[Click] CPU scheduling -> make elements interruptable.

[Roman Chertov]

In Click you have one or more threads of execution.  Most of the 
elements in Click are agnostic and do not generate any tasks themselves. 
  When an element like a Queue/PollDevice/ToDevice schedules a task and 
a packet is available, then this task will run through the entire chain 
of elements until it gets into another Queue/PollDevice/ToDevice 
element.  (Click SMP paper). So in order to what you are proposing, you 
would have to interrupt a Click thread in the middle of execution of 
some task and then to start a new one.  (obviously you have to remember 
the state so that you can come back).  I don't think this would be easy 
to do, as I don't think the executing threads (RouterThread) are 
designed to be interrupted.

Hopefully Eddie will shine more light on this.

Roman

Egi, Norbert wrote:
> Hi,
>  
> We are using Click for desinging a shared forwarding engine with separate forwarding paths (chain of elements) per customer. The customers share all the NICs, so after the packets are polled out from the NIC's queue a Classifier demultiplexes them by emitting them to the proper forwarding path. Our intention is to provide a fair CPU scheduling for each of the forwarding paths. The stride scheduling algorithm, implemented in Click, would be a perfect solution for our problem, but after checking the source code and the available documents I found out that the algorithm hasn't been implemented completely as it was proposed. If I understand it correctly from the source and its comments, there is no such thing like "quantums" (i.e. a discrete time slice when the scheduled task is entitled to use the CPU) and I guess that's the main reason while the operation of the elements can not be interrupted.
>  
> In the first comment of lib/task.cc it's mentioned that this may be addressed in the future, so I was wondering whether anyone is working on it and I may jump in and help or just could someone provide information on how to make elements interruptable the fastest way extending the current version of the code.
>  
> Regards,
> Norbert
> 
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