Messages in OpenMP

[Ibai Arrizabalaga]

Hi all,
I'm new on OpenMp and i would like to know if it's possible to make a communication between threads using messages in OpenMP. I've been reading many manuals but didn't find anything.
Thanks for help.

Ibai

[ejd]

I am not sure what you mean by "messages" - except in an MPI context. I am assuming that you want to do something like a "producer-consumer" program. With OpenMP, since memory is shared, you can pass information between threads using shared memory. You have to use flush, which is the tricky part. Not many people really understand how to use it correctly. Even the OpenMP experts have problems doing it correctly - though it can be done.

[Ibai Arrizabalaga]

First of all thanks for the answer.
The thing is, that i need to have a master thread and some slave threads in a specific part of my code. I know that I shouldn't use threads and UI all together but in this case I need to show a modal window in the middle of all those slave threads. To do that, i need those threads to send a message to the master so it can show the window making the other slave threads wait until this window is close to avoid any error. I have solved it using windows PostThreadMessage() and WaitMessage() functions and it works correctly, but I would like to know if OpenMP has any specific function to do this.

I have another question too: If I want a part of code been executed only by master thread I have to use #pragma omp master{ code } , but is there any directive for making the master thread NOT to execute that part?

Thanks again, and sorry for my English.

Ibai Arrizabalaga

[aksh_pai]

Hi
even am new to openmp n have some doubts regarding it......may i know incwhich platform your working....

thanks

akshatha

[Ibai Arrizabalaga]

I am working with Visual Studio 2005 C++

[ejd]

Ibai -

OpenMP doesn't have a facility to have multiple threads wait for some condition in a "nice" manner. You can use a shared variable and see if the value has changed, but it requires the threads to be active (and waste resource). You can also use locks, but then not all threads will restart immediately. This is a case where some functionality is really missing from OpenMP for it to really do what you want in a "nice" manner. You have to remember though, that OpenMP was originally designed to allow a user to add parallelism incrementally to their "workshared" program. Personally, I would say that this really meant loop based. With each release of a new spec, the types of programs that OpenMP can handle is increasing - but it still isn't the right thing for all types of parallel programs.

As for your second question, if you want a section of a program to be executed by all threads except the master thread then you have to do it with an if statement and a call to omp_get_thread_num().

Code: Select all

if (omp_get_thread_num() != 0) {
  ...
}


[aksh_pai]

Hi
Can i know from whehe cai i get openmp download to load in eclipse.........

thanks
akshatha

[Ibai Arrizabalaga]

Ibai -

OpenMP doesn't have a facility to have multiple threads wait for some condition in a "nice" manner. You can use a shared variable and see if the value has changed, but it requires the threads to be active (and waste resource). You can also use locks, but then not all threads will restart immediately. This is a case where some functionality is really missing from OpenMP for it to really do what you want in a "nice" manner. You have to remember though, that OpenMP was originally designed to allow a user to add parallelism incrementally to their "workshared" program. Personally, I would say that this really meant loop based. With each release of a new spec, the types of programs that OpenMP can handle is increasing - but it still isn't the right thing for all types of parallel programs.

Thanks ejd, that's just what i wanted to know, if it was posible or no. After seeing this I think I'll have to continue using windows threads messages with OpenMP, it may not be the best solution but it works for me.
Thanks for the help.

Ibai