Strange performance effects with unsafePerformIO

[Simon Marlow]

On 25/03/2011 08:56, Sebastian Fischer wrote:
> 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.

Incidentally this will be faster with GHC 7.2, because we implemented 
chunked stacks, so unsafePerformIO never has to traverse more than 32k 
of stack (you can tweak the chunk size with an RTS option).  This is 
still quite a lot of overhead, but at least it is bounded.

The example above runs in 1.45s for me with current HEAD, and I gave up 
waiting with 7.0.

Cheers,
	Simon



> 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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>
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