[Click] empty queue

[rchertov at cs.ucsb.edu]

Panos,

Was the NULL message printed just at the start of running the script? Since
you element issues a pull, it could have happened that it got scheduled
before Timedsource, and it attempted to pull packet from an empty Queue.

Roman

On 10:52 am 08/18/10 Panos Μatzakos  <magic_pan86 at yahoo.gr> wrote:
> Hello, as i had mentioned in a previous mail i try to implement a mac
> protocol (802.11 csma/ca) for my thesis and i am using the following
> flowgraph:
> 
> Timedsource -> Queue -> mysocket -> Discard
> 
> where mysocket is the main element of my router which implements the
> algorithm of the protocol and sends packets to PHY through sockets.
> Then, i needed a way to know from "mysocket" when the queue becomes
> empty because it is of great importance for my protocol. The simplest
> way that was suggested to me was this:
> 
> mysocket::pull() { 
>   Packet * p = input(0).pull();
>   if(p == NULL){
>       return 0;} 
> 
> } 
> p is NULL exactly if Queue is empty.  Everything seemed to work nice
> but when i tried to test my protocol in saturation conditions (in which
> case the queue is always non empty) by securing that the incoming rate
> of the queue is much higher than the outcoming rate i noticed that the
> queue got empty sometimes. Specifically i wrote the following code:
> 
> Packet * p = input(0).pull(); 
>     if(p == NULL){
>         cout<<"NULL!!"<<endl;
>         return 0;
>     }
> 
> and noticed that the "NULL!!" message was printed although it
> shouldn't. One thing that i am not sure about is what happens exactly
> when the queue holds capacity, which is a case that obviously interests
> me because as i said my incoming rate is greater than the outcoming
> rate. In the description of the queue element it says "Drops incoming
> packets if the queue already holds CAPACITY packets". What i understand
> from this is that the packets which are dropped are the new ones which
> are generated from timedsource and not the ones which are already in
> the queue, so the queue should still remain non empty. So do you have
> any idea about what happens and my queue still gets empty under these
> conditions?
> 
> One last thing i need to ask as i want to be sure if i have understood
> everything right is the following. In my flowgraph the element which
> initiates the packet flow after the queue element is the Discard
> element by a pull connection from its input. Is that right?
> 
> 
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