[Haskell-cafe] Re: multithreading speedup

[Simon Marlow]

Fawzi Mohamed wrote:
> 
> Il giorno Apr 14, 2007, alle ore 2:45 PM, Sebastian Sylvan ha scritto:
> 
>> I think you should probably consider the extremely lightweight forkIO 
>> threads as your "work items" and the GHC runtime as your thread pool 
>> system (it will find out how many threads you want using the RTS 
>> options and distribute it for you). If you're worried about memory 
>> efficiency you can tweak the initial stack sizes for threads etc. 
>> using runtime options.
>>
>> It's still true that you don't want to fork off trivial computations 
>> in a separate thread, BUT that's true for manual work item queues as 
>> well (you'd want each work item to be a substantial amount of 
>> computation because there is overhead per item). E.g. if you have a 
>> list you might not want one thread per element (and you wouldn't want 
>> one work item per element either) if the per element tasks are fairly 
>> trivial, so you'd first group the list into chunks, and then let each 
>> chunk be a work item ( i.e. spawn a forkIO thread to process it).
> 
> yes, but to build the optimal chunk size one would like to know the 
> number of working threads.
> So again, any way to know it at runtime? or it is  a bad practice to ask?

There's no way currently, but it would be a useful thing to have.

Cheers,
	Simon