[DGD] Multi Threading

[Felix A. Croes]

Kris Van Hees <aedil at alchar.org> wrote:

> Not to offer criticism (since I do believe that re-scheduling threads of
> execution is the right way to go - you convinced me of that in 1995 already),
> but isn't the worst case where one or more threads of execution may have to be
> rescheduled multiple times, due to getting undercut by other threads?  E.g. if
> all threads need to access a central object during their execution, it could be
> possible that thread n will get rescheduled (n - 1) times because all previous
> threads modifying that single central object?  It does of course depend on the
> scheduling algorithm, and it is very easy to guarantee at least completion of
> all threads, but it seems that the possible impact of a high degree of
> collisions in accesses (bad design, but nonetheless) could be quite a bit higher
> than sequential threads on a uni-processor driver.

The system guarantees that of all LPC threads running at any one time,
at least one will complete successfully.  Thus the worst case is
equivalent to a single-threaded system which executes LPC threads in
the same order of completion, but sequentially.

It is true that while the system as a whole still progresses, individual
threads may be rescheduled a number of times, and thereby delayed.
Making sure that eventually they do complete is not that hard a problem.
The most simple solution would be to run a thread which has been
rescheduled some given number of times in single-threaded mode.  That
would still not affect the worst case for the system as a whole.

Of course, there is some overhead for scheduling, making copies of
objects to work on for each thread, and so on.  But if that overhead
were significant, I would be doing a bad job as a programmer.

Regards,
Dworkin
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