[Haskell-cafe] Timing out a pure evaluation of an expression I did not write myself

Carter Schonwald carter.schonwald at gmail.com
Tue Nov 27 00:13:38 UTC 2018

so one way to handle no allocation things playing nicely with the scheduler
is to compile haskell code with -fno-omit-yields

this will generate code which has scheduler yields even in loops which dont


On Mon, Nov 19, 2018 at 3:27 PM Viktor Dukhovni <ietf-dane at dukhovni.org>

> On Mon, Nov 19, 2018 at 11:26:17AM -0800, Ryan Reich wrote:
> > I suppose my question concerns the more general question of how to create
> > OS-managed threads in GHC.  [...]
> > I would have expected there to
> > be a corresponding operation in GHC Haskell ("bound threads" seem not to
> be
> > it, as they are still scheduled by the RTS) but it does not appear that
> > there is.  Is this because of the need to keep the runtime unified?
> > Because it seems strange that we are prevented from operating truly
> > independent threads.
> I was just reading:
>     https://cs.nyu.edu/~mwalfish/classes/14fa/ref/boehm05threads.pdf
> which may offer some insight.  The runtime needs to be able to
> provide a consistent memory model to threads.  Just running something
> in an OS thread that's managed by the RTS could make that difficult,
> and it is not clear how that cooperates with garbage collection.
> But to your point, when adding threads to your example, I find that
> the infinite loop then runs concurrently in all the threads, and
> the timeout never happens.  While:
>     Replacing:   let x = 0:x in last x
>     with:        let ! x = 0 + x in x
> does make timeout work, so it is not entirely obvious which pure
> computations can be timed out.
> --
>         Viktor.
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