Thank you very much Thomas. This is the kind of explanation I needed! Janek
Dnia czwartek, 18 października 2012, Thomas Schilling napisał: > 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-measur > >e-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-Arra >y-Repa.html#v:deepSeqArray _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
