[Haskell-cafe] scheduling an alarm
briand at aracnet.com
Wed Jan 27 10:31:42 EST 2010
On Tue, 26 Jan 2010 22:41:44 -0800
Thomas DuBuisson <thomas.dubuisson at gmail.com> wrote:
> Brian Denheyer <briand at aracnet.com> wrote:
> > On Tue, 26 Jan 2010 10:54:03 -0800
> > Thomas DuBuisson <thomas.dubuisson at gmail.com> wrote:
> > > > doEvent f usDelay = forkIO $
> > > > threadDelay usDelay
> > > > doEvent f usDelay
> > > > f
> > Are you sure that's right ? It seems to be a memory-gobbling
> > infinite loop...
> Infinite loop? yes, that is what you wanted. Memory gobbling? Why
> would you think that? Are you assuming no TCO and a full stack push
> on every function call? Haskell compilers don't work that way.
Why would I think that ?
I think that because the following code:
f = putStrLn "foo"
doEvent f usDelay = do forkIO $ threadDelay usDelay
doEvent f usDelay
_really_ _does_ start to consume all of the memory on my system, that's
why. I don't know why, but that's what it does on my system. It's not
obvious to me that it should do that. So maybe ghci is not doing TCO.
> 1) There are a million ways to do this - why does one working make you
> suspicious of another? You can always test the other - it does work
> so long as you fix the missing 'do'.
The "suspicious" part, that was a joke.
Having something that works is nice, making sure I understand it is
better. I haven't used the Concurrent stuff before, so I'm just
> 2) It strikes me as funny you suspect the first way when there is zero
> fundamental difference between that and the way you posted except
> that: a) My version maintains the correct delay.
> b) My version forks the doEvent call and runs the action in the older
> thread while yours forks the action thread and keeps the doEvent in
> the older thread. I suppose keeping the doEvent as the old thread is
> good so you can kill it with the original ThreadID that would be
> returned to the caller.
Thanks for the explanation, as I said I'm a little fuzzy on what
constitutes a thread, so the two versions will help.
One interesting thing I noticed in the docs (which is not important
for what I am trying to do, just interesting):
There is no guarantee that the thread will be rescheduled promptly when
the delay has expired, but the thread will never continue to run
earlier than specified.
I expect that this means "promptly" in the sense of not accurately,
however I wonder what the upper bound on the (potential) delay is ?
> Some people miss the fact that threadDelay is a us value and an Int
> type - this limits the maximum delay to something like 35 minutes
> (assume a 32 bit Int) or even just 134 seconds if you go by Haskell
> 98 minimum of 27 bit Ints. Just making sure you realize this seeing
> as we are talking about delays in that order of magnitude. I advise
> the overly-complex but functional Control-Event package if you want
> longer delays.
I did notice that the delay is in us. 35 minutes is plenty of time. I
can always delay twice ;-)
Thanks for pointing me to control event, that looks useful !
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