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TL;DR: Based on the Haskell code and it's associated profiling data below, what conclusions can we draw that let us modify/improve it so we can narrow the performance gap vs. the same algorithm written in imperative languages (namely C++ / Python / C# but the specific language isn't important)?
I wrote the following piece of code as an answer to a question on a popular site which contains many questions of a programming and/or mathematical nature. (You've probably heard of this site, whose name is pronounced "oiler" by some, "yoolurr" by others.) Since the code below is a solution to one of the problems, I'm intentionally avoiding any mention of the site's name or any specific terms in the problem. That said, I'm talking about problem one hundred and three.
(In fact, I've seen many solutions in the site's forums from resident Haskell wizards :P)
This was the first problem (on said site) in which I encountered a difference in performance (as measured by execution time) between Haskell code vs. C++/Python/C# code (when both use a similar algorithm). In fact, it was the case for all of the problems (thus far; I've done ~100 problems but not sequentially) that an optimized Haskell code was pretty much neck-and-neck with the fastest C++ solutions, ceteris paribus for the algorithm, of course.
However, the posts in the forum for this particular problem would indicate that the same algorithm in these other languages typically require at most one or two seconds, with the longest taking 10-15 sec (assuming the same starting parameters; I'm ignoring the very naive algorithms that take 2-3 min+). In contrast, the Haskell code below required ~50 sec on my (decent) computer (with profiling disabled; with profiling enabled, it takes ~2 min, as you can see below; note: the exec time was identical when compiling with -fllvm). Specs: i5 2.4ghz laptop, 8gb RAM.
In an effort to learn Haskell in a way that it can become a viable substitute to the imperative languages, one of my aims in solving these problems is learning to write code that, to the extent possible, has performance that's on par with those imperative languages. In that context, I still consider the problem as yet unsolved by me (since there's nearly a ~25x difference in performance!)
In addition to the obvious step of streamlining the code itself (to the best of my ability), I've also performed the standard profiling exercises that are recommended in "Real World Haskell".
But I'm having a hard time drawing conclusions that that tell me which pieces need to be modified. That's where I'm hoping folks might be able to help provide some guidance.
I'd refer you to the website of problem one hundred and three on the aforementioned site but here's a brief summary: the goal is to find a group of seven numbers such that any two disjoint subgroups (of that group) satisfy the following two properties (I'm trying to avoid using the 's-e-t' word for reasons mentioned above...):
In particular, we are trying to find the group of seven numbers with the smallest sum.
A warning: some of these comments may well be totally wrong but I wanted to atleast take a stab at interpreting the profiling data based on what I read in Real World Haskell and other profiling-related posts on SO.
value sub-function, which determines values to fill in the dynamic programming ("DP") table, (ii) 29.1% in the table function, which generates the DP table and (iii) 12.4% in the rule1 function, which checks the resulting DP table to make sure that a given sum can only be calculated in one way (i.e., from one subgroup).value function relative to the table and rule1 functions given that it's the only one of the three which doesn't construct an array or filter through a large number of elements (it's really only performing O(1) lookups and making comparisons between Int types, which you'd think would be relatively quick). So this is a potential problem area. That said, it's unlikely that the value function is driving the high heap-allocationFrankly, I'm not sure what to make of the three charts.
Heap profile chart (i.e., the first char below):
Pinned. It makes sense that the dynamic function has a "spiky" memory allocation because it's called every time the construct function generates a tuple that meets the first three criteria and, each time it's called, it creates a decently large DP array. Also, I'd think that the allocation of memory to store the tuples (generated by construct) wouldn't be flat over the course of the program.Allocation by type and allocation by constructor:
ARR_WORDS (which represents a ByteString or unboxed Array according to the GHC docs) represents the low-level execution of the construction of the DP array (in the table function). Nut I'm not 100% sure.FROZEN and STATIC pointer categories correspond to.Without further ado, here's the code with comments explaining my algorithm. I've tried to make sure the code doesn't run off of the right-side of the code-box - but some of the comments do require scrolling (sorry).
{-# LANGUAGE NoImplicitPrelude #-}
{-# OPTIONS_GHC -Wall #-}
import CorePrelude
import Data.Array
import Data.List
import Data.Bool.HT ((?:))
import Control.Monad (guard)
main = print (minimum construct)
cap = 55 :: Int
flr = 20 :: Int
step = 1 :: Int
--we enumerate tuples that are potentially valid and then
--filter for valid ones; we perform the most computationally
--expensive step (i.e., rule 1) at the very end
construct :: [[Int]]
construct = {-# SCC "construct" #-} do
a <- [flr..cap] --1st: we construct potentially valid tuples while applying a
b <- [a+step..cap] --constraint on the upper bound of any element as implied by rule 2
c <- [b+step..a+b-1]
d <- [c+step..a+b-1]
e <- [d+step..a+b-1]
f <- [e+step..a+b-1]
g <- [f+step..a+b-1]
guard (a + b + c + d - e - f - g > 0) --2nd: we screen for tuples that completely conform to rule 2
let nn = [g,f,e,d,c,b,a]
guard (sum nn < 285) --3rd: we screen for tuples of a certain size (a guess to speed things up)
guard (rule1 nn) --4th: we screen for tuples that conform to rule 1
return nn
rule1 :: [Int] -> Bool
rule1 nn = {-# SCC "rule1" #-}
null . filter ((>1) . snd) --confirm that there's only one subgroup that sums to any given sum
. filter ((length nn==) . snd . fst) --the last column us how many subgroups sum to a given sum
. assocs --run the dynamic programming algorithm and generate a table
$ dynamic nn
dynamic :: [Int] -> Array (Int,Int) Int
dynamic ns = {-# SCC "dynamic" #-} table
where
(len, maxSum) = (length &&& sum) ns
table = array ((0,0),(maxSum,len))
[ ((s,i),x) | s <- [0..maxSum], i <- [0..len], let x = value (s,i) ]
elements = listArray (0,len) (0:ns)
value (s,i)
| i == 0 || s == 0 = 0
| s == m = table ! (s,i-1) + 1
| s > m = s <= sum (take i ns) ?:
(table ! (s,i-1) + table ! ((s-m),i-1), 0)
| otherwise = 0
where
m = elements ! i
Stats on heap allocation, garbage collection and time elapsed:
% ghc -O2 --make 103_specialsubset2.hs -rtsopts -prof -auto-all -caf-all -fforce-recomp
[1 of 1] Compiling Main ( 103_specialsubset2.hs, 103_specialsubset2.o )
Linking 103_specialsubset2 ...
% time ./103_specialsubset2.hs +RTS -p -sstderr
zsh: permission denied: ./103_specialsubset2.hs
./103_specialsubset2.hs +RTS -p -sstderr 0.00s user 0.00s system 86% cpu 0.002 total
% time ./103_specialsubset2 +RTS -p -sstderr
SOLUTION REDACTED
172,449,596,840 bytes allocated in the heap
21,738,677,624 bytes copied during GC
261,128 bytes maximum residency (74 sample(s))
55,464 bytes maximum slop
2 MB total memory in use (0 MB lost due to fragmentation)
Tot time (elapsed) Avg pause Max pause
Gen 0 327548 colls, 0 par 27.34s 41.64s 0.0001s 0.0092s
Gen 1 74 colls, 0 par 0.02s 0.02s 0.0003s 0.0013s
INIT time 0.00s ( 0.01s elapsed)
MUT time 53.91s ( 70.60s elapsed)
GC time 27.35s ( 41.66s elapsed)
RP time 0.00s ( 0.00s elapsed)
PROF time 0.00s ( 0.00s elapsed)
EXIT time 0.00s ( 0.00s elapsed)
Total time 81.26s (112.27s elapsed)
%GC time 33.7% (37.1% elapsed)
Alloc rate 3,199,123,974 bytes per MUT second
Productivity 66.3% of total user, 48.0% of total elapsed
./103_specialsubset2 +RTS -p -sstderr 81.26s user 30.90s system 99% cpu 1:52.29 total
Stats on time spent per cost-centre:
Wed Dec 17 23:21 2014 Time and Allocation Profiling Report (Final)
103_specialsubset2 +RTS -p -sstderr -RTS
total time = 15.56 secs (15565 ticks @ 1000 us, 1 processor)
total alloc = 118,221,354,488 bytes (excludes profiling overheads)
COST CENTRE MODULE %time %alloc
dynamic.value Main 41.6 17.7
dynamic.table Main 29.1 37.8
construct Main 12.9 37.4
rule1 Main 12.4 7.0
dynamic.table.x Main 1.9 0.0
individual inherited
COST CENTRE MODULE no. entries %time %alloc %time %alloc
MAIN MAIN 55 0 0.0 0.0 100.0 100.0
main Main 111 0 0.0 0.0 0.0 0.0
CAF:main1 Main 108 0 0.0 0.0 0.0 0.0
main Main 110 1 0.0 0.0 0.0 0.0
CAF:main2 Main 107 0 0.0 0.0 0.0 0.0
main Main 112 0 0.0 0.0 0.0 0.0
CAF:main3 Main 106 0 0.0 0.0 0.0 0.0
main Main 113 0 0.0 0.0 0.0 0.0
CAF:construct Main 105 0 0.0 0.0 100.0 100.0
construct Main 114 1 0.6 0.0 100.0 100.0
construct Main 115 1 12.9 37.4 99.4 100.0
rule1 Main 123 282235 0.6 0.0 86.5 62.6
rule1 Main 124 282235 12.4 7.0 85.9 62.6
dynamic Main 125 282235 0.2 0.0 73.5 55.6
dynamic.elements Main 133 282235 0.3 0.1 0.3 0.1
dynamic.len Main 129 282235 0.0 0.0 0.0 0.0
dynamic.table Main 128 282235 29.1 37.8 72.9 55.5
dynamic.table.x Main 130 133204473 1.9 0.0 43.8 17.7
dynamic.value Main 131 133204473 41.6 17.7 41.9 17.7
dynamic.value.m Main 132 132640003 0.3 0.0 0.3 0.0
dynamic.maxSum Main 127 282235 0.0 0.0 0.0 0.0
dynamic.(...) Main 126 282235 0.1 0.0 0.1 0.0
dynamic Main 122 282235 0.0 0.0 0.0 0.0
construct.nn Main 121 12683926 0.0 0.0 0.0 0.0
CAF:main4 Main 102 0 0.0 0.0 0.0 0.0
construct Main 116 0 0.0 0.0 0.0 0.0
construct Main 117 0 0.0 0.0 0.0 0.0
CAF:cap Main 101 0 0.0 0.0 0.0 0.0
cap Main 119 1 0.0 0.0 0.0 0.0
CAF:flr Main 100 0 0.0 0.0 0.0 0.0
flr Main 118 1 0.0 0.0 0.0 0.0
CAF:step_r1dD Main 99 0 0.0 0.0 0.0 0.0
step Main 120 1 0.0 0.0 0.0 0.0
CAF GHC.IO.Handle.FD 96 0 0.0 0.0 0.0 0.0
CAF GHC.Conc.Signal 93 0 0.0 0.0 0.0 0.0
CAF GHC.IO.Encoding 91 0 0.0 0.0 0.0 0.0
CAF GHC.IO.Encoding.Iconv 82 0 0.0 0.0 0.0 0.0
Heap profile:
Allocation by type:
Allocation by constructors:
919
There is a lot that can be said. In this answer I'll just comment on the nested list comprehensions in the construct function.
To get an idea on what's going on in construct we'll isolate it and compare it to a nested loop version that you would write in an imperative language. We've removed the rule1 guard to test only the generation of lists.
-- List.hs -- using list comprehensions
import Control.Monad
cap = 55 :: Int
flr = 20 :: Int
step = 1 :: Int
construct :: [[Int]]
construct = do
a <- [flr..cap]
b <- [a+step..cap]
c <- [b+step..a+b-1]
d <- [c+step..a+b-1]
e <- [d+step..a+b-1]
f <- [e+step..a+b-1]
g <- [f+step..a+b-1]
guard (a + b + c + d - e - f - g > 0)
guard (a + b + c + d + e + f + g < 285)
return [g,f,e,d,c,b,a]
-- guard (rule1 nn)
main = do
forM_ construct print
-- Loops.hs -- using imperative looping
import Control.Monad
loop a b f = go a
where go i | i > b = return ()
| otherwise = do f i; go (i+1)
cap = 55 :: Int
flr = 20 :: Int
step = 1 :: Int
main =
loop flr cap $ \a ->
loop (a+step) cap $ \b ->
loop (b+step) (a+b-1) $ \c ->
loop (c+step) (a+b-1) $ \d ->
loop (d+step) (a+b-1) $ \e ->
loop (e+step) (a+b-1) $ \f ->
loop (f+step) (a+b-1) $ \g ->
if (a+b+c+d-e-f-g > 0) && (a+b+c+d+e+f+g < 285)
then print [g,f,e,d,c,b,a]
else return ()
Both programs were compiled with ghc -O2 -rtsopts and run with prog +RTS -s > out.
Here is a summary of the results:
Lists.hs Loops.hs
Heap allocation 44,913 MB 2,740 MB
Max. Residency 44,312 44,312
%GC 5.8 % 1.7 %
Total Time 9.48 secs 1.43 secs
As you can see, the loop version, which is the way you would write this in a language like C, wins in every category.
The list comprehension version is cleaner and more composable but also less performant than direct iteration.
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