[fpc-devel] Light weight threads for FPC

[Mark Morgan Lloyd]

Mattias Gaertner wrote:
> On Fri, 14 Dec 2007 10:29:59 +0100
> Michael Schnell <mschnell at lumino.de> wrote:
> 
>> Mattias Gaertner wrote:
>>> Has someone already created a unit for light weight threads?
>>>   
>> What do you mean by "light weight threads" ? How can it get "lighter" 
>> than TThread, that offers close to no built-in
>> "comfort"-functionality ?
> 
> Hehe. I mainly choose the term to provoke.
> 
> TThread is a class and has more functionality than needed, but misses
> the ability to work in teams. For many parallel algorithms you don't
> need events, priority or synchronize. But you need to easily and fast
> start a set of threads with IDs 0..N. N can be computed from the
> number of cores or IO queues. But many algorithms simply compute N from
> the input size (e.g. N=n/log(n)). A pool with threads, each running and
> eating tasks in loops are 'lighter' than starting N real threads.
> For example starting a procedure "100 times in parallel" means on a dual
> core: Two threads execute the procedure 50 times. This is much lighter
> than 100 TThreads. And if one of the threads 'runs slower' than it will automatically
> load balance.
> 
> So, I hope that someone already implemented some 'light weight'
> threads for FPC and I can adapt and extend them.

On the other hand, particularly in the case where you are running a large 
number of iterations of a parallelised job it's useful to be able to time each 
thread so that the distribution of work can be optimised "on the fly".

As an example, looking at a big cellular automaton you might have a few 
hundred million cells and want to run for a billion or so generations. I think 
it's going to be significantly more efficient to have a set of N threads 
waiting for their worklists, on completion looking at the time each thread 
took and optimising the next generation than starting a pool of threads per 
generation.

One thing that would be useful would be a processor-affinity property on the 
basic TThread.

-- 
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]