这是为什么F#code这么慢? [英] Why is this F# code so slow?
问题描述
在C#和F#A莱文斯坦实现。 C#的版本是约1500个字符的两个字符串快10倍。 C#:69毫秒,F#867毫秒。为什么?据我所知,他们做同样的事情?如果是发布或调试版本没有关系。
编辑:如果有人来这里专为编辑距离实现来看,它坏了。工作code是<一个href=\"https://bitbucket.org/vgrit/clrsquirrel/src/82840c2ce95b/ClrSquirrel/ClrSquirrel.fs\">here.
C#
私有静态诠释MIN3(INT A,INT B,INT C)
{
返回Math.Min(Math.Min(A,B),C);
}公共静态INT EditDistance(串M,串N)
{
VAR D1 =新的INT [n.Length]
为(中间体X = 0; X&下; d1.Length; X ++)D1 [X] = X;
VAR D0 = INT新[n.Length]
的for(int i = 1; I&LT; m.Length;我++)
{
D0 [0] = I;
VAR UI = M [];
对于(INT J = 1; J&LT; n.Length; J ++)
{
D0 [J] = 1 + MIN3(D1 [J],D0 [J - 1],D1 [J - 1] +(UI == N [J] -1:0));
}
Array.Copy(D0,D1,d1.Length);
}
返回D0 [n.Length - 1];
}
F#
让MIN3(A,B,C)=分钟(分B C)让莱文斯坦(M:字符串)(N:字符串)=
让D1 = Array.init n.Length ID
让D0 = Array.create n.Length 0
对于i = 1到m.Length-1做
。D0 [0]&LT; - 我
让UI = M。[I]
对于j = 1至n.Length-1做
。D0 [J]&LT; - 1 + MIN3(。。D1 [J],D0 [J-1],D1 [J-1] +如果UI = N [J]。然后-1否则为0)
Array.blit D0 0 0 D1 n.Length
D0 [n.Length-1]
的问题是, MIN3
函数编译为使用通用的比较通用的功能(我想这仅使用 IComparable的
,但它实际上是更为复杂 - 它会使用结构比较F#的类型,它是相当复杂的逻辑)
&GT;让MIN3(A,B,C)=分钟(分B C);;
VAL MIN3:A * A * A - &GT; 当一个'A:比较
在C#版本,功能不是一般的(它只是需要 INT
)。
让MIN3(A:INT,B,C)=分钟(分B C)
...或者通过 MIN3
为在线
(在这种情况下,将专门为 INT
使用时):
让内联MIN3(A,B,C)=分钟(分B C);;
对于一个随机字符串 STR
长度300的,我得到以下数字:
&GT;莱文斯坦STR(富+ STR);;
房地产:00:00:03.938,CPU:00:00:03.900,GC gen0:275,第一代:1,第2代:0
VAL它:INT = 3&GT; levenshtein_inlined STR(富+ STR);;
房地产:00:00:00.068,CPU:00:00:00.078,GC gen0:0,第1代:0,第2代:0
VAL它:INT = 3
A Levenshtein implementation in C# and F#. The C# version is 10 times faster for two strings of about 1500 chars. C#: 69 ms, F# 867 ms. Why? As far as I can tell, they do the exact same thing? Doesn't matter if it is a Release or a Debug build.
EDIT: If anyone comes here looking specifically for the Edit Distance implementation, it is broken. Working code is here.
C#:
private static int min3(int a, int b, int c)
{
return Math.Min(Math.Min(a, b), c);
}
public static int EditDistance(string m, string n)
{
var d1 = new int[n.Length];
for (int x = 0; x < d1.Length; x++) d1[x] = x;
var d0 = new int[n.Length];
for(int i = 1; i < m.Length; i++)
{
d0[0] = i;
var ui = m[i];
for (int j = 1; j < n.Length; j++ )
{
d0[j] = 1 + min3(d1[j], d0[j - 1], d1[j - 1] + (ui == n[j] ? -1 : 0));
}
Array.Copy(d0, d1, d1.Length);
}
return d0[n.Length - 1];
}
F#:
let min3(a, b, c) = min a (min b c)
let levenshtein (m:string) (n:string) =
let d1 = Array.init n.Length id
let d0 = Array.create n.Length 0
for i=1 to m.Length-1 do
d0.[0] <- i
let ui = m.[i]
for j=1 to n.Length-1 do
d0.[j] <- 1 + min3(d1.[j], d0.[j-1], d1.[j-1] + if ui = n.[j] then -1 else 0)
Array.blit d0 0 d1 0 n.Length
d0.[n.Length-1]
The problem is that the min3
function is compiled as a generic function that uses generic comparison (I thought this uses just IComparable
, but it is actually more complicated - it would use structural comparison for F# types and it's fairly complex logic).
> let min3(a, b, c) = min a (min b c);;
val min3 : 'a * 'a * 'a -> 'a when 'a : comparison
In the C# version, the function is not generic (it just takes int
). You can improve the F# version by adding type annotations (to get the same thing as in C#):
let min3(a:int, b, c) = min a (min b c)
...or by making min3
as inline
(in which case, it will be specialized to int
when used):
let inline min3(a, b, c) = min a (min b c);;
For a random string str
of length 300, I get the following numbers:
> levenshtein str ("foo" + str);;
Real: 00:00:03.938, CPU: 00:00:03.900, GC gen0: 275, gen1: 1, gen2: 0
val it : int = 3
> levenshtein_inlined str ("foo" + str);;
Real: 00:00:00.068, CPU: 00:00:00.078, GC gen0: 0, gen1: 0, gen2: 0
val it : int = 3
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