Script-Fu
=========

Friday 22 January 2010 07:04

Trowel is written around AnyEvent, an event loop
programming framework. That means the program is
devided in units that can continue to run without
waiting for each other. This is convenient when
we're using network connections that are apt to
hang - like twitter - so the parts that have
nothing to do with the network connection can keep
going independent of the state of the network
connection (in theory). The downside is that you
never know in what order the various units interact
and to prevent race conditions in which two or more
units try to change the same central data at the
same time the interaction has to be atomic. For
Trowel this means one unit pushes the collected
tweets onto an array while the other unit pops them
of to display them, so at my level of abstraction
the units don't have to explicitly wait for each
other to finish changing the contents of the array.
I assume there's one processor that is changing
values in memory and in one processor cycle the
push and the pop can't be done simultaneously.

With linux on Cell Broadband Engine hardware it's
possible to do awesome things. But the trick is
that the Cell consists of a rather slow processor
and about 6 super fast specialised processor units.
To create programs that utilize all the processors
it is required to write them about the same as with
an event loop. The program would require multiple
threads that try to keep all the specialized
processors busy, and the hard part is to manage the
data that is used in all those threads. because if
6 units want to pop or push the same array at the
same time you're suddenly looking at one of those
that will have to wait up to 5 cycles, and a bigger
chance that it should be affected by what the other
5 units have been doing.

Getting bored of some repetitive task in the Gimp I
decided to write a script that would do the same
task with the push of a button. The challange was
that it had to be done in Scheme. Which is a pain.
Just like with the Cell you're confronted with how
the underlaying hardware really works and not how
your brain works. After a while your brain starts
to work like the hardware though and it gets
easier, but it shows that writing optimized code is
still hard. And why a lot of code is still slow on
today's blazingly fast hardware...

by Roland van Ipenburg
http://www.xs4all.nl/~ipenburg/blog/posts/play/2010/01/22/script-fu/