I did actually construct a workaround where I compute in a separate (forked, via System.Posix) process to time it out, then recomputed in the main process to get the value. It's got a significant overhead but I'm not sure yet if that is a real issue.
Using just a separate rts thread is how the standard timeout works; if you mean that those guys use a separate OS thread that gets scheduled outside the rts, that would be perfect. I'm just faking that with the fork.
Hi Ryan,
I'm not quite sure of your use case, but lambdabot/mueval do something like this I think. From a brief look at the source [1] it seems they compute in a separate thread, then in another thread, wait until the timeout then kill the computation thread. Would something like that work for you?
Erik
I expected something like that. I'm all the way back in ghc-8.2.2, but I think what this really shows is that the flag is unreliable and version-dependent. Unfortunately there doesn't seem to be a precise specification of where yield points should appear, and therefore, where they might be not-omitted.
On Wed, Nov 28, 2018 at 09:39:01AM -0800, Ryan Reich wrote:
> Thanks for the suggestion, which Arjen made also. Unfortunately, it does
> not appear to help. See this simple program:
>
> -- Loop.hs
> import Control.Exception
> import System.Timeout
>
> main :: IO (Maybe Integer)
> main = timeout 100000 $ evaluate $ last $ repeat 0
> -- end
>
> With either GHC invocation "stack exec ghc Loop[ -- -fno-omit-yields]",
> running ./Loop fails to terminate (it should do so in 0.1s).
>
> Based only on the very terse description of that flag in the User's Guide,
> and its name, I think the problem is simply that GHC doesn't normally
> *generate* yields in that loop, so there's nothing not to omit.
It times out for me with GHC 8.4.4 on FreeBSD 11.2, and "ghc -O -fno-omit-yields"
and does not time out with "ghc -O":
$ cat /tmp/foo.hs
import Control.Exception
import System.Timeout
main :: IO (Maybe Integer)
main = timeout 1000000 $ evaluate $ last $ repeat 0
$ ghc -O -fno-omit-yields /tmp/foo.hs
[1 of 1] Compiling Main ( /tmp/foo.hs, /tmp/foo.o ) [Optimisation flags changed]
Linking /tmp/foo ...
$ time /tmp/foo
real 0m1.033s
user 0m1.025s
sys 0m0.008s
$ rm /tmp/foo
$ ghc -O /tmp/foo.hs
[1 of 1] Compiling Main ( /tmp/foo.hs, /tmp/foo.o ) [Optimisation flags changed]
Linking /tmp/foo ...
$ time /tmp/foo
^C^C
real 0m5.864s
user 0m5.857s
sys 0m0.000s
On MacOS X with GHC 7.10.3, it does not time out either way. Perhaps
some versions of GHC don't make the timeout possible.
--
Viktor.
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