At the time when I got into application development it was common that GUIs became unresponsive when
applications (3D renderers for example) where busy calculating, or that you could not go on scrolling
in a text while it was being saved. Multi-threading was the cure to this and
threads were used to
build multi-tasking applications. Having Threads as a feature in Java was a big plus for moving to
it from C++. Off-the-shelve PCs had no more than one processor back then. Multi-core was unknown.
It was sufficient to have one thread for the GUI, one for I/O and one or two for computations and
you easily had maxed out your machine. Later came thread pools, for multi-plexing database calls,
or scaling your web server to more clients.
But eventually, one thread per socket connection was no longer scaling well for a web server, either.
There were too many concurrent threads to be handled efficiently by the OS.
Every context switch between two threads has an overhead. That's when
event-driven frameworks like MINA came to rescue. A thread was only assigned to a connection when
there is data to process.
Today, multicore multi-CPU machines are very common.
So we no longer need to optimize for our one-and-only processor.
Instead, we need to optimize for threads running truely in
parallel on the same machine.
New sorts of bottlenecks start to appear: One of them is sharing
variables between threads. Access to memory happens in a processor's local memory, so state has to be copied around between main memory and CPU-local caches (I am simplifying here).
There is a fascinating presentation by Joe Armstrong about how to scale in the multi-processor world
over at InfoQ.
He also wrote a book about it (amazon.de link).
It's about Erlang, a computer language optimized for running on lots of processors and lots of machines.
It is many years old and used in production, but only became better known to a wider public in the last years.
And if you don't have enough of it then already, listen to the Erlang interview
with Joe Armstrong by se-radio's Markus Voelter.
