Queuing disciplines in linux kernel

[Shrikrishna Khare]

Hi All, 

I have implemented a new queuing discipline in Linux kernel (as module 
in net/sched) and I want to test that. However, no matter what kind of 
network flows I run, the queue size never seems to build up even with 
default pfifo implementation. Here are the things that I tried. 

1.  Machine A and B connected using Ethernet. 
    Interface configured to run at 100Mbps. 
    Queuing discipline configured: pfifo (default) 
    UDP flow from sender A to receiver B. 
    Result: Queue size never builds up. Any queued packet is 
immediately dequeued! 

    Thus, I tried configuring ethernet to run at 10 Mbps with rest of 
the setup same as above. Now lesser number of packet gets transferred 
in same duration but the queue at sender still DOES NOT build up. 
Could anyone please tell me why this happens? How do I test my 
queueuing discipline implementation then? 

     I tried another thing: 
2. 
  Machine A connected to machine B over ethernet configured to run at 
100 Mbps 
- Machine B connected to machine C over ethernet configured to run at 
10 Mbps 
- When I run iperf flow from machine A to machine C, I expected 
congestion to occur at machine B since its being fed by 100 Mbps link 
while data can go out only at 10 Mbps. Thus, I expected the queue size 
(printed from pfifo queue in net/sched/sch_generic.c code) to grow. 
But it NEVER seems to grow. Machine A to C connection seems to get 
bandwidth of only 10 Mbps. 

  When I trace using dmesg, all I see is sequqnce of alternate Enqueue 
and dequeue operations and hence the queue never grows. 

Could anyone please provide me with any pointer as to why this 
happens? How to test queuing disciplines then? 
Thanks and Regards, 
Shrikrishna 



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