Re: General question about GNUStep

[David Chisnall]

Hi Simeon,

On 27 Aug 2009, at 08:54, Simeon Mitev wrote:

> Nice Nicola,
>
> Thank you for the good presentation!
>
> I have got answers for some of my questions. What I am still  
> interested in is multi threading support in objC compiler and  
> runtime. While the server application will run on unix with 2 quad  
> core processors I will definitively need to use multi threading to  
> get a real benefit from the hardware. In C++ I can use also openMP (http://openmp.org/wp/ 
> ) in example. Thoughts ?

OpenMP is intended to allow things like loops to be parallelized  
across threads.  It defines a series of pragmas that the user can use  
to provide hints to the compiler.  If you are going to use it, make  
sure you understand the characteristics of the target machine.  It is  
very easy to use OpenMP to make slower code than without using it if  
you do not understand the target's memory architecture, cache  
coherency model, and scheduling system.

Hyperthreading is Intel's brand name for their chip multithreading  
implementation, which splits the execution units between two  
contexts.  You absolutely and categorically do not want to be using  
OpenMP to parallelize loops like the one you gave as an example on a  
machine with hyperthreading, because it is likely to have very few  
cache misses (the compiler should be inserting prefetch instructions  
at the right points in the loop) and so you will just be confusing the  
operating system scheduler and starving one of the threads of  
execution units.

There are currently two Objective-C compilers that you can use with  
GNUstep; gcc and clang/llvm (llvm-gcc only works on OS X).  GCC  
supports OpenMP, Clang does not.  Clang supports a number of Objective- 
C 2 constructs (e.g. fast enumeration, declared properties), GCC does  
not.

OpenMP attempts to implicitly create threads for you.  You can, of  
course, explicitly create threads if you wish. At the lowest level,  
you can use the POSIX theading APIs.  GNUstep provides NSThread on top  
of these, which gives a simpler abstraction, along with a number of  
support classes, like NSLock, NSCondition, and so on.  You can also  
use Distributed Objects between processes or threads, which provides  
quite a heavy-weight system for concurrency.  If your program consists  
of largely-independent parts, then you should consider splitting it  
into different programs and using DO for communication.  This makes it  
easy to add fault tolerance (you can easily restart a crashed process;  
a crashed thread will kill the whole program) and lets you scale to  
multiple machines in a cluster in future.

EtoileThread is part of EtoileFoundation, and provides another  
abstraction using higher-order messaging.  You can read a bit more  
about it here:

http://etoileos.com/news/archive/2008/07/29/0231/

David