Re: [PATCH] loop unrolling in net/sched/sch_generic.c

[Thomas Graf <tgraf@xxxxxxx>]

* Eric Dumazet <42CB2698.2080904@xxxxxxxxxxxxx> 2005-07-06 02:32
> Hum... shouldnt it be :
> 
> +                     skb = __qdisc_dequeue_head(qdisc, list + prio);
> 

Correct.


> Anyway, the branches misprediction come from the fact that most of packets 
> are queued in the prio=2 list.
> 
> So each time this function is called, a non unrolled version has to pay 2 
> to 5 branches misprediction.
> 
> if ((!skb_queue_empty(list + prio))  /* branch not taken, mispredict when 
> prio=0 */

The !expr implies an unlikely so the prediction should be right and
equal to your unrolling version.

> Maybe we can rewrite the whole thing without branches, examining prio from 
> PFIFO_FAST_BANDS-1 down to 0, at least for modern cpu with conditional mov 
> (cmov)

This would break the whole thing, the qdisc is supposed to try and
dequeue from the highest priority queue (prio=0) first.