On Mon, 2008-05-12 at 19:30 -0700, Don Stewart wrote:
I offer up the following example:
mean xs = sum xs / length xs
Now try, say, "mean [1.. 1e9]", and watch GHC eat several GB of RAM. (!!)
But you know why, don't you?
sat down and spent the best part of a day writing an MD5 implementation. Eventually I got it so that all the test vectors work right. (Stupid little-endian nonsense... mutter mutter...) When I tried it on a file containing more than 1 MB of data... ooooohhhh dear...
Did you use Data.Binary or the other libraries for efficient access to gigabytes of data?
Of course, the first step in any serious attempt at performance improvement is to actually profile the code to figure out where the time is being spent.
Well, actually, for our 'mean()' case, it means just using the right structures. Arrays for example:
import Data.Array.Vector import Text.Printf
mean :: UArr Double -> Double mean arr = b / fromIntegral a where k (n :*: s) a = n+1 :*: s+a a :*: b = foldlU k (0 :*: 0) arr :: (Int :*: Double)
main = printf "%f\n" . mean $ enumFromToFracU 1 1e9
For example,
$ time ./A 5.00000000067109e8 ./A 3.53s user 0.00s system 99% cpu 3.532 total
Try allocating an array of doubles in C, and getting similar results. (The compiler is optimising this to:
Main_zdszdwfold_info: leaq 32(%r12), %rax cmpq %r15, %rax movq %rax, %r12 ja .L10 movsd .LC0(%rip), %xmm0 ucomisd %xmm5, %xmm0 jae .L12 movq %rsi, (%rax) movq $base_GHCziFloat_Dzh_con_info, -24(%rax) movsd %xmm6, -16(%rax) movq $base_GHCziBase_Izh_con_info, -8(%rax) leaq -7(%rax), %rbx leaq -23(%rax), %rsi jmp *(%rbp) .L12: movapd %xmm6, %xmm0 addq $1, %rsi subq $32, %r12 addsd %xmm5, %xmm0 addsd .LC2(%rip), %xmm5 movapd %xmm0, %xmm6 jmp Main_zdszdwfold_info
Note even any garbage collection. This should be pretty much the same performance-wise as unoptimised C.
almost any nontrivial program you write spends 60% or more of its time doing GC rather than actual work.
Ok, you're doing something very wrong. GC time is typically less than 15% of the running time of typical work programs I hack on.
I bet you're using lists inappropriately?
I find it completely impossibly to write code that doesn't crawl along at a snail's pace. Even when I manage to make it faster, I usually have no clue why.
I think there is a problem that few people are taking the time to understand the compilation model of Haskell, while they've had the C runtime behaviour drilled into their brains since college.
Until you sit down and understand what your Haskell code means, it will be very hard to reason about optimisations, unfortunately.
GHC does what it does well, and its predictable. As long as you understand the operations your trying to make predictions about.
I suggest installing ghc-core, and looking at how your code is compiled. Try many examples, and have a good knowledge of the STG paper.
My experience has been that simply understanding lazy/normal order evaluation is enough to catch most of the "big performance problems" beginners complain about. If you can't evaluate Haskell by hand at the source level, you have no hope of understanding performance issues. For going more for C-like speed, your advice seems more appropriate.