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Distributed Debugging Tool (DDT) from Allinea Software is a parallel debugger installed on Edison and Cori.


DDT is a parallel debugger which can be run with up to 8192 processors. It can be used to debug serial, OpenMP, MPI, Coarray Fortran (CAF), UPC (Unified Parallel C) codes.

Please note that these 8192 license tokens are shared among NERSC computers and users.

Totalview users will find DDT has very similar functionality and an intuitive user interface. All of the primary parallel debugging features from Totalview are available with DDT.

The Arm Forge (previously, Allinea Forge) web page and 'Arm Forge User Guide' (available as $ALLINEA_TOOLS_DOCDIR/userguide.pdf after loading an allinea-forge module) is a good resource for learning more about some of the advanced DDT features.

Change of Module Name

As we are now using the package manger tool called Spack to install Allinea Tools, we have changed the module name to 'allinea-forge' from 'allineatools', following the standard Spack package name. Although the old name will still work for some time, we encourage users to switch to the new name as new versions will be installed under the new name and the old modules will be phased out.

Note also that software or tools installed with Spack go to the new installation location under /global/common/sw, instead of the previous location in /usr/common/software.

After the major OS upgrade on Edison in July, 2017, the old name is simply a symlink to the new name there.

Loading the Allinea Tools Module

To use DDT at NERSC, first load the 'allinea-forge' module to set the correct environment settings:

$ module load allinea-forge

Note that the name of the module has changed. The old module name 'allineatools' still works:

$ module load allineatools

Again, we encourage users to use the new module name.

Compiling Code to Run with DDT

In order to use DDT, code must be compiled with the -g option. Add the -O0 flag with the Intel compiler. We also recommend that you do not run with optimization turned on, flags such as -fast.

A Fortran example:

$ ftn -g -O0 -o testDDT_ex testDDT.f             # on Edison or Cori

A C example:

$ cc -g -O0 -o testDDT_ex testDDT.c             # on Edison or Cori

Starting a Job with DDT

Running an X window GUI application can be painfully slow when it is launched from a remote system over internet. NERSC recommends to use the free NX software because the performance of the X Window-based DDT GUI can be greatly improved. Another way to cope with the problem is to use Allinea's remote client, which will be discussed in the next section.

You can also start Be sure to log in with an X window forwarding enabled. This could mean using the -X or -Y option to ssh. The -Y option often works better for Mac OSX.

$ ssh -Y

After loading the allinea-forge module and compiling with the -g option, request an interactive session on Edison or Cori.

$ salloc -q debug -N numNodes 

Then launch the debugger with the ddt command followed by the name of the executable to debug:

$ ddt ./testDDT_ex

or, starting from the 5.x version,

$ forge ./testDDT_ex

The Allinea Forge GUI will pop up, showing a start up menu for you to select what to do. For basic debugging choose the option Run with the 'allinea DDT' tool. A user can also choose 'ATTACH'  to attach DDT to an already running program, or 'OPEN CORE' to view a core dump file from a previous job.

Then a submission window will appear with a prefilled path to the executable to debug. Select the number of processors on which to run and press run. To pass command line arguments to a program enter them in the aprun arguments box.

DDT Submit window

Note for UPC users: The current version (6.0.1-46365) has a problem with starting a UPC (Unified Parallel C) code built with the Cray compiler. A UPC code built with BUPC works fine. A temporary workaround is to set the environment variable ALLINEA_STOP_AT_MAIN to 1 before you run DDT. When DDT starts, it will say that it cannot find your source code. Go to the Project Files panel on the left side of the DDT window, and select your code.

Reverse Connect Using Remote Client

Note: Using the remote client of version 18.3 on a Mac can generate a segmentation fault.

Allinea provides remote clients for Windows, macOS and Linux that can run on your local desktop to connect via SSH to NERSC systems to debug, profile, edit and compile files directly on the remote NERSC machine. You can download the clients from Arm Forge download page and install on your laptop/desktop. Please note that the client version must be the same as the Allinea version that you're going to use on the NERSC machines.

First, we need to configure the client for running a debugging session on a NERSC machine. Start the client, and select 'Configure...' in the 'Remote Launch' pull-down menu. 

allinea remoteclient1

That will open the 'Configure Remote Connections' window.

Using the 'Add', 'Edit' and other buttons, create configuration for Cori, as shown in the following example.

arm forge remote client cori 183

Make sure that you uncheck the 'Proxy through login node' box.

The configuration settins for Edison should be as follows:

arm forge remote client edison 183

For versions prior to 18.3:

allinea forge remote launch settings cori


allinea forge remote launch settings edison

Note that in the Host Name field, we need enter two account entries for the Cray machines, one with the machine name itself and the other with one of its MOM nodes ('' in the above example).

Edison has 6 MOM nodes: edimom01, edison02, ..., and edimom06.

Cori's MOM node names are cmom02 and cmom05.

For the 'Remote Installation Directory', use the path for the default allinea-forge module. The value for the 'Remote Script' field should be exactly the same as shown above.

The Remote Installation Directory and Remote Script values for Cori for the new 'allinea-forge' module are /global/common/sw/cray/cnl6/haswell/allinea-forge/default and /global/common/sw/cray/cnl6/haswell/allinea-forge/remote-init, respectively.

For Edison the correct paths are /global/common/sw/cray/cnl6/ivybridge/allinea-forge/default and /global/common/sw/cray/cnl6/ivybridge/allinea-forge/remote-init.

To start a debugging session on a machine, you need to login to the corresponding machine. This choose the configuration for the machine from the same 'Remote Launch' menu.

allinea remoteclient3

You'll be prompted to enter the password even when you have set up passwordless connections by storing your ssh public key in NIM.

allinea remoteclient4

Starting with release 6.x, Allinea recommends to use the Reverse Connection method with the remote client. To do this, put aside the remote client window that you have been working with, and login to the corresponding machine from a window on your local machine, as you would normally do. Then, start an interactive batch session there, and run ddt with with the option '--connect' as follows:

$ ssh
$ salloc -N 1 -t 30:00 -q debug
$ module load alline-forge
$ ddt --connect srun -n 24 ./jacobi_mpi

The remote client will ask you whether to accept a Reverse Connect request. Click 'Accept'.

allinea reverseconnect1 

The usual Run window will appear where you can change or set run configurations and debugging options. Click 'Run'.

allinea reverseconnect2

Now, you can start debugging in the remote client:

allinea reverseconnect3

Trouble Shooting

If you are having trouble launching DDT try these steps.

Make sure you have the most recent version of the system.config configuration file. The first time you run DDT, you pick up a master template which then gets stored locally in your home directory in ~/.allinea/${NERSC_HOST}/system.config where ${NERSC_HOST} is the machine name: edison or cori. If you are having problems launching DDT you could be using an older verion of the system.config file and you may want to remove the entire directory:

$ rm -rf ~/.allinea/${NERSC_HOST}  

Remove any stale processes that may have been left by DDT.

$ rm -rf $TMPDIR/allinea-$USER 

In case of a font problem where every character is displayed as a square, please delete the .fontconfig directory in your home directory and restart ddt.

$ rm -rf ~/.fontconfig

Make sure you are requesting an interactive batch session on Edison or Cori. NERSC has configured DDT to run from the interactive batch jobs.

$ salloc -q debug -N numNodes 

Finally make sure you have compiled your code with -g. A large number of users who are having trouble running with parallel debuggers forget to compile their codes with debugging flags turned on. If none of these tips help, please contact the consultants at

Basic Debugging Functionality

The DDT GUI interface should be intuitive to anyone who has used a parallel debugger like Totalview before. Users can set breakpoints, step through code, set watches, examine and change variables, dive into arrays, dereference pointers, view variables across processors, step through processors etc. Please see the Arm Forge User Guide if you have trouble with any of these basic features.                              

Useful DDT Features

Process Groups

With DDT, the user can easily change the debugger to focus on a single process or group of processes. If Focus on current Processor is chosen, then stepping through the code, setting a breakpoint etc will occur only for a given processor. If Focus on current Group is chosen then the entire group of processors will advance when stepping forward in a program and a breakpoint will be set for all processors in a group.

Similary, when Focus on current Thread is chosen, then all actions are for an OpenMP thread. DDT doesn't allow to create a thread group. However, one can click the Step Threads Together box to make all threads to move together inside a parallel region. In the image shown above, this box is grayed out simply because the code is not an OpenMP code.

A user can create new sub-groups of processors in several ways. One way is to click on the Create Group button at the bottom of the Process Group Window. Another way is to right-click in the Process Group Window to create a group and then drag the desired processors to the group. Groups can also be created more efficiently using sub-groups from the Parallel Stack View described below. The below image shows 3 different groups of processors, the default All group, a group with only a single master processor Master and a group with the remaining Workers processors.

Parallel Stack View

A feature which should help users debug at high concurrencies is DDT's Parallel Stack View window found in the lower left area, which allows the user to see the position of all processors in a code at the same time from the main window. A program is displayed as a branching tree with the number and location of each processor at each point. Instead of clicking through windows to determine where each processor has stopped, the Parallel Stack View presents a quick overview which easily allows users to identify stray processes. Users can also create sub-groups of processors from a branch of the tree by right clicking on the branch. A new group will appear in the Process Group Window at the top of the GUI.

Memory Debugging

DDT has a memory debugging tool that can show heap memory usage across processors.

To access the memory debugging feature, you must first build your code for memory debugging. On Edison and Cori, you have to follow certain steps. Below is a table showing steps for building a static executable using different compilers for memory debugging on Edison and Cori. For the compilers other than PGI, the linking step is made of two parts. The first is to run in verbose mode using the -v flag to show all the linking steps taken. The second step is to rerun the last linker line after inserting some more options.

Compiler For static linking

% ftn -g -c prog.f
% ftn -v -o prog prog.o          # -v to get the last linker line

Rerun the last linker line after inserting '-zmuldefs' right after the command and putting ${DDT_LINK_DMALLOC} just before -lc:
% /opt/gcc/4.7.1/snos/libexec/gcc/x86_64-suse-linux/4.7.1/collect2 -zmuldefs ... ${DDT_LINK_DMALLOC} -lc ...


% ftn -g -c prog.f
% ftn -v -o prog prog.o

Do similarly as above:
% /opt/cray/cce/8.0.7/cray-binutils/x86_64-unknown-linux-gnu/bin/ld -zmuldefs ... ${DDT_LINK_DMALLOC} -lc ...


% ftn -g -c prog.f
% ftn -v -o prog prog.o

Do similary as above. There are two locations to put ${DDT_LINK_DMALLOC} as there are two -lc's:
% ld -zmuldefs ... ${DDT_LINK_DMALLOC} -lc ... ${DDT_LINK_DMALLOC} -lc ...

The example commands are shown for a Fortran case. cc and CC should be used similarly for C and C++ codes. In case of a C++ code, ${DDT_LINK_DMALLOCXX} is to be used instead of ${DDT_LINK_DMALLOC}.

A simple script, static_linking_ddt_md, is provided in your $PATH to help you complete the somewhat complicated steps shown above.

$ module load allinea-forge
$ ftn -g -c prog.f
$ static_linking_ddt_md ftn -o prog prog.o # instead of 'ftn -o prog prog.o'
$ ls -l prog
-rwx------ 1 wyang wyang 6701908 2012-10-15 15:19 prog

You need to separate the compile and link stages. That is, you need to create *.o files using the -c compile flag first; otherwise, you can see the following message:

/usr/bin/ld: cannot find /scratch/scratchdirs/wyang/ifortnr7R21.o: No such file or directory

 For multi-threaded codes, DDT_LINK_DMALLOCTH and DDT_LINK_DMALLOCTHCXX are used in place of DDT_LINK_DMALLOC and DDT_LINK_DMALLOCXX, respectively. Again, a utility script, static_linking_ddt_md_th, is provided to help with linking:

$ static_linking_ddt_md_th ftn -mp -o prog prog.o   # instead of 'ftn -mp -o prog prog.o' 

Below is a table showing how to prepare your code using dyanmic linking on Edison and Cori. The example is provided for a Fortran code case. Adjustments should be made for C and C++ codes as above. Again, in case of a C++ code, ${DDT_LINK_DMALLOC} must be repalced with ${DDT_LINK_DMALLOCXX} .

Compiler For dynamic linking
PGI, Cray

% ftn -g -c prog.f
% ftn -dyanmic -o prog prog.o ${DDT_LINK_DMALLOC} --Wl,--allow-multiple-definition

GNU, Intel

% ftn -g -c prog.f
% ftn -dynamic -o prog.o ${DDT_LINK_DMALLOC} -zmuldefs

For multi-threaded codes, ${DDT_LINK_DMALLOCTH} or ${DDT_LINK_DMALLOCTHCXX} should be used instead.

Next, when DDT starts, you must click the "Memory Debugging" checkbox in the DDT run menu that first comes up

DDT Groups

To set detailed memory debugging options, click the 'Details...' button on the far right side, which will open the 'Memory Debugging Options' window. There you can set the heap debugging level, the number of guard pages before or after arrays (but not both) for detection of heap overflow or underflow in the program, etc. The default page size is 4 KB.

DDT - memory debugging option

When running ddt with a statically built code, please deselect the 'Preload the memory debugging library' item. Otherwise, ddt can hang indefinitely during startup on Cray machines.

Also, leave the 'Preload ...' checkbox with a dynamically linked executable unchecked on the Cray machines if a C++ version of Allinea's dmalloc library was used (that is, when $DDT_LINK_DMALLOCXX or $DDT_LINK_DMALLOCTHCXX was used). Otherwise, ddt hangs during startup.

Several features are enabled with memory debugging. Select Current Memory Usage or  Memory Statistics under the Tools menu. With the following buggy code that generates memory leaks:

      program memory_leaks

!...  Buggy code prepared by NERSC User Service Group for a debugging tutorial
!...  February, 2012

      implicit none
      include 'mpif.h'
      integer, parameter :: n = 1000000
      real val
      integer i, ierr
      call mpi_init(ierr)
      val = 0.
      do i=1,10
         call sub_ok(val,n)
      end do
      do i=1,10
         call sub_bad(val,n)
      end do
      do i=1,10
         call sub_badx2(val,n)
      end do
      print *, val
      call mpi_finalize(ierr)

      subroutine sub_ok(val,n)      ! no memory leak
      integer n
      real val
      real, allocatable :: a(:)
      allocate (a(n))
      call random_number(a)
      val = val + sum(a)
!     deallocate(a)                 ! ok not to deallocate

      subroutine sub_bad(val,n)     ! memory leak of 4*n bytes per call
      integer n
      real val
      real, pointer :: a(:)
      allocate (a(n))
      call random_number(a)
      val = val + sum(a)
!     deallocate(a)                 ! not ok not to deallocate

      subroutine sub_badx2(val,n)   ! memory leak of 8*n bytes per call
      integer n
      real val
      real, pointer :: a(:)
      allocate (a(n))
      call random_number(a)
      val = val + sum(a)
      allocate (a(n))               ! not ok to allocate again
      call random_number(a)
      val = val + sum(a)
!     deallocate(a)                 ! not ok not to deallocate

you can easily see heap memory information (such as how much is being used, how much has been allocated, how much is freed, etc.), from which you can deduce where memory leaks occur. Below is a window shown when the Current Memory Usage menu is selected:

DDT - Current Memory Usage

It displays current heap memory usage of the program and the routines where it is allocated. Clicking on a histogram bar on the right, you will see the 'Allocation Details' box on the left filled up with information about where the memory allocation was made. By clicking on one of the pointers in the 'Allocation Details' list you can get information mapped to source code:

DDT - Pointer Details

It shows how much It is known that memory debugging can fail with the error message "A tree node closed prematurely. One or more proceses may be unusable.", especially with MPI_Bcast. A workaround is to disable 'store stack backtraces for memory allocations' option in the 'Enable Memory Debugging' setting. This problem will be fixed in the next release.

Offline Debugging

Offline debugging is to run DDT in a command-line mode, without using GUI. This mode may be useful if all you want is to get tracepoint (a specified location in the code where requested values are printed) output or stack backtraces without directly interacting with DDT. This can be good for a "parameter study" where you want to check for an error condition for a range of a parameter value, which would become a tedious task if GUI is used.

To run DDT in this mode, you submit a batch job using a batch script that looks like:

$ cat runit

module load allinea-forge
ddt --offline -o filename.html --np=4 myprogram arg1 ... # to get HTML output file
ddt --offline -o filename      --np=4 myprogram arg1 ... # to get plain text output file

$ sbatch runit

Please note that we are using 'ddt -offline ...' in place of 'srun' or 'mpirun' for launching an application. Output of the debugging session is saved in the specified file ('filename.html' or 'filename' in the above example).

Some options can be used for the ddt command:

  • --session=sessionfile: run using settings saved using the Save Session option during a previous GUI run session
  • --np=numTasks: run with numTasks (MPI) tasks
  • --mem-debug: enable memory debugging
  • --trace-at=LOCATION[,N:M,P],VAR1,VAR2,... [if CONDITION]: set a tracepoint at location LOCATION (given by either 'filename:linenumber' or 'functionname' as in 'main.c:22' or 'myfunction'), beginning recording after the N-th visit of each process to the location, and recording every M-th subsequent pass until it has been triggered P times; record the value of variable VAR1, VAR2, ...; the if clause allows to specify a boolean CONDITION that must be satisfied to trigger the tracepoint
  • --break-at=LOCATION[,N:M:P] [if CONDITION]: set a breakpoint at a location using the format explained above; the stack back traces of pausing processes will be recorded at the breakpoint before they are then made to continue

An example using the following simple code is shown below:

      program offline
!... Prepared for a debugger tutorial by NERSC
include 'mpif.h'
integer, parameter :: n = 24
real, allocatable :: a(:)
integer i, me
call mpi_init(ierr)
call mpi_comm_rank(mpi_comm_world,me,ierr)
allocate (a(n))
call random_number(a)
do i=1,n
if (mod(i,2) == 1) call sub(i,n,a) ! 'sub' called when i=1,3,5,...
end do
print *, me, sum(a)
call mpi_finalize(ierr)
subroutine sub(i,n,a)
integer n, i, j
real a(n)
do j=1,n
a(j) = cos(a(j))
end do

 The following is to set a tracepoint at the beginning of the routine 'sub' where values of i and a(1) are to be printed; and to set a breakpoint at line 23, using the activation scheme of '5:3:2':

ddt --offline -o offline.html --np=4 --trace-at=sub,i,a\(1\) --break-at=offline.f:23,5:3:2 ./offline

The output file is broken into three sections: Messages (showing process activities such as startup and termination etc., as well as call backtrace at breakpoints), Tracepoints (showing output from activated tracepoints), and Output (program output).

Introductory Video Tutorial

Watch the video.