On 18 October 2012 13:15, Janek S. <fremenz...@poczta.onet.pl> wrote: >> Something like this might work, not sure what the canonical way is. >> (...) > > This is basically the same as the answer I was given on SO. My concerns about > this solutions are: > - rnf requires its parameter to belong to NFData type class. This is not the > case for some data > structures like Repa arrays.
For unboxed arrays of primitive types WHNF = NF. That is, once the array is constructed all its elements will be in WHNF. > - evaluate only evaluates its argument to WHNF - is this enough? If I have a > tuple containing two > lists won't this only evaluate the tuple construtor and leave the lists as > thunks? This is > actually the case in my code. That is why you use "rnf" from the NFData type class. You use "evaluate" to kick-start rnf which then goes ahead and evaluates everything (assuming the NFData instance has been defined correctly.) > > As I said previously, it seems that Criterion somehow evaluates the data so > that time needed for > its creation is not included in the benchmark. I modified my dataBuild > function to look lik this: > > dataBuild gen = unsafePerformIO $ do > let x = (take 6 $ randoms gen, take 2048 $ randoms gen) > delayThread 1000000 > return x > > When I ran the benchmark, criterion estimated the time needed to complete it > to over 100 seconds > (which means that delayThread worked and was used as a basis for estimation), > but the benchamrk > was finished much faster and there was no difference in the final result > comparing to the normal > dataBuild function. This suggests that once data was created and used for > estimation, the > dataBuild function was not used again. The main question is: is this > observation correct? In this > question on SO: > http://stackoverflow.com/questions/6637968/how-to-use-criterion-to-measure-performance-of-haskell-programs > one of the aswers says that there is no automatic memoization, while it looks > that in fact the > values of dataBuild are memoized. I have a feeling that I am misunderstanding > something. If you bind an expression to a variable and then reuse that variable, the expression is only evaluated once. That is, in "let x = expr in ..." the expression is only evaluated once. However, if you have "f y = let x = expr in ..." then the expression is evaluated once per function call. > >> I don't know if you have already read them, >> but Tibell's slides on High Performance Haskell are pretty good: >> >> http://www.slideshare.net/tibbe/highperformance-haskell >> >> There is a section at the end where he runs several tests using Criterion. > I skimmed the slides and slide 59 seems to show that my concerns regarding > WHNF might be true. It's usually safe if you benchmark a function. However, you most likely want the result to be in normal form. The "nf" does this for you. So, if your benchmark function has type "f :: X -> ([Double], Double)", your benchmark will be: bench "f" (nf f input) The first run will evaluate the input (and discard the runtime) and all subsequent runs will evaluate the result to normal form. For repa you can use deepSeqArray [1] if your array is not unboxed: bench "f'" (whnf (deepSeqArray . f) input) [1]: http://hackage.haskell.org/packages/archive/repa/3.2.2.2/doc/html/Data-Array-Repa.html#v:deepSeqArray _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
