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T-man
10-15-2012, 04:29 AM
Hello everyone,

So I was thinking the best way to go at this is just attach my code and explain what is going on.

http://www.2shared.com/file/qJPty_V1/hello.html
http://www.2shared.com/file/ShsAHPXx/defs.html
http://www.2shared.com/file/1b97qaQd/BCkernel_test.html (I cant seem to have attach option on this forum).

So what I am doing is I want to parallelize the betweenness algorithm of Barnes.

For the example: ./hello_world -parallel -grid 3 4 with two work_groups(6 threads per workgroup) the following graph is constructed
0 1 2 3
4 5 6 7
8 9 10 11 so a grid.

Now the value of sigma for node 0 is 1, and for all other nodes is 0. The kernel computes the sigma for all the other nodes in parallel, which means that sigma of a node will be the sum of his own sigma and the sigma of the root.

So sigma of node 1 and 4, will be value 1, sigma of node 5 will be 2 etc.

Now the problem is that in the while loop "while(count_priv<nr_roots)" count_priv will be initially 0 and it so happens that once every 4-5 runs of the program, count_priv does not get incremented for one of the work_groups. So what happens is that the iteration gets done with count_priv = 0 , gets incremented and when the while test (while(count_priv<nr_roots)) is done the value is still 0. Any ideas why that might be?

Thank you for your time and hope someone has a solution for me!

notzed
10-15-2012, 02:11 PM
Well i notice you're not using a local barrier at the start of the loop (the first 2 global barriers). Remember global barriers only work within the workgroup - they are not 'global'.

Semaphores in general are problematic for gpu code since work-items within a workgroup are not independent threads - see the archives.

T-man
10-15-2012, 02:47 PM
I know, but that's not what is wrong. I know exactly what's wrong, but it's really hard to explain exactly. What are my options? I really need to get this done, its master thesis dependent :(. Do you know anyone that I can talk to ? Thanks!

notzed
10-15-2012, 10:44 PM
I know, but that's not what is wrong. I know exactly what's wrong, but it's really hard to explain exactly. What are my options? I really need to get this done, its master thesis dependent :(. Do you know anyone that I can talk to ? Thanks!

If you know "exactly what's wrong", then why did you ask why it isn't working?

That's no way to get help.

Anyway - masters or homework, any school work is yours and yours alone.

T-man
10-16-2012, 12:50 AM
Well the problem is that the program has a behavior that I dont understand. I just need someone that can help me in finding the misunderstanding. Thank you for your time!