[Haskell-cafe] Real-time code in Haskell (Was: Can a GC delay TCP connection formation?)

timothyhobbs at seznam.cz timothyhobbs at seznam.cz
Tue Nov 27 20:52:09 CET 2012

What triggers GC in haskell?  We obviously aren't using Java's method of GC 
as needed(for good reasons, Java's method is terrible because you get slow 
downs when you need speed the most).  But we should be able to learn 
something from Java and have a gc::IO() method that one could call BEFORE a 
critical region of code...

---------- Původní zpráva ----------
Od: Mike Meyer <mwm at mired.org>
Datum: 27. 11. 2012
Předmět: [Haskell-cafe] Real-time code in Haskell (Was: Can a GC delay TCP 
connection formation?)
"On Tue, Nov 27, 2012 at 3:45 AM, Gregory Collins
<greg at gregorycollins.net> wrote:
> If you have a hard real-time requirement then a garbage-collected
> language may not be appropriate for you.

This is a common meme, but frankly, it isn't true. When writing
real-time code, you just need to make sure that everything that
happens takes a known maximum amount of time. Then, you can sum up the
maximums and verify that you do indeed finish in the real-time window
of the task.

GC is a problem because it's not predictable, and may not have a
maximum. However, it's no worse than a modern version of the C
function malloc. Some of those even do garbage collection internally
before doing an OS call if they're out of memory. The solution is the
same in both cases - make sure you don't do GC (or call malloc) in the
critical region. Both require knowing implementation details of
everything you call, but it isn't impossible, or even particularly

Lazyness, on the other hand ... I haven't thought about. I suspect you
need to force the evaluation of everything you're going to need before
you start the critical region, but I wonder if that's enough? Has
anyone out there investigated this?


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