[Haskell-cafe] Re: Joels Time Leak

Chris Kuklewicz haskell at list.mightyreason.com
Tue Jan 3 12:32:07 EST 2006

Thanks for the answer, but I should I written a longer comment. I have
added such a longer comment below:

Simon Marlow wrote:
> Chris Kuklewicz wrote:
>> Another comment: between 1000's of threads and writing a custom
>> continuation based scheduler, what about using a thread pool?  Does
>> anyone have a library with a "fork-IO-Pool" command?
> You don't need a thread pool, because threads are so cheap.  Thread
> pools are just a workaround for lack of lightweight concurrency.
> Cheers,
>     Simon

Since the round-robin scheduler has (0.02 * N) seconds of delay for N
therads, then one could trade off latency between time spent waiting for
the thread pool to start a job and time spend running the job and
getting interrupted.

In the limit of 1 worker thread, all the latency is waiting to get run,
and there are no interruptions, so the time taken *while running* is
very short.  With 10 threads, there can be a delay to start, and each
interruption adds 0.2 seconds to the job's run time once it has started.

For a server, the client requests queue up and wait for room in the
thread pool, and the pool is kept small enough that the
round-robin-schedular-delay keeps requests from timing out while being
serviced.  Otherwise 1000 client requests would cause 20 seconds of
reschedule penalty for all threads and they could all timeout.  With a
thread pool, one can drop threads that have been waiting for too long
instead of running them, so those threads will timeout. But the pool
keeps servicing at least some of the client requests on time.

All hypothetical to me, of course.


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