Multicore Watershed Delineation

MapWindow version 4.8 adopted version 5 of the TauDEM software. TauDEM, Terrain Analysis Using Digital Elevation Maps, is the main component of the MapWindow Watershed Delineation plugin.

TauDEM version 5 is designed for modern multicore architectures. A multicore CPU has more than one processor: 2, 4, even more. By using an implementation of the Message Passing Interface (MPI), TauDEM version 5 can take advantage of multiple cores, splitting tasks into several processes distributed between them, and speeding up calculations. It is also able (provided your machine has enough memory) to process much bigger DEMs: up to 4GB.

The implementation of MPI used by TauDEM is MPICH2 from the Argonne National Laboratory in the USA. MPICH2 is free, open source software. It is a little tricky to install on Windows, and not all users will need it, so the approach adopted for MapWindow is as follows:

• By default MPICH2 is not installed. TauDEM will run without it, in "single process" mode. Note that in this mode you may initially see Windows Security Alerts, as illustrated below, for some of the TauDEM programs. Just click Keep Blocking (XP) or Cancel (Vista and 7).
• You may obtain and install MPICH2, and we give some instructions below. Then MapWindow will discover that MPICH2's program manager is running, and will take advantage of it. There is no point in doing this unless you have a multicore CPU, and probably not much unless you have large DEMs. To give an example, on a dual-core machine I managed to get a reduction of just over 30% on the 36 minutes need to calculate the D8 flow directions for a DEM with 36 million cells. The best number of processes for this calculation was 8.

Installing MPICH2

• Download the installer from the MPICH2 web site. The current stable release is 1.4.1p1: it is almost always advisable to choose the latest stable release. Make sure you get an installer for Windows. Get the 64-bit version if you have a 64-bit machine.

• MPICH2 supports multi-process computations across several machines. So you may get a Windows firewall query as to whether you want to allow the MPICH2 program manager, smpd, access to networks. Since TauDEM only runs multiple processes inside your machine you may Keep Blocking access (Windows XP) or Cancel access (Vista and Windows 7) (unless, of course, you are using MPICH2 for other purposes for which it needs network access).

  
  

• During installation you are asked for a secret word to be used for security in inter-machine communication. Again, for TauDEM you are not interested, so you can leave the default secret word of "behappy".

  
  

• You need to add MPICH2's bin folder to your Path environment variable. To do this, in Windows 7: select Start (the icon in the bottom left of your screen) -> Computer -> Properties -> Advanced System Settings -> Advanced -> Environment Variables -> Path (in System variables) -> Edit, and add ;C:\Program Files\MPICH2\Bin to the end of the Path. (The initial ";" is the separator between entries.) In Windows XP, select Start -> right click on My Computer -> Properties -> Advanced -> Environment Variables and then as in Windows 7.

  
  

• You need to register a program mpiexec, supplying it with your login password. To do this you need to start a command window in administrator mode. In Windows 7, select Start -> All Programs -> Accessories, right click on Command Prompt -> Run as Administrator. In Windows XP, select Start -> All Programs -> Accessories -> Command Prompt. Then use the command

  mpiexec -register

  At the prompts supply your user name (or just hit Enter if the default is you) and then provide and confirm your login password.

  
  

• The moment has come to check all is well. In the same command window, cd to C:\Program Files\MPICH2\examples, and try the command

  mpiexec -n 2 cpi

  cpi is a small program for calculating π. As you can see, this failed the first time for me. I tried

  smpd -install

  which reinstalled the MPICH2 program manager smpd, and then I was able to run the cpi program with 2 processes. Now it is ready for use with MapWindow: installation is complete.