[GridSim-developers] AllocPolicy's synchronisation (or lack of) problem

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

Hi All,

adding something to a previous discussion about the way simjava2  
manages the threads (i.e. entities), I think that you have probably  
faced the same problems.

As simjava2 restarts all entities that have events scheduled for the  
same time (i.e. a clock tick), it may happen that both a GridResource  
and its policy have two separate events to be handled at the same  
simulation time. The event received by the GridResource may be a  
gridlet submission whereas the event received by the policy may be an  
internal event used to update some list.

(From sim_system.run_tick()) :

   int entities_size = entities.size();
   for (int i=0; i < entities_size; i++) {
       ent = (Sim_entity)entities.get(i);
       if (ent.get_state() == Sim_entity.RUNNABLE) {
	ent.restart();
	num_started++;
       }
    }

If you have used java 5's features such as "for(Object obj :  
Iterable) {" you will notice that if you try to iterate an iterable  
object in the gridletSubmit method and update it somewhere else, you  
are likely to receive a ConcurrentModificationException. The reason  
is that the methods:

public void gridletSubmit(Gridlet gl, boolean ack);
public void gridletCancel(int gridletId, int userId);
public void gridletPause(int gridletId, int userId, boolean ack);
public void gridletResume(int gridletId, int userId, boolean ack);
public int gridletStatus(int gridletId, int userId);
public void gridletMove(int gridletId, int userId, int destId,  
boolean ack);
public void processOtherEvent(Sim_event ev)

are not synchronised. This way, GridResource may be invoking some of  
the policie's method running in one thread while the policy is  
updating the list in another thread.

Although this may look obvious, a user trying to implement its policy  
might get quite puzzled about it. I am also wondering, another  
problem is that the results may look different if a simulation is run  
on different computers with different numbers of cores.

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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