> >  */
> >  static inline struct sfq_head *sfq_dep_head(struct sfq_sched_data *q, sfq_index val)
> >  {
> > -       if (val < SFQ_SLOTS)
> 
> 
> why not use q->maxflows here rather than SFQ_MAX_FLOWS ?
> 

Debug-ability ? val indexes for dep[] array are not dependent on
q->maxflows : 0xFF80 -> 0xFFFF

> looking at sfq_change(), q->maxflows may be <= to SFQ_MAX_FLOWS

yep... but values between q->maxflows and SFQ_MAX_FLOWS wont be
possible.

> 
> the initial creation of the slots array in sfq_init() uses q->maxflows.
> q->maxflows is initialized to SFQ_DEFAULT_FLOWS which is / maybe less
> than SFQ_MAX_FLOWS
> 

It really doesnt matter, and code is faster on most arches.

if (val < SFQ_MAX_FLOWS)
      return &q->slots[val].dep;
return &q->dep[val - SFQ_MAX_FLOWS];

faster than : (because of extra memory reference)

if (val < q->maxflows)
      return &q->slots[val].dep;
return &q->dep[val - q->maxflows];


As a matter of fact, maxflows could be moved at the end of struct
sfq_sched_data since its not used in fast path.



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