在 Matlab 中并行处理期间出错

Question

我有这个（相当长的）带有嵌套循环的 Matlab 代码，我想在其中并行化主要的耗时迭代。唯一（显然）给我带来问题的变量是 DMax，在那里我得到错误：

Error: The variable DMax in a `parfor` cannot be classified.
See Parallel for Loops in MATLAB, "Overview".

这是我的代码草稿：

t0=matrix (Maxiter,1); % This is a big matrix whose dimensions are reported in brachets
Maxiter = 1E6;
DMax = zeros(Maxiter,40);
% Other Stuff
for j=1:269 
     % Do more stuff
     for soil=1:4
        parfor i =1:Maxiter        
            k(i,soil) = a %k is a real number
            a(i,soil) = b %similar to k
            % Do a lot of stuff
            for t= (floor(t0(i,soil))+1):40
                DMax(i,t) = k(i,soil)*((t-t0(i,soil))^a(i,soil));
                % Do some more stuff
            end
        end
    end
end
for time=1:40
   % Do the final stuff
end

我想问题出在我定义 DMax 的方式上，但我不知道它可能更准确。我已经在网上看了，但结果不是很令人满意。

Answer 1

在documentation that each variable inside parfor must be classified into one of several types. Your DMax variable should be a sliced variable (arrays whose segments are operated on by different iterations of the loop), but in order to be classified as such, all the following conditions must hold中描述的很清楚：

Type of First-Level Indexing — The first level of indexing is either parentheses, (), or braces, {}.

Fixed Index Listing — Within the first-level parenthesis or braces, the list of indices is the same for all occurrences of a given variable.

Form of Indexing — Within the list of indices for the variable, exactly one index involves the loop variable.

Shape of Array — The array maintains a constant shape. In assigning to a sliced variable, the right-hand side of the assignment cannot be [] or '', because these operators attempt to
delete elements.

显然，固定索引列表 属性不成立，因为您将其引用为 DMax(i,t)，其中 t 更改了它的值。文档中描述了一个相同的示例，请注意。因此，一种解决方法是在内循环中使用一个临时变量，然后将整行分配回 DMax.

另请注意，变量a也不能归入任何类别。更不用说它根本没有在您的示例中定义。请仔细阅读指南并确保它可以归入其中一类。如果需要重写代码，例如引入新的临时变量。

这是更正 DMax 用法的代码：

Maxiter = 1E6;
t0 = randn(Maxiter,1); % This is a big matrix whose dimensions are reported in brachets
DMax = zeros(Maxiter,40);
% Other Stuff
for j = 1:269 
     % Do more stuff
     for soil = 1:4
        parfor i = 1:Maxiter        
            k(i,soil) = a %k is a real number
            a(i,soil) = b %similar to k
            % Do a lot of stuff
            tmp = zeros(1,40);
            for t = (floor(t0(i,soil))+1):40
                tmp(t) = k(i,soil)*((t-t0(i,soil))^a(i,soil));
                % Do some more stuff
            end
            DMax(i,:) = tmp;
        end
    end
end
for time = 1:40
   % Do the final stuff
end

在 Matlab 中并行处理期间出错

Error during parallel processing in Matlab

parallel-processing

matlab

loops

parfor