我的代码中有几个辅助函数,对于给定的数值计算,它们会被多次调用。这些辅助函数使用一些常数值进行计算。多个辅助函数可能使用相同的常量值。
这似乎是用常量值定义类属性的理想方案。但是,我做了一些基准测试,结果让我感到非常惊讶。
考虑以下类,例如 ( Consts.m
):
classdef Consts
properties (Constant)
A = 0.5
B = 3
end
properties
VariableA
VariableB
end
methods
function obj = Consts()
obj.VariableA = 0.5;
obj.VariableB = 3;
end
end
end
以及以下文件 ( speed_tests.m
):
function speed_tests()
tic;
for i = 1:200000
direct_constant_access(1, 2);
end
fprintf('Direct constant access: ');
toc;
tic;
c = Consts();
for i = 1:200000
passing_extra_obj(1, 2, c);
end
fprintf('Passing extra object: ');
toc;
tic;
for i = 1:200000
persistent_constants(1, 2);
end
fprintf('Persistent constants: ');
toc;
% Let's assume this code is executed at some point externally:
% global A B;
% A = 0.5;
% B = 3;
tic;
for i = 1:200000
defined_globally(1, 2);
end
fprintf('Defined globally: ');
toc;
tic;
for i = 1:200000
hardcoded(1, 2);
end
fprintf('Hardcoded: ');
toc;
tic;
for i = 1:200000
hardcoded_v2(1, 2);
end
fprintf('Hardcoded v2: ');
toc;
end
function val = direct_constant_access(a, b)
val = (a + Consts.A)^2 + log(b * Consts.B);
end
function val = passing_extra_obj(a, b, obj)
val = (a + obj.VariableA)^2 + log(b * obj.VariableB);
end
function val = persistent_constants(a, b)
persistent A B;
if isempty(A)
A = Consts.A^2;
B = Consts.B;
end
val = (a + A)^2 + log(b * B);
end
function val = defined_globally(a, b)
global A B;
val = (a + A)^2 + log(b * B);
end
function val = hardcoded(a, b)
val = (a + 0.5)^2 + log(b * 3);
end
function val = hardcoded_v2(a, b)
A = 0.5;
B = 3;
val = (a + A)^2 + log(b * B);
end
当我speed_tests()
在 MATLAB R2010b 上运行时,这是我得到的(您的里程可能会有所不同):
>> speed_tests()
Direct constant access: Elapsed time is 5.973690 seconds.
Passing extra object: Elapsed time is 1.760897 seconds.
Persistent constants: Elapsed time is 1.594263 seconds.
Defined globally: Elapsed time is 1.559441 seconds.
Hardcoded: Elapsed time is 0.673995 seconds.
Hardcoded v2: Elapsed time is 0.661189 seconds.
也许我太习惯于其他编程语言(真正的常量可能在编译时简单地被文字替换),但是在 MATLAB 中访问类常量真的那么慢还是我错过了什么?
当我在 MATLAB R2013a(同一台计算机)中尝试相同的操作时,这种直接常量访问似乎已经改进了很多:
>> speed_tests()
Direct constant access: Elapsed time is 2.168146 seconds.
Passing extra object: Elapsed time is 1.593721 seconds.
Persistent constants: Elapsed time is 2.302785 seconds.
Defined globally: Elapsed time is 1.404252 seconds.
Hardcoded: Elapsed time is 0.531191 seconds.
Hardcoded v2: Elapsed time is 0.493668 seconds.
尽管如此,没有一个非硬编码版本接近硬编码版本。这是我在工作中仅有的两个 MATLAB 版本,所以我不知道这些年来这是否一直在改进(这与我自己无关,因为无论如何我都无法使用更新的版本)。
CPU 时间对于我正在开发的东西来说是一个非常重要的因素,但如果可以的话,我想避免用硬编码的文字填充代码。类常量不是所谓的避免这种情况的方法吗?
还有什么我可以考虑的吗?
注意:真正的辅助函数每次都会用不同的参数调用,所以缓存结果对我来说没有帮助。