net_device: limit rate ot tx packets

[michi1 at michaelblizek.twilightparadox.com]

Hi!

On 14:11 Sat 13 Apr     , christian+kn at wwad.de wrote:
> Hi All,
> 
> can someone please explain me, how the kernel handles different
> transfer rates of different net_devices? Or in other words: How does
> the systemcall send() know, when to block?
> 
> An example:
>   cat /dev/zero | pv | nc -u <someip>
> will show different throughput speeds depending on the network device,
> the packets are sent over (wlan0 will be slower than eth0).
> 
>  - Can someone point out the location in the linux kernel source, where
>    this is handled?
> 
>  - If I register a net_device. How do I signal to the upper
>    network layers that my driver can only accept packets at a
>    certain rate? I tried stopping the egress queue by calling
>    netif_stop_queue(), but this only has the effect that the queue
>    overruns.
> 
> 
> I have the feeling that I'm missing out a very vital point on how the
> kernel's networking subsystem works. Unfortunately, Understanding Linux
> Network Internals couldn't help me out here.

I think this is what causes the behaviour:

1) If the network device is congested, packets will queue up in the qdisc

2) Socket memory gets used up, semdmsg will probably sleep here:
udp_sendmsg
ip_make_skb
__ip_append_data
sock_alloc_send_skb
sock_alloc_send_pskb
sock_wait_for_wmem

3) When packets are sent, the socket destructor will be called and wake up
the sender:
sock_wfree()
sk->sk_write_space(sk); (which is sock_def_write_space)


It would be interesting to see what will happen if the qdisc is smaller than
the socket memory...

	-Michi
-- 
programing a layer 3+4 network protocol for mesh networks
see http://michaelblizek.twilightparadox.com