【发布时间】：2018-06-22 22:21:50
【问题描述】：
我尝试测量缓存/非缓存内存访问时间，结果让我感到困惑。
代码如下：
1 #include <stdio.h>
2 #include <x86intrin.h>
3 #include <stdint.h>
4
5 #define SIZE 32*1024
6
7 char arr[SIZE];
8
9 int main()
10 {
11 char *addr;
12 unsigned int dummy;
13 uint64_t tsc1, tsc2;
14 unsigned i;
15 volatile char val;
16
17 memset(arr, 0x0, SIZE);
18 for (addr = arr; addr < arr + SIZE; addr += 64) {
19 _mm_clflush((void *) addr);
20 }
21 asm volatile("sfence\n\t"
22 :
23 :
24 : "memory");
25
26 tsc1 = __rdtscp(&dummy);
27 for (i = 0; i < SIZE; i++) {
28 asm volatile (
29 "mov %0, %%al\n\t" // load data
30 :
31 : "m" (arr[i])
32 );
33
34 }
35 tsc2 = __rdtscp(&dummy);
36 printf("(1) tsc: %llu\n", tsc2 - tsc1);
37
38 tsc1 = __rdtscp(&dummy);
39 for (i = 0; i < SIZE; i++) {
40 asm volatile (
41 "mov %0, %%al\n\t" // load data
42 :
43 : "m" (arr[i])
44 );
45
46 }
47 tsc2 = __rdtscp(&dummy);
48 printf("(2) tsc: %llu\n", tsc2 - tsc1);
49
50 return 0;
51 }
输出：
(1) tsc: 451248
(2) tsc: 449568
我预计，第一个值会大得多，因为在情况 (1) 中缓存被 clflush 无效。
关于我的 cpu（Intel(R) Core(TM) i7 CPU Q 720 @ 1.60GHz）缓存的信息：
Cache ID 0:
- Level: 1
- Type: Data Cache
- Sets: 64
- System Coherency Line Size: 64 bytes
- Physical Line partitions: 1
- Ways of associativity: 8
- Total Size: 32768 bytes (32 kb)
- Is fully associative: false
- Is Self Initializing: true
Cache ID 1:
- Level: 1
- Type: Instruction Cache
- Sets: 128
- System Coherency Line Size: 64 bytes
- Physical Line partitions: 1
- Ways of associativity: 4
- Total Size: 32768 bytes (32 kb)
- Is fully associative: false
- Is Self Initializing: true
Cache ID 2:
- Level: 2
- Type: Unified Cache
- Sets: 512
- System Coherency Line Size: 64 bytes
- Physical Line partitions: 1
- Ways of associativity: 8
- Total Size: 262144 bytes (256 kb)
- Is fully associative: false
- Is Self Initializing: true
Cache ID 3:
- Level: 3
- Type: Unified Cache
- Sets: 8192
- System Coherency Line Size: 64 bytes
- Physical Line partitions: 1
- Ways of associativity: 12
- Total Size: 6291456 bytes (6144 kb)
- Is fully associative: false
- Is Self Initializing: true
两条rdtscp指令之间的代码反汇编
400614: 0f 01 f9 rdtscp
400617: 89 ce mov %ecx,%esi
400619: 48 8b 4d d8 mov -0x28(%rbp),%rcx
40061d: 89 31 mov %esi,(%rcx)
40061f: 48 c1 e2 20 shl $0x20,%rdx
400623: 48 09 d0 or %rdx,%rax
400626: 48 89 45 c0 mov %rax,-0x40(%rbp)
40062a: c7 45 b4 00 00 00 00 movl $0x0,-0x4c(%rbp)
400631: eb 0d jmp 400640 <main+0x8a>
400633: 8b 45 b4 mov -0x4c(%rbp),%eax
400636: 8a 80 80 10 60 00 mov 0x601080(%rax),%al
40063c: 83 45 b4 01 addl $0x1,-0x4c(%rbp)
400640: 81 7d b4 ff 7f 00 00 cmpl $0x7fff,-0x4c(%rbp)
400647: 76 ea jbe 400633 <main+0x7d>
400649: 48 8d 45 b0 lea -0x50(%rbp),%rax
40064d: 48 89 45 e0 mov %rax,-0x20(%rbp)
400651: 0f 01 f9 rdtscp
看起来我缺少/误解了一些东西。你能推荐一下吗？
【问题讨论】：
标签： performance caching assembly memory x86