Strange performance effects with unsafePerformIO

[Sebastian Fischer]

2011/3/25 Thomas Schilling <nominolo at googlemail.com>:
> unsafePerformIO traverses the stack to perform blackholing.  It could
> be that your code uses a deep stack and unsafePerformIO is repeatedly
> traversing it.  Just a guess, though.

Sounds reasonable. Here is a variant of the program without intermediate lists.

    import System.IO.Unsafe

    main = run (10^5)

    run 0 = return ()
    run n = (unsafePerformIO . return) (run (n - 1)) >> return ()

I think it does not do much more than producing a large stack and
(like the original program) is much faster if the unsafe-return
combination or the final return (which probably prohibits tail-call
optimization) is removed.

Sebastian

> 2011/3/24 Björn Peemöller <bjp at informatik.uni-kiel.de>:
>> Hello,
>>
>> we have a strange performance behaviour when we use unsafePerformIO, at
>> least with GHC 6.12.3 and 7.0.1.
>>
>> Please consider the example program following at the end of this post.
>> Running the original code the execution time is about 26 seconds, while
>> uncommenting one (or both) of the comments shrinks it to about 0.01
>> seconds on our machine.
>>
>> Is there an explanation for this effect?
>>
>> Regards,
>> Bjoern
>>
>> -- ---------------
>>
>> module Main where
>>
>> import System.IO.Unsafe
>>
>> traverse []     = return ()
>> -- traverse (_:xs) = traverse xs
>> traverse (_:xs) = traverse xs >> return ()
>>
>> makeList 0 = []
>> -- makeList n = () : (makeList (n - 1))
>> makeList n = () : (unsafePerformIO . return) (makeList (n - 1))
>>
>> main = traverse $ makeList (10^5)
>>
>>
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>
>
>
> --
> Push the envelope. Watch it bend.
>
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