Debugging Chapel Programs

This document discusses support for debugging your Chapel program and a set of experimental flags and configuration constants to enable task monitoring and memory tracking.

Running in gdb

The compiler-generated executable has a --gdb flag that can be used to launch the program within a gdb session. For best results, make sure that your program has been compiled using the chpl compiler's -g flag. With CHPL_COMM=gasnet when using the amudprun launcher, you can launch gdb by setting the environment variable CHPL_COMM_USE_GDB when running the program. This will open up a separate xterm for each locale, running the designated debugger. On the Mac OS X (darwin) platform, you can launch lldb instead, by setting the CHPL_COMM_USE_LLDB environment variable.

The utility of this feature depends greatly on your familiarity with the Chapel generated code. However, if your program is crashing or running into a runtime error, you can often determine where that is taking place by looking at a stack trace within gdb.

When debugging Chapel, it is useful to know that in generating its code, the Chapel compiler renames user identifiers. By default, the Chapel compiler munges all user identifiers, such that a variable named x would be code generated as x_chpl. This munging can be controlled using the --[no-]munge-user-idents flag (see the chpl man page for more information). In some cases, additional munging may be required or applied that cannot be turned off.

The net effect of this is that Chapel variables can often be inspected using p name_chpl (or p name_chpl<TAB> in cases where the compiler has further renamed the variable). If the --no-munge-user-idents flag is used, p name or p name<TAB> should work in most cases.

Over time, we plan to improve our ability to debug the generated C code for a Chapel program. If you find yourself debugging the generated code a lot and need help or have requests for better support, please let us know so that we can prioritize accordingly.

Flags for Tracking Tasks

For certain tasking layers, Chapel supports a few experimental capabilities for tracking the status of tasks, primarily designed for use in a single-locale execution. To enable this capability, your program must be compiled with the --task-tracking flag. These flags add a fair amount of runtime overhead to task-parallel programs. The flags are as follows:

-b, --blockreport
 When <CTRL-C> is entered during a program executing under this flag, it will display a list of where tasks are blocked on a synchronization variable. Running with this flag will also cause the executable to attempt to automatically detect deadlock for single-locale executions. This is only supported with CHPL_TASKS=qthreads or CHPL_TASKS=fifo.
-t, --taskreport
 When <CTRL-C> is entered during a program executing under this flag, a list of pending and executing tasks will be printed to the console, giving an indication of which tasks are at which source locations. This is only supported with CHPL_TASKS=fifo.

Configuration Constants for Tracking Memory

Chapel supports a number of configuration constants related to dynamic memory allocation for the compiler-generated executable, currently designed for use primarily by the development team to track memory usage in tests. Please note that our generated code currently contains memory leaks, so you should not be surprised if your program requires more memory than it seems it should.

For full information on these configuration constants consult Memory.

A brief synopsis of these configuration constants is as follows:

--memTrack turn on memory tracking and enable reporting
--memStats call printMemAllocStats() on normal termination
 call printMemAllocsByType() on normal termination
--memLeaks call printMemAllocs() on normal termination
--memMax=int set maximum level of allocatable memory
 set minimum threshold for memory tracking
 file to contain all memory reporting
 if set, append final stats and leaks-by-type here