BLUR(1)                                                   BLUR(1)

          blur - an example program to demonstrate splitting a task
          over several machines.

          grid/demo/blur [imagefile]              - master process

          grid/demo/blur                          - slave process

          Blur is a small program that works in two parts to manipu-
          late an image(6). The master process takes an image file as
          an argument and displays the image in a window on screen
          whilst waiting for and displaying results from the slave
          processes. Each slave process takes a block of the image at
          a time and blurs it, reduces the contrast and overlays the
          result of a simple edge detection analysis.

          The only requirement for a master and slave process to work
          together is that they both have a common /tmp directory.
          Within this a /blur directory is created and used by both
          processes. All communication and synchronisation is done
          through files in this directory. There can be many slave
          processes running concurrently in order to improve perfor-

          All the communication takes place through files in the
          /tmp/blur directory which is common to all the processses.
          Once the master process has started, it creates:

          image.bit        the image being processed

          data.dat         processing parameters e.g. block size, blur
                           radius etc

          working          tells slave processes to continue reading
                           todo/ directory

          todo/            contains files showing which blocks still
                           need processing

          Within the todo/ directory, the master process creates a
          file for each block to be processed. Starting at block.0.a,
          the name of each file denotes the block number and the ver-
          sion id. The version id is used to indicate the current ver-
          sion of a block being worked on. This is to allow the master
          process to ask another slave process to work on a block if
          it believes the current one has crashed or is taking too

     BLUR(1)                                                   BLUR(1)

          long. Once a block has been processed, the master removes
          the file from todo/.

          Each slave process reads the list of files in todo/ and for
          each block in turn, attempts to create a directory with the
          same name e.g.  block.1.a/ in /tmp/blur.  It is not possible
          to create a directory if it already exists and as the slave
          will only process blocks for which it has been able to cre-
          ate a directory, no two slave processes can be simultane-
          ously working on the same block. Once a slave has managed to
          create a directory, it processes the relevant block of the
          image and writes the result to img.bit in the directory.
          Having completed a block, the slave creates a file called
          done in order to let the master process know that it can
          read the completed result.

          The master process keeps track of all the blocks being pro-
          cessed and if any of them have not been completed after a
          certain time, it creates a new file for that block in todo/
          with a new version id. This guarantees that all blocks will
          be processed as long as at least one working slave process
          remains. Once all the blocks have been processed, the master
          process removes all the files created in /tmp/blur.  Each
          slave, upon seeing that the working file has gone, will then