Re: [GridSim-developers] [GridSim-users] Strange error

[Marcos D. de A. <ma...@cs...>]

Dear Giancarlo,

> I think the exception is brought by a race condition between a call to
> gridletSubmit() method of SpaceShared class (or whatever class extends
> AllocPolicy) made by GridResource thread and events treated in the  
> body
> of SpaceShared thread


You are right, the race condition is probably there.

The problem is that each entity in GridSim is a thread. Simjava2  
works with two event queues, namely the future event queue and the  
deferred event queue. Every time Sim_system class reads the queue of  
future events (i.e. at each clock tick), it puts all the next events  
to be handles in the deferred queue. If there are multiple events  
scheduled for the very same simulation time, they will all be placed  
in the deferred queue. After that, Sim_system wakes up all the  
threads to consume events. The problem arises when there are two  
events in the deferred queue, one for GridResource and another for  
the policy, both for the same time and both will result in making the  
entities access internal variables of the policy. This problem is  
noticed more often if you use the send method without the port  
because in this case there will be no network. When you use the send  
with the port, the network features introduce very small delays  
between the events, which will prevent them from being scheduled at  
exactly the same time, so you will not notice the problem.

Yes, this is a bug and you are not the first one to experience this  
problem. I think people had not noticed it before because they were  
using Java 1.4. One solution to this problem would consist in making  
GridResource send an event to the policy instead of calling a method  
from it. By the way, this is the approach suggested in SimJava2's  
documentation. The other solution, if you are using the the source  
code of GridSim instead of the jar files, is to add some  
synchronisation to the allocation policy.

Regards,

Marcos

Marcos Dias de Assuncao
Grid Computing and Distributed Systems (GRIDS) Laboratory
Department of Computer Science and Software Engineering
The University of Melbourne, Australia
Email: ma...@cs...

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