[Haskell-cafe] Thread priority?

Neil Davies semanticphilosopher at googlemail.com
Sat Apr 25 05:09:28 EDT 2009

Count me in too

I've got a library that endeavours to deliver 'rate-equivalence' - i.e  
there may be some jitter in when the events should have occurred but  
their long term rate of progress is stable.

Testing has shown that I can get events to occur at the right time  
within 1ms (99%+ of the time, with 50%+ of the events < 0.5ms) -  
choose your O/S carefully though. And this is without resorting to  
marking the threads as real time with the kernel. If it matters you  
can tune the scheduling to further reduce the timer scheduling latency  
(at the cost of more CPU) and increase the fidelity of event timing  
another order of magnitude or more.

As for 'garbage collection ruins my real time properties' argument -  
I've been pragmatic about this, you can configure the scheduler to go  
and perform a GC if it knows that it is going to sleep for a while  
(yes, this doesn't resolve the issues of external events arriving in  
that time but read on) but not perform that too often. Turning off  
GHCs timers (to stop it garbage collecting after it has actually been  
idle for a while) and having code that doesn't create vast amounts of  
garbage means that a GC takes <0.1ms.

Now if I could see within Haskell the amount of heap that has been  
recently mutated and some other GC statistics when it was running I  
could even decide to schedule that more effectively.

We use this to create synthetic network traffic sources and other  
timing/performance related activities in the wide area context - it  
serves our needs.


On 25 Apr 2009, at 03:48, Christopher Lane Hinson wrote:

> Is there any interest or movement in developing thread priority or  
> any other realtime support in Haskell?
> Right now, if I have tasks that need to be responsive in real time,  
> even if the realtime needs are very soft, it seems that the only  
> option is to try to ensure that at least one hardware thread is kept  
> clear of any other activity.
> To be very useful to me, thread priority would not need to come with  
> very strict guarantees, but there would need to be a way to make  
> sure that `par` sparks and DPH inherit the priority of the sparking  
> thread.
> In part I ask because I'm working on a small library to support a  
> degree of cooperative task prioritization.
> Friendly,
> --Lane
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