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BLUR(1) BLUR(1)
NAME
blur - an example program to demonstrate splitting a task over several
machines.
SYNOPSIS
grid/demo/blur [imagefile] - master process
grid/demo/blur - slave process
DESCRIPTION
Blur is a small program that works in two parts to manipulate 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 dis‐
playing 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 direc‐
tory is created and used by both processes. All communication and syn‐
chronisation is done through files in this directory. There can be many
slave processes running concurrently in order to improve performance.
COMMUNICATION
All the communication takes place through files in the /tmp/blur direc‐
tory 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/ direc‐
tory
todo/ contains files showing which blocks still need pro‐
cessing
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 version id. The version id is used to
indicate the current version 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 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 create a directory, no two slave processes can be
simultaneously 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 processed 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 exit.
SOURCE
/appl/grid/demo/blur.b
/appl/grid/demo/block.b
BLUR(1)
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