TotalView Users Guide - CI Wiki

Debugging OpenMP Applications
� The compiler splits variables between the original routine and the outlined
routine. Normally, shared variables reside in the master thread’s
original routine, and private variables reside in the outlined routine.
� The master thread creates threads to share the workload. As the master
thread begins to execute a parallel region in the OpenMP code, it creates
the worker threads, dispatches them to the outlined routine, and then
calls the outlined routine itself.
About TotalView OpenMP Features
TotalView interprets the changes that the OpenMP compiler makes to your
code so that it can display your program in a coherent way. Here are some
things you should know:
� The compiler can generate multiple outlined routines from a single parallel
region. This means that a single line of source code can generate multiple
blocks of machine code inside different functions.
� You can’t single step into or out of a parallel region. Instead, set a breakpoint
inside the parallel region and let the process run to it. After execution
reaches the parallel region, you can single step in it.
� OpenMP programs are multi-threaded programs, so the rules for debugging
multi-threaded programs apply.
Figure 94 on page 134 shows a sample OpenMP debugging session.
About OpenMP Platform Differences
In general, TotalView smooths out the differences that occur when you execute
OpenMP platforms on different platforms. The following list discusses
these differences:
� The OpenMP master thread has logical thread ID number 1. The OpenMP
worker threads have a logical thread ID number greater than 1.
� Select or dive on the stack parent token line to view the original routine’s
stack frame in the OpenMP master thread.
� When you stop the OpenMP worker threads in a PARALLEL DO outlined
routine, the stack backtrace shows the following call sequence:
� Outlined routine called from the special stack parent token line.
� The OpenMP runtime library called from.
� The original routine (containing the parallel region).
� On HP Alpha Tru64 UNIX, the system manager threads have a negative
thread ID; since these threads are created by OpenMP, they are not part of
the code you wrote. Consequently, you should never manipulate them.
� On HP Alpha Tru64 UNIX and on the Guide compilers, the OpenMP
threads are implemented by the compiler as pthreads. On SGI IRIX, they
are implemented as sprocs. TotalView shows the threads’ logical and/or
system thread ID, not the OpenMP thread number.
� SGI OpenMP uses the SIGTERM signal to terminate threads. Because
TotalView stops a process when the process receives a SIGTERM, the
OpenMP process doesn’t terminate. If you want the OpenMP process to
terminate instead of stop, set the default action for the SIGTERM signal
to Resend.
TotalView Reference Guide: version 8.7 133