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 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-
mance.
COMMUNICATION
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
Page 1 Plan 9 (printed 12/21/24)
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
exit.
SOURCE
/appl/grid/demo/blur.b
/appl/grid/demo/block.b
Page 2 Plan 9 (printed 12/21/24)