add _posts/2024-11-05-perf-tools-06-perf-practice-01.md

_posts/2024-11-05-perf-tools-06-perf-practice-01.md

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+---
+title: perf性能分析(6) -- perf实战(1) -- 分支预测
+date: 2024-11-05 +0800 # 2022-01-01 13:14:15 +0800 只写日期也行；不写秒也行；这样也行 2022-03-09T00:55:42+08:00
+categories: [perf]
+tags: [perf, vtune, tbb]      # TAG names should always be lowercase
+
+# 以下默认false
+math: true
+mermaid: true
+# pin: true
+---
+
+使用`Debug`模式编译 `sort` 和 `unsort` 代码:
+
+```c++
+#include <algorithm>
+#include <ctime>
+#include <iostream>
+
+int main(int argc, char* argv[]) {
+  constexpr int kArrLen = 1024 * 1024;
+  int* data = new int[kArrLen];
+
+  for (int c = 0; c < kArrLen; c++)
+    data[c] = std::rand() % 256;
+
+  // std::sort(data, data + kArrLen);  // 是否排序
+
+  long long sum = 0;
+  for (int i = 0; i < 30000; i++) {
+    for (int c = 0; c < kArrLen; c++) {
+      if (data[c] >= 128) {  // 故意選 256 一半
+        sum += data[c];
+      }
+    }
+  }
+
+  delete[] data;
+  std::cout << "sum = " << sum << std::endl;
+}
+```
+
+## 1. sort/unsort 性能测试及比较: perf stat ##
+
+The CPU is `Intel(R) Core(TM) i5-8260U CPU @ 1.60GHz`.
+
+`perf stat ./sort` 测试结果:
+
+```bash
+$ perf stat ./sort
+
+sum = 3016043160000
+
+ Performance counter stats for './sort':
+
+         63,995.28 msec task-clock                       #    1.000 CPUs utilized
+             1,039      context-switches                 #   16.236 /sec
+                 7      cpu-migrations                   #    0.109 /sec
+             1,151      page-faults                      #   17.986 /sec
+   240,342,649,070      cycles                           #    3.756 GHz
+   473,524,864,500      instructions                     #    1.97  insn per cycle
+    63,163,134,842      branches                         #  986.997 M/sec
+         7,402,185      branch-misses                    #    0.01% of all branches
+
+      64.008866998 seconds time elapsed
+
+      63.983827000 seconds user
+       0.010999000 seconds sys
+```
+
+`perf stat ./unsort` 测试结果:
+
+```bash
+$ perf stat ./unsort
+
+sum = 3016043160000
+
+ Performance counter stats for './unsort':
+
+        206,441.99 msec task-clock                       #    1.000 CPUs utilized
+               484      context-switches                 #    2.344 /sec
+                44      cpu-migrations                   #    0.213 /sec
+             1,150      page-faults                      #    5.571 /sec
+   771,936,412,315      cycles                           #    3.739 GHz
+   473,410,337,438      instructions                     #    0.61  insn per cycle
+    63,144,272,701      branches                         #  305.869 M/sec
+    15,732,825,641      branch-misses                    #   24.92% of all branches
+
+     206.439026771 seconds time elapsed
+
+     206.440760000 seconds user
+       0.002000000 seconds sys
+```
+
+### 1.1. Result comparison ###
+
+`unsort`的分支预测失败率为`24.92%`, 且`CPU IPC`为`0.61`, 小于1.0.
+
+`sort`分支预测失败率为`0.01%`, `CPU IPC`为`1.97`.
+
+## 2. analysis using `perf record` and `perf ananotate` ##
+
+### 2.1. program unsort ###
+
+运行 `perf record` 和 `perf annotate` 命令:
+
+```bash
+perf record ./unsort # 结束后生产 perf.data 文件
+perf annotate        # 自动定位到最耗时的地方
+```
+
+![perf_annotate_unsort](/assets/images/perf/20241105_perf_practice_branch_prediction/perf_annotate_unsort.png)
+
+`if`语句比较失败跳转至循环体, 循环体占用`24.44%`的CPU时间. 原因是`分支预测失败` -- `if`语句在`for`循环中失败次数较多.
+
+## 2.2. program sort ##
+
+![perf_annotate_sort](/assets/images/perf/20241105_perf_practice_branch_prediction/perf_annotate_sort.png)
+
+分支预测失败率为`.051%` + `4.31%`, 且`CPU IPC`为`1.97`.
+
+* 内存`load`操作: `41.68%`.
+* `add`指令: `10.54%`.
+
+## 3. When build with `Release` ##
+
+如果使用`-O2`或者`-O3`编译，编译器使用了`vector`相关指令(针对32位的 `pcmpgtd`)，消除了`if`分支语句。其结果就是，`sort`与`unsort`的性能差距不大，甚至`sort`的性能更好。
+
+## references ##
+
+* [在 Linux 上使用 Perf 做效能分析(入門篇)](https://tigercosmos.xyz/post/2020/08/system/perf-basic/)
+* [Category:Architecture/x86-assembly](https://www.aldeid.com/wiki/Category:Architecture/x86-assembly)
+* [StackOverflow: Why is processing a sorted array faster than processing an unsorted array?](https://stackoverflow.com/questions/11227809/why-is-processing-a-sorted-array-faster-than-processing-an-unsorted-array)
+* [StackOverflow: Why is processing an unsorted array the same speed as processing a sorted array with modern x86-64 clang?](https://stackoverflow.com/questions/66521344/why-is-processing-an-unsorted-array-the-same-speed-as-processing-a-sorted-array)
+* [知乎 -- 分支对代码性能的影响和优化](https://zhuanlan.zhihu.com/p/487690985)
+* [Modern Microprocessors -- A 90-Minute Guide!](https://www.lighterra.com/papers/modernmicroprocessors/)