提升精神业力真正的发电机性能

Question

当我对实数使用策略时性能下降感到有些惊讶时，我正在检查 boost spirit karma generator 性能。 生活在 Coliru
代码取自 boost spirit 并添加了几个测试函数。 Coliru 示例替换了使用的计时器。请注意，Coliru 会中止长 运行 progs，因此它可能不会结束所有测试。
正如您所见，使用策略会使性能降低 2-3 倍（在 coliru 上为 x10）。这是预期的行为吗？

我的数字：

sprintf: 0.367
iostreams: 0.818
format: 1.036
karma: 0.087
(string): 0.152
karma (string) with policy: 0.396
karma (rule): 0.12
karma (direct): 0.083
karma (direct) string: 0.089
karma (direct) string with policy: 0.278

使用 x64 VC14 构建

Answer 1

如果你比较苹果和梨，这不是回归。在这种情况下，两次。

第一个apple/pear对

这里fixed是苹果，scientific是梨。

不仅结果输出明显不同，而且到达结果需要不同的步骤。

重要的是，scientific 涉及对输入值进行 log10 运算，以确定小数点前 10 位数字的大小：

默认情况下，real_policies 调用 "cheap" 判定：

    static int floatfield(T n)
    {
        if (traits::test_zero(n))
            return fmtflags::fixed;

        T abs_n = traits::get_absolute_value(n);
        return (abs_n >= 1e5 || abs_n < 1e-3) 
          ? fmtflags::scientific : fmtflags::fixed;
    }

因此，如果您选择一种无论如何都会切换为科学格式，您可以看到差异消失：123456.123456 而不是 12345.12345...:[=​​26=]

clock resolution: mean is 16.9199 ns (40960002 iterations)

benchmarking format_performance_direct_string
collecting 100 samples, 1 iterations each, in estimated 4.7784 ms
mean: 238.81 ns, lb 187.22 ns, ub 493.46 ns, ci 0.95
std dev: 507.559 ns, lb 5.36317 ns, ub 1111.94 ns, ci 0.95
found 11 outliers among 100 samples (11%)
variance is severely inflated by outliers

benchmarking format_performance_direct_string_with_policy
collecting 100 samples, 96 iterations each, in estimated 1699.2 μs
mean: 173.927 ns, lb 172.764 ns, ub 176.939 ns, ci 0.95
std dev: 8.33706 ns, lb 0.256875 ns, ub 16.9312 ns, ci 0.95
found 2 outliers among 100 samples (2%)
variance is moderately inflated by outliers

benchmarking format_performance_string
collecting 100 samples, 84 iterations each, in estimated 1705.2 μs
mean: 312.646 ns, lb 311.027 ns, ub 314.819 ns, ci 0.95
std dev: 9.42479 ns, lb 7.32668 ns, ub 15.2546 ns, ci 0.95
found 1 outliers among 100 samples (1%)
variance is moderately inflated by outliers

benchmarking format_performance_string_with_policy
collecting 100 samples, 31 iterations each, in estimated 1736 μs
mean: 193.572 ns, lb 192.257 ns, ub 200.032 ns, ci 0.95
std dev: 12.8586 ns, lb 0.322008 ns, ub 30.6708 ns, ci 0.95
found 4 outliers among 100 samples (4%)
variance is severely inflated by outliers

如您所见，自定义策略（可预测）快得多

作为Interactive Link:

第二 apples/pears 对

这出现在您将精度固定为 15 位数字的地方。

通过使用两个策略的单独的头对头基准测试，这两个策略实际上增加了精度：http://paste.ubuntu.com/13087371/ 你可以看到，这不仅仅是失去了将格式固定为 scientific 见上图：

clock resolution: mean is 18.6041 ns (40960002 iterations)

benchmarking format_performance_direct_string_with_policy
collecting 100 samples, 1 iterations each, in estimated 1892.9 μs
mean: 228.83 ns, lb 179.9 ns, ub 471.84 ns, ci 0.95
std dev: 483.67 ns, lb 2.29965 ns, ub 1153.98 ns, ci 0.95
found 14 outliers among 100 samples (14%)
variance is severely inflated by outliers

benchmarking format_performance_direct_string_with_policy15
collecting 100 samples, 45 iterations each, in estimated 1858.5 μs
mean: 418.697 ns, lb 410.976 ns, ub 438.865 ns, ci 0.95
std dev: 58.0984 ns, lb 24.1313 ns, ub 115.549 ns, ci 0.95
found 6 outliers among 100 samples (6%)
variance is severely inflated by outliers

benchmarking format_performance_string_with_policy
collecting 100 samples, 87 iterations each, in estimated 1870.5 μs
mean: 262.057 ns, lb 254.73 ns, ub 269.354 ns, ci 0.95
std dev: 37.2502 ns, lb 31.1261 ns, ub 50.5813 ns, ci 0.95
found 17 outliers among 100 samples (17%)
variance is severely inflated by outliers

benchmarking format_performance_string_with_policy15
collecting 100 samples, 42 iterations each, in estimated 1898.4 μs
mean: 458.505 ns, lb 453.626 ns, ub 481.044 ns, ci 0.95
std dev: 45.5401 ns, lb 4.30147 ns, ub 108.045 ns, ci 0.95
found 4 outliers among 100 samples (4%)
variance is severely inflated by outliers

或在图表中：Interactive Link