[fpc-devel] Multithreading wait for

[Mattias Gaertner]

On Wed, 03 Dec 2008 15:41:55 +0000
Martin Friebe <fpc at mfriebe.de> wrote:

> Mattias Gärtner wrote:
> > Hi all,
> >
> > I'm looking for a high performance 'wait-for'.
> >
> > The situation:
> > There are m threads working in parallel on n chunks of work (n>=m).
> > The n chunks are indexed 0..n-1.
> > Sometimes one thread needs the result of some of the lower indexed
> > chunks. For example the chunk number 5 needs chunks 0..2.
> > So I have a function WaitForIndex(LowerIndex: integer) which should
> > wait until all chunks with less or equal to LowerIndex have
> > finished.
> >
> > My current approach is this:
> > EnterCriticalSection (try..finally)
> > Check if all lower chunks have finished, if yes then exit
> > LeaveCriticalSection
> > RTLeventWaitFor(AnEventOfTheCurrentThread);
> >
> > And each time a thread has finished a chunk, it wakes all
> > corresponding threads.
> >
> > The problem is the gap between LeaveCriticalSection and
> > RTLeventWaitFor. During this time the other threads may finish. So
> > there is no one left to wake up the waiting thread.
> >
> > Moving the RTLeventWaitFor into the CritialSection creates a
> > deadlock. I can use the timeout of RTLeventWaitFor and check in
> > intervals, but criticalsections and low response times don't fit
> > together.
> >
> >   
> "high performance 'wait-for'. "
> First think that came to mind was Spin-Locks, so you never enter a
> wait state. But that only works, if you know you will never have to
> wait for long.

Yes. Maybe doable with sleep.

 
> Another possibility is a set of Semaphores. Each thread needs one 
> semaphore. You can use zero wait on all semaphores of "lower"
> threads. Each thread sets it sem to 1 while working and to zero once
> finished.
> 
> You can reverse the logic: 0 eq thread is busy,  1 (via increment,
> not via set-value) means thread is done. Then the waitng thread tries
> to decrement all semaphores

Thanks, I will try.

Mattias