[Cbc] CbcEventHandler in multithreading

[John Forrest]

Alexis,

On 15/09/16 01:21, Alexis wrote:
>
> Hi,
>
> With the help of Tobias, I managed to compile CBC standalone in 
> VS2015. Inspection of the CPU usage on my machine indicates that 
> multithreading seems to work.
>
> I have a few more questions:
>
> 1 - Does the parameter with -threads indicates the number of extra 
> threads? 0 would mean that there is only the master thread, 
> essentially no multithreading?
>
Yes.
>
> 2 - I am not sure I understand at this point the parameter 
> threadMode_, but I am going to look more into it.
>
Mostly of secondary importance.  The deterministic option switch on by 
-threads 1nn is important.
> 3 - More importantly, I am trying to use multithreading with my 
> application. Compilation is ok. I am mostly wondering about the 
> behavior of the handlers in the multithreading settings.
>
> My code is essentially an extended version of driver6.cpp/driver4.cpp 
> so I used to have
>
>   // Pass to Cbc initialize defaults
>   CbcModel modelA(solver1);
>   CbcModel * model = &modelA;
>   CbcMain0(modelA);
>   // Event handler
>   MyEventHandler3 eventHandler;
>   model->passInEventHandler(&eventHandler);
>   /* Now go into code for standalone solver
>      Could copy arguments and add -quit at end to be safe
>      but this will do
>   */
>     const char * argv2[]={"driver4","-solve","-quit"};
>     CbcMain1(3,argv2,modelA);
>
> Now, I have
>
> *    const char * argv2[]={"driver4","-threads, "X", "-solve","-quit"};*
> *    CbcMain1(5,argv2,modelA);*
>
> - Is eventHandler shared between all the threads? Or is there one 
> handler (copy) per thread?
I may get this wrong, but ... Normally shared - just one copy - so 
CbcModel pointer is to master. There is option to copy - then points to 
thread CbcModel.
> - In the first case, is eventHandler thread safe?
Threadsafe for locals.  I would try having copies so each can get local 
CbcModel and global one.  I would use three globals (global model, best 
objective and stopflag) + mutex.  Then if event handler wants to do 
something it grabs mutex, looks at flag - if set releases mutex and 
returns.  If not set then sets it, releases mutex and returns saying do 
something.  If you just want to save a good solution then similar.  I 
have probably got that all wrong.
> - What would be the best way to get the best solution in the tree? 
> Currently, I just have to call
>
> CbcEventHandler::CbcAction
> MyEventHandler3::event(CbcEvent whichEvent)
> {
>   // If in sub tree carry on
>   if (!model_->parentModel()) {
>     if (whichEvent==solution||whichEvent==heuristicSolution) {
>       printf("value of solution is %g\n",model_->getObjValue());
>   } else {
>       return noAction; // carry on
>   }
> }

see above
>
> Thanks!
>
> Alexis
>
> -- 
> Alexis Guigue, E.I.T., B.Eng., M.A.Sc., Ph.D.
> Software Engineer/Research Engineer
> 604-519-6222, ext. 107
>
> Softree Technical Systems Inc. 
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