TotalView

From HPC Wiki
Jump to: navigation, search

TotalView is a debugging software designed for high-scale HPC applications. It supports debugging serial as well as parallel applications. Supported parallel programming models include OpenMP, MPI, OpenACC and CUDA.
This page briefly describes how to debug serial and parallel programs written in C, C++ or Fortran using the TotalView debugger.

Debugging Serial Programs

Before debugging the program needs to be compiled with debug information using the -g flag and without any optimization (-O0 flag).

Starting TotalView

There are three ways to start debugging your program:

  • by starting TotalView with your program as a parameter
    $ totalview a.out [ -a options ]
    
  • by starting your program first and then attaching TotalView to it. In this case just start
    $ totalview
    
    . You will be asked what you would like to debug. Choose A running program (attach) and look for your already started program.
  • by analyzing the core dump after your program crashed using
    $ totalview a.out <corefile>
    

If the program requires startup parameters like runtime arguments, environment variables or standard IO these can be set in the ProcessStartup Parameters... menu.
After starting your programm Total View opens the Process Window which consists of different parts summarized by the table below:

Source Pane displays the program's source code
Stack Trace Pane displays the call stack
Stack Frame Pane displays all variables associated with the selected stack routine
Tabbed Pane displays all breakpoints, action points and evaluation points in the Action Points subpane
displays all (MPI) processes in the Processes subpane
displays all threads of the current process in the Threads subpane
Status Bar displays the status of the current process and thread
Toolbar contains all action buttons

Starting, Stopping and Restarting a program

A program can be started by selecting the Go button and stopped by clicking on the Halt button in the Toolbar. If a breakpoint is set the program will also stop upon reaching that breakpoint. It is also possible to select a program line in the Source pane and let the program run until it reaches that point during execution by clicking on the Run To button in the Toolbar. You can also manually execute the program line by line. By clicking on the Step button you can step into a function call and by clicking on the Next button you may jump over function calls. In order to leave the current function just use the Out button in the Toolbar. A program can be restarted with the Restart button.

Printing variables

The Stack Frame Pane provides values of simple variables. You can also search for a specific value using the ViewLookup Variable command. This will bring up a new window, where you need to enter the name of the variable you are interested in. The value of the variable as well as its address and its type are then shown in the Variable Window. Alternatively, you can also dive (middle click) on a variable to open the Variable Window.
If an array is examined in the Variable Window the Slice and Filter fields can be specified to only show a subset of all entries of the array. For example, Slice: 1:10:2 will show every second entry starting from index 1 and ending with index 10. Specifying Filter: > 30 will only show array entries that have a value larger than 30. Arrays can also be visualized using the ToolsVisualize command in the Variable Window as long as they array is one- or two-dimensional.
If you are examining a structure variable that contains other structures you can dive in the hierarchy of structures and navigate through it using the left and right arrow buttons in the top right of the Variable Window.

Action Points: Breakpoints, Evaluation Points, Watchpoints

A breakpoint can easily be set by clicking on a boxed line number of an executable statement in the Source Pane. To remove a breakpoint just click on the corresponding boxed line number again.
After creating a breakpoint, an Evaluation Point can be created by right-clicking on the STOP sign and selecting PropertiesEvaluate. Such an Evaluation Point can be used to temporarily add some program lines. Some basic examples are illustrated by the table below:

Additional print statement (FORTRAN write not accepted) printf("x = %f\n", x/20)
Conditional breakpoint if(i == 20) $stop
Stop after every 20 executions $count 20
Jump to program line 78 goto $78
Visualize an array $visualize a

A watchpoint can be used to monitor the value of a variable. Each time the value at the corresponding memory location of the variable is changed the program is stopped. Watchpoints can be created in the Variable Window using the ToolsWatchpoint command.

Memory Debugging

Different memory debugging features are offered by TotalView. Dynamically allocated memory can be guarded. If a memory access outside the boundaries of an allocated block occurs the program will stop. Memory can also be hoarded to avoid program crashes if the program accesses a memory block that has already been freed. TotalView can also detect memory leaks.
Memory debugging needs to be enabled before starting the debugging process using the DebugEnable memory debugging command. After that you can set a breakpoint and let the program run in to it. The memory debugging window can be displayed by the DebugOpen MemoryScape menu entry. In order to detect memory leaks select the Memory ReportsLeak Detection tab and choose either Source report or Backtrace report to get a list of leaking memory blocks.

ReplayEngine

Another interesting feature of Totalview is the ReplayEngine which records the program execution. As a result each program state can be restored afterwards. This feature allows to step forward and backwards through the code. So you can track errors causing a failure back from their occurence without restarting the application. In order to enable the ReplayEngine just click on the Record button in the Toolbar or use the DebugEnable ReplayEngine menu entry. Note that the program needs to be stopped for that. If the program was only loaded but not yet executed the ReplayEngine will start recording as soon as the process starts to execute the program. If the program is restarted the ReplayEngine will again record data from the beginning of the process execution. However, if the program is already executing but was halted enabling the ReplayEngine will only record execution data from that point on and only for a single time.
The ReplayEngine offers different commands in the Toolbar shown in the table below:

Record A toggle to enable/disable the ReplayEngine.
GoBack Displays the state that existed at the last action point or at the start of its recording if no action point is encountered.
Prev Displays the state that existed when the previous statement executed. Function calls are skipped over.
Unstep Displays the state that existed when the previous statement executed. In case of a function call moves to the last statement of that function.
Caller Displays the state that existed before the current routine was called.
BackTo Displays the program state for the selected line (must have executed prior to the currently displayed line).
Live Shift from replay mode to record mode.
Save Saves the current replay recording session to a file

Debugging Parallel Programs

Before debugging a parallel application you should first get familier with TotalView by debugging a serial toy program first.

Debugging MPI Programs

Starting TotalView

You can either start TotalView in the New Launch or the Classic Launch way.
The New Launch is similar to the launch of a serial program. After starting TotalView with your program as an argument the Startup Parameters window appears. In the 'Parallel' pane you can specify the Parallel system (e.g. Open MPI, Intel MPI) and the number of Tasks (i.e. the number of MPI processes to start).
If you want to specify these parameters from the commandline instead of in the GUI choose the Classic Launch way. Depending on the MPI implementation used the arguments that are passed to the mpiexec command may differ. For Intel MPI and Open MPI the flag -tv needs to be set as the following example shows:

$ mpiexec -tv -np 2 a.out < input

A dialog window may appear stating: "Process ... is a parallel job. Do you want to stop the job now?". Clicking Yes will open the TotalView debugger window with the source pane.
In order to display the source code of your MPI program look for the main function in the Stack Trace Pane and click on it.
By default the process with MPI rank 0 is selected for debugging. Clicking on another process in the root window selects another process. Alternatively, the buttons P- and P+ in the bottom right of the Process Window can be used as well. You can also open a separate Process Window by right-clicking on a process in the root window and selecting Dive in New Window.

Debugging of large jobs

For each MPI process a separate TotalView license token is consumed. These licenses may be limited at your site. Hence, it is recommended to reduce the number of MPI processes to a minimum. With only a few processes the debugging process becomes easier to handle. If your problem only occurs with a large number of MPI processes you can attach the TotalView debugger only to a subset of your MPI processes. Therefor navigate to the FilePreferencesParallel menu and under When a job goes parallel set the checkbox on Ask what to do instead of the default Attach to all. At the next start up of a parallel job a Attach Subset window will open where you can check the processes that TotalView should be attached to. This subset of processes can be changed at any time using the GroupAttach Subset menu.

Breakpoints

Breakpoints can be set in the same way as for debugging serial programs. However, for debugging parallel programs you can specify the properties of the breakpoint by right-clicking on it and selecting Properties. In the Properties Window you can specify whether the breakpoint will stop the whole process group or only one process. It is also possible to turn a breakpoint into a barrier to synchronize all processes by right-clicking the breakpoint and selecting Set Barrier.

Starting, Stopping and Restarting your Program

Program start, stop and restart are identical to debugging serial programs. However, you can choose to control the whole group of processes or only a single one using the drop-down menu on the left of the Toolbar.

Printing variables

In contrast to a serial program each process stores its own copy of a variable in its memory. Thus, a variable may have different values across different processes. These values can be examined by diving in a variable to open the Variable Window and then selecting ViewShow AcrossProcesses. Alternatively, you can right-click on a variable and select Across processes which has the same effect. A graphical visualization of these values can be displayed by selecting ToolsVisualize in the Variable Window.

Message Queues

You can have a look at outstanding message passing operations like pending sends and receives by inspecting the Message Queue which can be found under ToolsMessage Queue. There is also a visualization in form of a graph available under ToolsMessage Queue Graph. This graph can be useful to detect deadlocks in your program. Therefor open the Message Queue Graph Window and select OptionsCycle Detection.

Debugging OpenMP Programs

Before you start debugging an OpenMP program it is important to verify that the corresponding serial program runs correctly. Typical errors when parallelizing a program with OpenMP are data races. These errors are hard to detect with a debugging session because the timing behavior of the program is heavily influenced by the debugger. In this case it might be better to use a Thread Sanitizer tool.
Moreover, you should make sure that the program code is compiled with debug information (-g flag) and without any optimization because specifying the corresponding OpenMP compiler flag causes the compiler to automatically turn on optimization. The table below provides the corresponding flags for some commonly used compilers:

Intel compiler -qopenmp -O0 -g
GCC compiler -fopenmp -O0 -g
Oracle Studio compiler -xopenmp=noopt -g
PGI compiler -mp -Minfo=mp -O0 -g

Starting TotalView

In order to start a debugging session for your OpenMP program specify the number of threads you want to use and call the TotalView command with your program as an argument as follow:

$ OMP_NUM_THREADS=<nthreads> totalview a.out

The original source program is transformed by the interpretation of the OpenMP compiler directives. Each parallel region becomes a separate subroutine. Shared variables are passed as call parameters and private variables are defined locally. You cannot enter a parallel region by stepping into it but only by running into a breakpoint.
To switch between different threads in the Process Window you can either select another thread in the root window or by using the T- and T+ buttons in the bottom right of the Process Window.

Breakpoints

Breakpoints are set by clicking on a boxed line number in the Source Pane. For parallel programs you can specify the properties of a breakpoint. By default running your program into a breakpoint will stop the whole process (group). But it is also possible to only stop the thread for which the breakpoint is defined. Moreover a breakpoint can be used to synchronize all threads. Therefor right-click on the breakpoint and select Set Barrier.

Starting, Stopping and Restarting your Program

Program start, stop and restart work exactly like for serial programs. However, in a parallel OpenMP application you can choose to control the whole Group, the Process or the individual Thread using the drop-down menu on the left of the Toolbar.

Printing variables

In a parallel OpenMP program each thread may have an own copy of a variable. This is the case for private variables. Hence, the value of such a variable might be different across different threads. In order to examine these values you can dive in a variable to open the Variable Window. There you can select ViewShow AcrossThreads and will get a list of the values for each thread. Alternatively, you can right-click on a variable and select Across Threads which has the same effect. These values can also be visualized graphically by opening the Variable Window and selecting ToolsVisualize. Note that only one-dimensional arrays or array slices can be visualized in this way and that the thread ID is interpreted as an additional dimension.

Site-specific notes

RWTH

In order to use TotalView on the RWTH Cluster you need to load the corresponding module using the module system. The TotalView module is part of the DEVELOP module group. You can load this group with the following command:

$ module load DEVELOP

If you like to get an overview about the installed Versions use the command:

$ module avail totalview

To load a specific version of the TotalView module type:

$ module load totalview/<version>

By omitting the <version> the module system will load a default version of the TotalView module.