Hi! On 14:11 Sat 13 Apr , christian+kn@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