parallel_do? Parallel done!

parallel_do is a new TBB construct. It isn’t even in the Commercial Aligned or Stable releases; I had to install a Development release (tbb20_20080226oss) in order to get access to it.

The parallel_do construct is used when you don’t know how much data you have to process. parallel_do starts up tasks from a list, but these tasks can add further work to the list. parallel_do only shuts down when the list is empty and all the tasks are done.

Parallel sorting

After my problems with parallel_scan, I approached parallel_sort with some trepidation. I was pleasantly surprised when parallel_sort worked as advertised. (I did have a few problems, but these were related to my C++ skills and not to TBB directly.)

parallel_scan works … kinda, sorta

In a previous post, I showed a program that uses the parallel_scan construct but gets the wrong result. Since then, I received a working, running sum example for parallel_scan (from Mike Deskevich of ITT) that I could poke at and observe the results. Doing that I found a mistake I was making, and I found a mistake that TBB is making.

Scanners? Aren’t those the guys that make your head explode?

Back in the 80’s, there was a movie called “Scanners”. Scanners were mutants that could think intensely about you with a very constipated look and veins standing out on their faces. Then your head would explode. That’s the way the parallel_scan construct makes me feel.

There is no example for parallel_scan in the TBB tutorial, but there is a terse explanation in the reference manual. Basically, parallel_scan breaks a range into subranges and computes a partial result in each subrange in parallel. Then, the partial result for subrange k is used to update the information in subrange k+1, starting from k=0 and proceeding sequentially up to the last subrange. Then each subrange uses its updated information to compute its final result in parallel with all the other subranges.