[Haskell-cafe] 3GB allocation, 75% CPU usage from a callback function

[Edward Amsden]

Thanks much!

On Sat, Jan 29, 2011 at 12:01 PM, Daniel Fischer
<daniel.is.fischer at googlemail.com> wrote:
> On Saturday 29 January 2011 17:29:02, Edward Amsden wrote:
>> (Note that the function produced by newSinWave is the one actually
>> registered as a callback:
>>
>> newSinWave :: Int -> Float -> IO (CFloat -> IO CFloat)
>> newSinWave sampleRate freq =
>>  do ioref <- newIORef (0::Integer)
>>     let multiplier = 2 * pi * freq /
>>                      (fromIntegral sampleRate)
>>
>>     return (\_ -> {-# SCC "sinWave" #-}
>>              do t <- readIORef ioref
>>                 modifyIORef ioref (+1)
>>                 return $ fromRational $
>>                   toRational $
>
> fromRational . toRational is a big no-no here. It will still be slow when
> the new implementation makes it into base, but currently it's
> slooooooooooooow.
> If you replace it with realToFrac and compile with optimisations, it should
> be rewritten to a no-op.

And suddenly I get a very reasonable 3% cpu load when I use realToFrac. Thanks!

>> I profiled this since when my program registered with the jack server
>> and started taking callbacks, it was using about 75% cpu
>> (in contrast, the echo program included with the jack package uses
>> less than 2%). The following two lines are of concern to me:
>>
>> "total alloc = 3,040,397,164 bytes  (excludes profiling overheads)"
>> My program uses 3GB of virtual memory over a 15 second run?
>
> Sequentially. With 44100 calls per second, you'd have 661500 calls.
> Each call to toRational allocates several Integers, the calls to
> fromRational even more. I wouldn't expect it to be 4k per wave call, but
> see what figures you get with realToFrac instead of fromRational .
> toRational.
And with realToFrac and -O3, 58MB total allocation instead.


Thanks for clearing that up.


-- 
Edward Amsden
Undergraduate
Computer Science
Rochester Institute of Technology
www.edwardamsden.com