StackThreads/MP: version 0.77 User's Guide

3.9: Workers and Worker Groups

A worker refers to an underlying OS-thread such as a thread of Solaris Thread library or Pthreads library. Normally, StackThreads/MP program creates a fixed number of workers at program initialization and they share threads created by st_main, directly or indirectly. In this simple use of StackThreads/MP, you do not have to understand the following procedures and do not even have to know the notion of workers or worker groups.

StackThreads/MP allows other ways to access the provided multithreading facility. Specifically, you can dynamically create another group of workers and add workers to an existing group. Creating a group of workers dynamically is useful in several circumstances. First, when you want to have multiple groups of workers and to keep them isolated; threads are shared only among workers within a single group and threads are never migrated from one group to another. Second, creating a group of workers at runtime allows you to increase or decrease the number of OS-threads (the number of threads visible to OS) according to the application's need. In particular, an application can spend most of the time with a single thread and create a group of workers to perform a compute-intensive task in parallel. When the task is finished, all workers will die and the application goes back to its single-threaded state. Actually, such an application could be built without dynamically creating worker groups, if you do not care about the fact that many idle threads may consume CPU time when the application has no parallelism. Third, unlike other StackThreads/MP procedures, the procedure that creates a worker group (stf_create_worker_group) can be called from programs that is not compiled by stgcc. Thus, it can be used as a callback from library whose source is not available.