问题描述
我在 MATLAB 中编程，并且按照建议，我总是尝试使用矢量化.但最终程序还是很慢.所以我发现在一个地方，使用循环时代码明显更快(下面的例子).
I was programming something in MATLAB and, as recommended, I am always trying to use vectorization. But in the end the program was quite slow. So I found out that in one place the code is significantly faster when using loops (example below).
我想知道我是否误解或做错了什么，因为在这种情况下性能很重要，我不想一直猜测矢量化或循环是否会更快.
I would like to know if I misinterpreted something or did something wrong, because performance is important in this case, and I don't want to keep guessing if vectorization or loops are going to be faster.
% data initialization
k = 8;
n = 2^k+1;
h = 1/(n-1);
cw = 0.1;
iter = 10000;
uloc = zeros(n);
fploc = uloc;
uloc(2:end-1,2:end-1) = 1;
vloc = uloc;
ploc = ones(n);
uloc2 = zeros(n);
fploc2 = uloc2;
uloc2(2:end-1,2:end-1) = 1;
vloc2 = uloc2;
ploc2 = ones(n);
%%%%%%%%%%%%%%%%%%%%%%
% vectorized version %
%%%%%%%%%%%%%%%%%%%%%%
tic
for it=1:iter
il=2:4;
jl=2:4;
fploc(il,jl) = h/6*(-uloc(il-1,jl-1) + uloc(il-1,jl)...
-2*uloc(il,jl-1)+2*uloc(il,jl+1)...
-uloc(il+1,jl) + uloc(il+1,jl+1)...
...
-vloc(il-1,jl-1) - 2*vloc(il-1,jl)...
+vloc(il,jl-1) - vloc(il,jl+1)...
+ 2*vloc(il+1,jl) + vloc(il+1,jl+1))...
...
+cw*h^2*(-ploc(il-1,jl)-ploc(il,jl-1)+4*ploc(il,jl)...
-ploc(il+1,jl)-ploc(il,jl+1));
end
toc
%%%%%%%%%%%%%%%%%%%%%%
% loop version %
%%%%%%%%%%%%%%%%%%%%%%
tic
for it=1:iter
for il=2:4
for jl=2:4
fploc2(il,jl) = h/6*(-uloc2(il-1,jl-1) + uloc2(il-1,jl)...
-2*uloc2(il,jl-1)+2*uloc2(il,jl+1)...
-uloc2(il+1,jl) + uloc2(il+1,jl+1)...
...
-vloc2(il-1,jl-1) - 2*vloc2(il-1,jl)...
+vloc2(il,jl-1) - vloc2(il,jl+1)...
+ 2*vloc2(il+1,jl) + vloc2(il+1,jl+1))...
...
+cw*h^2*(-ploc2(il-1,jl)-ploc2(il,jl-1)+4*ploc2(il,jl)...
-ploc2(il+1,jl)-ploc2(il,jl+1));
end
end
end
toc
推荐答案
MATLAB 的即时编译器 (JIT) 在过去几年中得到了显着改进.即使您认为通常应该对代码进行矢量化是正确的，但根据我的经验，这仅适用于某些操作和函数，并且还取决于您的函数处理的数据量.
MATLAB's just in time compiler (JIT) has been improved significantly over the last couple years. And even though you are right that one should generally vectorize code, from my experience this is only true for certain operations and functions and also depends on how much data your functions are handling.
要找出最有效的方法，最好的方法是使用和不使用矢量化分析您的 MATLAB 代码.
The best way for you to find out what works best, is to profile your MATLAB code with and without vectorization.
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