Subject: Re: HTB accuracy for high speed

From: Jarek Poplawski <[email-masked]>
Date: Mon, 18 May 2009 19:23:49 +0200

On Mon, May 18, 2009 at 06:40:56PM +0200, Eric Dumazet wrote:
> Jarek Poplawski a écrit :
> > On Fri, May 15, 2009 at 03:49:31PM +0100, Antonio Almeida wrote:
> > ...
> >> I also note that, for HTB rate configurations over 500Mbit/s on leaf
> >> class, when I stop the traffic, in the output of "tc -s -d class ls
> >> dev eth1" command, I see that leaf's rate (in bits/s) is growing
> >> instead of decreasing (as expected since I've stopped the traffic).
> >> Rate in pps is ok and decreases until 0pps. Rate in bits/s increases
> >> above 1000Mbit and stays there for a few minutes. After two or three
> >> minutes it becomes 0bit. The same happens for it's ancestors (also for
> >> root class).Here's tc output of my leaf class for this situation:
> >>
> >> class htb 1:108 parent 1:10 leaf 108: prio 7 quantum 1514 rate
> >> 555000Kbit ceil 555000Kbit burst 70901b/8 mpu 0b overhead 0b cburst
> >> 70901b/8 mpu 0b overhead 0b level 0
> >>  Sent 120267768144 bytes 242475339 pkt (dropped 62272599, overlimits 0
> >> requeues 0)
> >>  rate 1074Mbit 0pps backlog 0b 0p requeues 0
> >>  lended: 242475339 borrowed: 0 giants: 0
> >>  tokens: 8 ctokens: 8
> > 
> > This looks like a regular bug. I guess it's an overflow in
> > gen_estimator(), but I'm not sure there is nothing more. Could you
> > try the patch below? (An offset warning when patching 2.6.25 is OK)
> > 
> > Thanks,
> > Jarek P.
> > ---
> > 
> >  net/core/gen_estimator.c |    6 +++++-
> >  1 files changed, 5 insertions(+), 1 deletions(-)
> > 
> > diff --git a/net/core/gen_estimator.c b/net/core/gen_estimator.c
> > index 9cc9f95..87f0ced 100644
> > --- a/net/core/gen_estimator.c
> > +++ b/net/core/gen_estimator.c
> > @@ -127,7 +127,11 @@ static void est_timer(unsigned long arg)
> >  		npackets = e->bstats->packets;
> >  		rate = (nbytes - e->last_bytes)<<(7 - idx);
> >  		e->last_bytes = nbytes;
> > -		e->avbps += ((long)rate - (long)e->avbps) >> e->ewma_log;
> > +		if (rate > e->avbps)
> > +			e->avbps += (rate - e->avbps) >> e->ewma_log;
> > +		else
> > +			e->avbps -= (e->avbps - rate) >> e->ewma_log;
> > +
> >  		e->rate_est->bps = (e->avbps+0xF)>>5;
> >  
> >  		rate = (npackets - e->last_packets)<<(12 - idx);
> 
> With a typical estimator "1sec 8sec", ewma_log value is 3
> 
> At gigabit speeds, we are very close to overflow yes, since
> we only have 27 bits available, so 134217728 bytes per second
> or 1073741824 bits per second.
> 
> So formula :
> e->avbps += ((long)rate - (long)e->avbps) >> e->ewma_log;
> is going to overflow.
> 
> One way to avoid the overflow would be to use a smaller estimator, like "500ms 4sec" 
> 
> Or use a 64bits rate & avbps, this is needed fo 10Gb speeds I suppose...

Yes, I considered this too, but because of an overhead I decided to
fix as designed (according to the comment) for now. But probably you
are right, and we should go further, so I'm OK with your patch.

Jarek P.

> 
> diff --git a/net/core/gen_estimator.c b/net/core/gen_estimator.c
> index 9cc9f95..150e2f5 100644
> --- a/net/core/gen_estimator.c
> +++ b/net/core/gen_estimator.c
> @@ -86,9 +86,9 @@ struct gen_estimator
>  	spinlock_t		*stats_lock;
>  	int			ewma_log;
>  	u64			last_bytes;
> +	u64			avbps;
>  	u32			last_packets;
>  	u32			avpps;
> -	u32			avbps;
>  	struct rcu_head		e_rcu;
>  	struct rb_node		node;
>  };
> @@ -115,6 +115,7 @@ static void est_timer(unsigned long arg)
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(e, &elist[idx].list, list) {
>  		u64 nbytes;
> +		u64 brate;
>  		u32 npackets;
>  		u32 rate;
>  
> @@ -125,9 +126,9 @@ static void est_timer(unsigned long arg)
>  
>  		nbytes = e->bstats->bytes;
>  		npackets = e->bstats->packets;
> -		rate = (nbytes - e->last_bytes)<<(7 - idx);
> +		brate = (nbytes - e->last_bytes)<<(7 - idx);
>  		e->last_bytes = nbytes;
> -		e->avbps += ((long)rate - (long)e->avbps) >> e->ewma_log;
> +		e->avbps += ((s64)(brate - e->avbps)) >> e->ewma_log;
>  		e->rate_est->bps = (e->avbps+0xF)>>5;
>  
>  		rate = (npackets - e->last_packets)<<(12 - idx);
>