c - L2 HW预取器真的有用吗？

Question

我在威士忌湖i7-8565U 上，分析性能计数器和复制512 KiB数据的时间(是L2高速缓存大小的两倍)，并且对L2硬件预取器的工作遇到了一些误解。

在Intel Manual Vol.4 MSR中，有MSR 0x1A4的位0用于控制L2硬件预取器(禁用1)。

考虑以下基准:
memcopy.h:

void *avx_memcpy_forward_lsls(void *restrict, const void *restrict, size_t);

memcopy.S:

avx_memcpy_forward_lsls:
    shr rdx, 0x3
    xor rcx, rcx
avx_memcpy_forward_loop_lsls:
    vmovdqa ymm0, [rsi + 8*rcx]
    vmovdqa [rdi + rcx*8], ymm0
    vmovdqa ymm1, [rsi + 8*rcx + 0x20]
    vmovdqa [rdi + rcx*8 + 0x20], ymm1
    add rcx, 0x08
    cmp rdx, rcx
    ja avx_memcpy_forward_loop_lsls
    ret

main.c:

#include <string.h>
#include <stdlib.h>
#include <inttypes.h>
#include <x86intrin.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include "memcopy.h"

#define ITERATIONS 1000
#define BUF_SIZE 512 * 1024

_Alignas(64) char src[BUF_SIZE];
_Alignas(64) char dest[BUF_SIZE];

static void __run_benchmark(unsigned runs, unsigned run_iterations,
                    void *(*fn)(void *, const void*, size_t), void *dest, const void* src, size_t sz);

#define run_benchmark(runs, run_iterations, fn, dest, src, sz) \
    do{\
        printf("Benchmarking " #fn "\n");\
        __run_benchmark(runs, run_iterations, fn, dest, src, sz);\
    }while(0)

int main(void){
    int fd = open("/dev/urandom", O_RDONLY);
    read(fd, src, sizeof src);
    run_benchmark(20, ITERATIONS, avx_memcpy_forward_lsls, dest, src, BUF_SIZE);
}

static inline void benchmark_copy_function(unsigned iterations, void *(*fn)(void *, const void *, size_t),
                                               void *restrict dest, const void *restrict src, size_t sz){
    while(iterations --> 0){
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
        fn(dest, src, sz);
    }
}

static void __run_benchmark(unsigned runs, unsigned run_iterations,
                    void *(*fn)(void *, const void*, size_t), void *dest, const void* src, size_t sz){
    unsigned current_run = 1;
    while(current_run <= runs){
        benchmark_copy_function(run_iterations, fn, dest, src, sz);
        printf("Run %d finished\n", current_run);
        current_run++;
    }
}

考虑编译的 main.c的2次运行

我。
MSR:

$ sudo rdmsr -p 0 0x1A4
0

Run:

$ taskset -c 0 sudo ../profile.sh ./bin 

 Performance counter stats for './bin':

    10 486 164 071      L1-dcache-loads                                               (12,13%)
    10 461 354 384      L1-dcache-load-misses     #   99,76% of all L1-dcache hits    (12,05%)
    10 481 930 413      L1-dcache-stores                                              (12,05%)
    10 461 136 686      l1d.replacement                                               (12,12%)
    31 466 394 422      l1d_pend_miss.fb_full                                         (12,11%)
   211 853 643 294      l1d_pend_miss.pending                                         (12,09%)
     1 759 204 317      LLC-loads                                                     (12,16%)
            31 007      LLC-load-misses           #    0,00% of all LL-cache hits     (12,16%)
     3 154 901 630      LLC-stores                                                    (6,19%)
    15 867 315 545      l2_rqsts.all_pf                                               (9,22%)
                 0      sw_prefetch_access.t1_t2                                      (12,22%)
         1 393 306      l2_lines_out.useless_hwpf                                     (12,16%)
     3 549 170 919      l2_rqsts.pf_hit                                               (12,09%)
    12 356 247 643      l2_rqsts.pf_miss                                              (12,06%)
                 0      load_hit_pre.sw_pf                                            (12,09%)
     3 159 712 695      l2_rqsts.rfo_hit                                              (12,06%)
     1 207 642 335      l2_rqsts.rfo_miss                                             (12,02%)
     4 366 526 618      l2_rqsts.all_rfo                                              (12,06%)
     5 240 013 774      offcore_requests.all_data_rd                                     (12,06%)
    19 936 657 118      offcore_requests.all_requests                                     (12,09%)
     1 761 660 763      offcore_response.demand_data_rd.any_response                                     (12,12%)
       287 044 397      bus-cycles                                                    (12,15%)
    36 816 767 779      resource_stalls.any                                           (12,15%)
    36 553 997 653      resource_stalls.sb                                            (12,15%)
    38 035 066 210      uops_retired.stall_cycles                                     (12,12%)
    24 766 225 119      uops_executed.stall_cycles                                     (12,09%)
    40 478 455 041      uops_issued.stall_cycles                                      (12,05%)
    24 497 256 548      cycle_activity.stalls_l1d_miss                                     (12,02%)
    12 611 038 018      cycle_activity.stalls_l2_miss                                     (12,09%)
        10 228 869      cycle_activity.stalls_l3_miss                                     (12,12%)
    24 707 614 483      cycle_activity.stalls_mem_any                                     (12,22%)
    24 776 110 104      cycle_activity.stalls_total                                     (12,22%)
    48 914 478 241      cycles                                                        (12,19%)

      12,155774555 seconds time elapsed

      11,984577000 seconds user
       0,015984000 seconds sys

II。
MSR:

$ sudo rdmsr -p 0 0x1A4
1

Run:

$ taskset -c 0 sudo ../profile.sh ./bin

 Performance counter stats for './bin':

    10 508 027 832      L1-dcache-loads                                               (12,05%)
    10 463 643 206      L1-dcache-load-misses     #   99,58% of all L1-dcache hits    (12,09%)
    10 481 296 605      L1-dcache-stores                                              (12,12%)
    10 444 854 468      l1d.replacement                                               (12,15%)
    29 287 445 744      l1d_pend_miss.fb_full                                         (12,17%)
   205 569 630 707      l1d_pend_miss.pending                                         (12,17%)
     5 103 444 329      LLC-loads                                                     (12,17%)
            33 406      LLC-load-misses           #    0,00% of all LL-cache hits     (12,17%)
     9 567 917 742      LLC-stores                                                    (6,08%)
     1 157 237 980      l2_rqsts.all_pf                                               (9,12%)
                 0      sw_prefetch_access.t1_t2                                      (12,17%)
           301 471      l2_lines_out.useless_hwpf                                     (12,17%)
       218 528 985      l2_rqsts.pf_hit                                               (12,17%)
       938 735 722      l2_rqsts.pf_miss                                              (12,17%)
                 0      load_hit_pre.sw_pf                                            (12,17%)
         4 096 281      l2_rqsts.rfo_hit                                              (12,17%)
     4 972 640 931      l2_rqsts.rfo_miss                                             (12,17%)
     4 976 006 805      l2_rqsts.all_rfo                                              (12,17%)
     5 175 544 191      offcore_requests.all_data_rd                                     (12,17%)
    15 772 124 082      offcore_requests.all_requests                                     (12,17%)
     5 120 635 892      offcore_response.demand_data_rd.any_response                                     (12,17%)
       292 980 395      bus-cycles                                                    (12,17%)
    37 592 020 151      resource_stalls.any                                           (12,14%)
    37 317 091 982      resource_stalls.sb                                            (12,11%)
    38 121 826 730      uops_retired.stall_cycles                                     (12,08%)
    25 430 699 605      uops_executed.stall_cycles                                     (12,04%)
    41 416 190 037      uops_issued.stall_cycles                                      (12,04%)
    25 326 579 070      cycle_activity.stalls_l1d_miss                                     (12,04%)
    25 019 148 253      cycle_activity.stalls_l2_miss                                     (12,03%)
         7 384 770      cycle_activity.stalls_l3_miss                                     (12,03%)
    25 442 709 033      cycle_activity.stalls_mem_any                                     (12,03%)
    25 406 897 956      cycle_activity.stalls_total                                     (12,03%)
    49 877 044 086      cycles                                                        (12,03%)

      12,231406658 seconds time elapsed

      12,226386000 seconds user
       0,004000000 seconds sys

我注意到柜台:
12 611 038 018 cycle_activity.stalls_l2_miss v/s 25 019 148 253 cycle_activity.stalls_l2_miss
提示正在应用禁用M2硬件预取器的MSR。其他与L2/LLC相关的内容也存在显着差异。 差异在不同的运行之间是可重现的。问题在于 total time和周期几乎没有区别:
48 914 478 241 cycles v/s 49 877 044 086 cycles 12,155774555 seconds time elapsed v/s 12,231406658 seconds time elapsed
问题: L2遗漏是否被其他性能限制因素隐藏？如果是，您能建议看一下哪些计数器来理解它吗？

Answer 1

是的，L2拖缆在很多时候确实很有帮助。

memcpy没有任何隐藏的计算延迟，因此我想它可以让OoO exec资源(ROB大小)处理您从更多L2未命中获得的额外负载延迟，至少在这种情况下，您会从中获得所有L3命中使用适合L3的中型工作集(1MiB)，不需要进行预取就可以使L3命中。

唯一的指令是加载/存储(和循环开销)，因此OoO窗口包含了相当遥远的需求负载。

如果L2空间预取器和L1d预取器在此方面有所帮助，则为IDK。

预测以检验该假设:增大阵列大小，以便您错过L3，一旦OoO exec不足以掩盖一路直入DRAM的负载延迟，您可能会发现总体时间有所不同。硬件预取触发更远的距离可以有所帮助。

硬件预取的另一个大好处是，它可以跟上您的计算速度，因此您获得了L2命中率。 (在具有中等长度计算但没有循环携带的依赖链的循环中。)

当ROB容量没有其他压力时，需求负载和OoO exec可以利用可用(单线程)内存带宽做很多事情。

还应注意，在Intel CPU上，每个高速缓存未命中可能会花费一定量的相关uops 后端回放(来自RS/调度程序)，当预期数据到达时，L1d和L2未命中一个。在那之后，显然核心在等待数据从L3到达时乐观地发送垃圾邮件。

(请参阅https://chat.stackoverflow.com/rooms/206639/discussion-on-question-by-beeonrope-are-load-ops-deallocated-from-the-rs-when-th和Are load ops deallocated from the RS when they dispatch, complete or some other time?)

不是缓存未命中本身。在这种情况下，将是商店的说明。更具体地说，端口4的存储数据uop。使用32字节的存储区以及L3带宽上的瓶颈意味着每个时钟我们不接近1个端口4个uop。