netdev - Re: [PATCH net-next V3] htb: improved accuracy at high rates

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Date:	Mon, 05 Nov 2012 08:58:39 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vimalkumar <j.vimal@...il.com>
Cc:	davem@...emloft.net, Jamal Hadi Salim <jhs@...atatu.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next V3] htb: improved accuracy at high rates

On Wed, 2012-10-31 at 09:04 -0700, Vimalkumar wrote:
> Current HTB (and TBF) uses rate table computed by the "tc"
> userspace program, which has the following issue:
> 
> The rate table has 256 entries to map packet lengths
> to token (time units).  With TSO sized packets, the
> 256 entry granularity leads to loss/gain of rate,
> making the token bucket inaccurate.
> 
> Thus, instead of relying on rate table, this patch
> explicitly computes the time and accounts for packet
> transmission times with nanosecond granularity.
> 
> This greatly improves accuracy of HTB with a wide
> range of packet sizes.
> 
> Example:
> 
> tc qdisc add dev $dev root handle 1: \
>         htb default 1
> 
> tc class add dev $dev classid 1:1 parent 1: \
>         rate 5Gbit mtu 64k
> 
> Here is an example of inaccuracy:
> 
> $ iperf -c host -t 10 -i 1
> 
> With old htb:
> eth4:   34.76 Mb/s In  5827.98 Mb/s Out -  65836.0 p/s In  481273.0 p/s Out
> [SUM]  9.0-10.0 sec   669 MBytes  5.61 Gbits/sec
> [SUM]  0.0-10.0 sec  6.50 GBytes  5.58 Gbits/sec
> 
> With new htb:
> eth4:   28.36 Mb/s In  5208.06 Mb/s Out -  53704.0 p/s In  430076.0 p/s Out
> [SUM]  9.0-10.0 sec   594 MBytes  4.98 Gbits/sec
> [SUM]  0.0-10.0 sec  5.80 GBytes  4.98 Gbits/sec
> 
> The bits per second on the wire is still 5200Mb/s with new HTB
> because qdisc accounts for packet length using skb->len, which
> is smaller than total bytes on the wire if GSO is used.  But
> that is for another patch regardless of how time is accounted.
> 
> Many thanks to Eric Dumazet for review and feedback.
> 
> Signed-off-by: Vimalkumar <j.vimal@...il.com>
> ---
>  net/sched/sch_htb.c |  128 ++++++++++++++++++++++++++++++++++++---------------
>  1 files changed, 90 insertions(+), 38 deletions(-)
> 

> @@ -684,7 +729,7 @@ static psched_time_t htb_do_events(struct htb_sched *q, int level,
>  			return cl->pq_key;
>  
>  		htb_safe_rb_erase(p, q->wait_pq + level);
> -		diff = psched_tdiff_bounded(q->now, cl->t_c, cl->mbuffer);
> +		diff = min_t(s64, q->now - cl->t_c, cl->mbuffer);
>  		htb_change_class_mode(q, cl, &diff);
>  		if (cl->cmode != HTB_CAN_SEND)
>  			htb_add_to_wait_tree(q, cl, diff);
> @@ -834,7 +879,6 @@ next:
>  	} while (cl != start);
>  
>  	if (likely(skb != NULL)) {
> -		bstats_update(&cl->bstats, skb);
>  		cl->un.leaf.deficit[level] -= qdisc_pkt_len(skb);
>  		if (cl->un.leaf.deficit[level] < 0) {
>  			cl->un.leaf.deficit[level] += cl->quantum;
> @@ -871,10 +915,10 @@ ok:
>  

Why this bstats_update() call was removed ?



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