netdev - Re: large divisor for flow classifier

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Date:	Fri, 15 Oct 2010 22:01:45 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jonathan Thibault <jonathan@...igue.com>
Cc:	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: large divisor for flow classifier

Le vendredi 15 octobre 2010 à 14:14 -0400, Jonathan Thibault a écrit :
> It appears that when setting a fairly large divisor on the flow classifier for sfq, traffic stops altogether.
> 
> On my machine, anything above divisor 2200 seems to stop all traffic.  If I want to be fair between hosts (but not flows) for a large network (say 6000 hosts), I run into problems.  Obviously the rates here are quite low but this is just an example.
> 
> I also tested on a real interface with the same results.
> 
> Example that works:
> 
> tc qdisc add dev ifb0 root handle 3: htb default 10
>   tc class add dev ifb0 parent 3: classid 3:1 htb rate 80kbit
>     tc class add dev ifb0 parent 3:1 classid 3:10 htb rate 80kbit
>       tc qdisc add dev ifb0 parent 3:10 handle 310: sfq perturb 10
>       tc filter add dev ifb0 parent 3: protocol all prio 1 u32 match mark 0x000 0xf00 flowid 3:10
>       tc filter add dev ifb0 parent 310: protocol all handle 0x310 flow hash keys dst divisor 1024
> 
> Example that doesn't work:
> 
> tc qdisc add dev ifb0 root handle 3: htb default 10
>   tc class add dev ifb0 parent 3: classid 3:1 htb rate 80kbit
>     tc class add dev ifb0 parent 3:1 classid 3:10 htb rate 80kbit
>       tc qdisc add dev ifb0 parent 3:10 handle 310: sfq perturb 10
>       tc filter add dev ifb0 parent 3: protocol all prio 1 u32 match mark 0x000 0xf00 flowid 3:10
>       tc filter add dev ifb0 parent 310: protocol all handle 0x310 flow hash keys dst divisor 6144
> 

SFQ is limited to a 1024 divisor

You might try following patch :

(8192 is the smallest power of two greater than 6144)

sizeof(struct sfq_sched_data) becomes 0x2ccc instead of 0x10cc

keep in mind hash distribution is not perfect.

What would be the real rate ?


diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 3cf478d..c4a53d6 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -77,7 +77,7 @@
 	It is easy to increase these values, but not in flight.  */
 
 #define SFQ_DEPTH		128
-#define SFQ_HASH_DIVISOR	1024
+#define SFQ_HASH_DIVISOR	8192
 
 /* This type should contain at least SFQ_DEPTH*2 values */
 typedef unsigned char sfq_index;


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