[Nomen-dev] Cilk

[Brian H. <bh...@sp...>]

I've done some reading on Cilk, and have my preliminary responses:

It's an interesting idea and very usefull, in a certain domain of
problems.  I am, in fact, strongly thinking of using it for another one of
my projects (which is in that domain).  It is not a general purpose
threading paradigm, however.  

1) Their response to data race conditions is the same as the old joke
"Doctor, it hurts when I do this!"  "Well, don't do that, then!"  Indeed,
several of their theoretical papers say that in the precesence of locks,
their proofs of perfection go out the window.  I notice a distinct lack of
producer-consumer or shared-resource problems among the programs they brag
about- it's all programs that are very easy to split up the data among
different processes and run with no synchronization whatsoever.

2) Their model of multithreading doesn't provide real asynchronicity, or 
any sort of latency bounding.  This makes it unsuitable for anything where 
you need to interrupt a computation- for example, what GUIs use 
multithreading for (allowing one thread to go off and do some expensive 
computation while another thread polls for GUI events, and can abort the 
expensive computation when the user hits cancel).  In cilk on a 
monoprocessor, the expensive computation would start and not let the GUI 
thread have any clock cycles until it was finished- by which point the 
user has aborted the program and rebooted the computer, because obviously 
the program locked.  Even in a SMP/SMT environment, the expensive 
computation could (should) easily fill up all the processors with sub 
parts of it's expensive computation and the same problem exists.

3) On the other hand, for problems in it's domain, cilk-style 
multithreading is within a loud shout of the optimal solution.  Capable of 
keeping a large number of processors busy, and with near-optimal 
scheduling.  For the problems it serves, it's the best at serving them.  
Which means that it may be good for Nomen to support more than one style 
of multithreading- old style "he-man" threads, and cilk-style computation 
threads.  The disadvantage of this is that it makes for a new style of bug 
(using the wrong type of threading).

Brian