RE: qdisc tbf dropping many packets

[Madhukar.Mythri]

Check whether Jumbo frames was set on network-interface (i.e, May be MTU
size was increased more than 1500).

Regards,
Madhukar.
-----Original Message-----
From: linux-net-owner@xxxxxxxxxxxxxxx
[mailto:linux-net-owner@xxxxxxxxxxxxxxx] On Behalf Of Abdul Basit
Sent: Wednesday, October 15, 2008 7:07 AM
To: linux-net@xxxxxxxxxxxxxxx
Cc: abasit@xxxxxxxxx
Subject: qdisc tbf dropping many packets 

Hi,

I am running linux kernel 2.6.23.1 on PowerPC and trying to set qdisc
tbf  to limit the rate:-
*
*==> tc qdisc add dev eth0 root tbf rate 2048kbit buffer 10kb latency
50ms

The problem is no matter what rate i specify, if I download a file it
always get down to ~16-18Kbps download speed, after investigating a bit
more, i found by tc -s qdisc show that many dropped packets show up in
statistic s, looking further in kernel code, I found that in
linux/net/sched/sch_tbf.c:126

static int tbf_enqueue(struct sk_buff *skb, struct Qdisc* sch) {
       struct tbf_sched_data *q = qdisc_priv(sch);
       int ret;
 
        if (skb->len > q->max_size) {
                printk (KERN_INFO "tbf_enqueue, drop skb->len = %d, 
q->max_size = %d\n", skb->len, q->max_size);
                sch->qstats.drops++;

        ....
}

whenever it try to enqueue the packet, the skb->len ( range from 2980 -
2977 ) is usually greator than q->max_size ( 2907 ) hence it drops the
packet.

On a different machine with same source code, skb->len never become >
1500 and there the tbf works fine.

Could someone please provide more insight or help where should i look
more?

Thanks,
~Basit

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