X86 三级缓存未命中的PMC（性能监视计数器）值过高

我正在寻找一种方法来估计三级缓存未命中数，方法是在装有英特尔CPU（英特尔i7 6700 skylake）的Linux PC上使用“IA32_PERFEVTSELx”和“IA32_PMCx”MSR对。 为此，我在内核中安装了一个计时器，它定期（1秒）报告PMC的值。 在代码中，我在写入“0x41412E”后读取IA32_PMC1 MSR的值（映射到0xC2），其中事件选择部分为0x2E，UMask部分为0x41，第16位为用户，第22位为相对于IA32_PERFEVTSEL1 MSR的启用位（映射到0x187）：

即使我期望该值表示三级缓存未命中的数量，但这似乎很奇怪。它的值太高，因此，我认为它不是三级缓存未命中数，我在手册（英特尔64和IA-32体系结构软件开发人员手册第3B卷：系统编程指南）中找不到它的含义。我观察到的数值如下：

rdmsr: 0 at start_shscan(56) in mcsched.c
rdmsr: 0 at start_shscan(56) in mcsched.c
rdmsr: 8595908 at start_shscan(56) in mcsched.c
rdmsr: 17274482 at start_shscan(56) in mcsched.c
rdmsr: 21449216 at start_shscan(56) in mcsched.c
rdmsr: 26305745 at start_shscan(56) in mcsched.c
rdmsr: 26511242 at start_shscan(56) in mcsched.c
rdmsr: 33316291 at start_shscan(56) in mcsched.c
rdmsr: 34736360 at start_shscan(56) in mcsched.c
rdmsr: 35151932 at start_shscan(56) in mcsched.c
rdmsr: 43806356 at start_shscan(56) in mcsched.c
rdmsr: 51132302 at start_shscan(56) in mcsched.c
rdmsr: 59797757 at start_shscan(56) in mcsched.c
rdmsr: 0 at start_shscan(56) in mcsched.c
rdmsr: 0 at start_shscan(56) in mcsched.c
rdmsr: 6820029 at start_shscan(56) in mcsched.c
rdmsr: 8322078 at start_shscan(56) in mcsched.c
rdmsr: 63313471 at start_shscan(56) in mcsched.c
rdmsr: 397962 at start_shscan(56) in mcsched.c
rdmsr: 9429026 at start_shscan(56) in mcsched.c
rdmsr: 18124455 at start_shscan(56) in mcsched.c
rdmsr: 23706367 at start_shscan(56) in mcsched.c
rdmsr: 27087960 at start_shscan(56) in mcsched.c
rdmsr: 68769660 at start_shscan(56) in mcsched.c
rdmsr: 69110424 at start_shscan(56) in mcsched.c
rdmsr: 78216541 at start_shscan(56) in mcsched.c
rdmsr: 87385467 at start_shscan(56) in mcsched.c
rdmsr: 95083478 at start_shscan(56) in mcsched.c
rdmsr: 101347654 at start_shscan(56) in mcsched.c
rdmsr: 8327692 at start_shscan(56) in mcsched.c
rdmsr: 27377092 at start_shscan(56) in mcsched.c
rdmsr: 36316258 at start_shscan(56) in mcsched.c
rdmsr: 45323291 at start_shscan(56) in mcsched.c
rdmsr: 54366010 at start_shscan(56) in mcsched.c
rdmsr: 63135801 at start_shscan(56) in mcsched.c
rdmsr: 72037000 at start_shscan(56) in mcsched.c
rdmsr: 81032798 at start_shscan(56) in mcsched.c
rdmsr: 89975340 at start_shscan(56) in mcsched.c
rdmsr: 98661287 at start_shscan(56) in mcsched.c
rdmsr: 107482921 at start_shscan(56) in mcsched.c
rdmsr: 116290561 at start_shscan(56) in mcsched.c
rdmsr: 125135979 at start_shscan(56) in mcsched.c
rdmsr: 133920103 at start_shscan(56) in mcsched.c
rdmsr: 142695638 at start_shscan(56) in mcsched.c
rdmsr: 151456156 at start_shscan(56) in mcsched.c
rdmsr: 160171239 at start_shscan(56) in mcsched.c
rdmsr: 168879495 at start_shscan(56) in mcsched.c
rdmsr: 177788861 at start_shscan(56) in mcsched.c
rdmsr: 186589920 at start_shscan(56) in mcsched.c
rdmsr: 195331675 at start_shscan(56) in mcsched.c
rdmsr: 204166715 at start_shscan(56) in mcsched.c
rdmsr: 213045449 at start_shscan(56) in mcsched.c
rdmsr: 221942627 at start_shscan(56) in mcsched.c
rdmsr: 231073520 at start_shscan(56) in mcsched.c

代码中有我犯的错误吗？或者请给我一个建议的价值观

=====================================添加了以下内容==========================

@Peter Cordes，我参考了《英特尔手册》（英特尔64和IA-32体系结构软件开发人员手册第3B卷：系统编程指南），我打算使用“LLC未命中”，这是下表中预定义的体系结构性能事件之一：

---

我认为在perf中给出一个示例更有助于您理解：我可以在perf中使用“perf stat-e r412e ls”来估计“ls”命令期间的三级缓存未命中。“r412e”可分为“r”+“41”+“2e”，r表示“[原始硬件evnet事件描述符]，41表示UMask（0x41），2e表示事件选择（0x2e）。您可以通过“性能列表”查看它。

您认为您正在测量的事件的名称是什么？

mem\u load\u retired.l3\u miss

，或其他事件之一？您正在记录的CPU内核上运行的是什么？这些数字是否接近您为tha获得的

perf stat--per core-C$core\u number-e LLC load miss

t core？（这个命令可能不太正确，但你明白了。）@PeterCordes我看到了您的评论并添加了上述文章。请再次检查。谢谢。您是否尝试过使用

perf stat-e LLC load misses

进行测量，以查看其计数是否与您使用自己的测量代码看到的计数一样高？对于此事件或任何其他事件？参考周期很容易验证，因为它具有精确的相关性pondence to wall clock time，这样你就知道应该有多少个周期。（特别是如果你使用

rdtsc

）@PeterCordes啊，对不起。我刚刚执行了一个微基准测试，它创建了4个线程（适合物理核的数量），我将每个线程固定在一个物理核中，每个线程访问自己的数据（大尺寸int数组）。基准测试用于对三级缓存进行阈值筛选。结合您估计的结果和我上面的结果，我的测量值似乎大致符合三级缓存未命中率。非常感谢！是的，对于刷新缓存的工作负载来说，每秒6900万次三级缓存未命中率似乎完全合理。即每57个时钟周期1次（使用4GHz CPU）.内存操作是流水线的，每个核心可以为不同的缓存线保留10个未完成的请求。（因此，如果没有遇到DRAM吞吐量瓶颈，吞吐量=最大并发/延迟。）请参阅。

rdmsr: 0 at start_shscan(56) in mcsched.c
rdmsr: 0 at start_shscan(56) in mcsched.c
rdmsr: 8595908 at start_shscan(56) in mcsched.c
rdmsr: 17274482 at start_shscan(56) in mcsched.c
rdmsr: 21449216 at start_shscan(56) in mcsched.c
rdmsr: 26305745 at start_shscan(56) in mcsched.c
rdmsr: 26511242 at start_shscan(56) in mcsched.c
rdmsr: 33316291 at start_shscan(56) in mcsched.c
rdmsr: 34736360 at start_shscan(56) in mcsched.c
rdmsr: 35151932 at start_shscan(56) in mcsched.c
rdmsr: 43806356 at start_shscan(56) in mcsched.c
rdmsr: 51132302 at start_shscan(56) in mcsched.c
rdmsr: 59797757 at start_shscan(56) in mcsched.c
rdmsr: 0 at start_shscan(56) in mcsched.c
rdmsr: 0 at start_shscan(56) in mcsched.c
rdmsr: 6820029 at start_shscan(56) in mcsched.c
rdmsr: 8322078 at start_shscan(56) in mcsched.c
rdmsr: 63313471 at start_shscan(56) in mcsched.c
rdmsr: 397962 at start_shscan(56) in mcsched.c
rdmsr: 9429026 at start_shscan(56) in mcsched.c
rdmsr: 18124455 at start_shscan(56) in mcsched.c
rdmsr: 23706367 at start_shscan(56) in mcsched.c
rdmsr: 27087960 at start_shscan(56) in mcsched.c
rdmsr: 68769660 at start_shscan(56) in mcsched.c
rdmsr: 69110424 at start_shscan(56) in mcsched.c
rdmsr: 78216541 at start_shscan(56) in mcsched.c
rdmsr: 87385467 at start_shscan(56) in mcsched.c
rdmsr: 95083478 at start_shscan(56) in mcsched.c
rdmsr: 101347654 at start_shscan(56) in mcsched.c
rdmsr: 8327692 at start_shscan(56) in mcsched.c
rdmsr: 27377092 at start_shscan(56) in mcsched.c
rdmsr: 36316258 at start_shscan(56) in mcsched.c
rdmsr: 45323291 at start_shscan(56) in mcsched.c
rdmsr: 54366010 at start_shscan(56) in mcsched.c
rdmsr: 63135801 at start_shscan(56) in mcsched.c
rdmsr: 72037000 at start_shscan(56) in mcsched.c
rdmsr: 81032798 at start_shscan(56) in mcsched.c
rdmsr: 89975340 at start_shscan(56) in mcsched.c
rdmsr: 98661287 at start_shscan(56) in mcsched.c
rdmsr: 107482921 at start_shscan(56) in mcsched.c
rdmsr: 116290561 at start_shscan(56) in mcsched.c
rdmsr: 125135979 at start_shscan(56) in mcsched.c
rdmsr: 133920103 at start_shscan(56) in mcsched.c
rdmsr: 142695638 at start_shscan(56) in mcsched.c
rdmsr: 151456156 at start_shscan(56) in mcsched.c
rdmsr: 160171239 at start_shscan(56) in mcsched.c
rdmsr: 168879495 at start_shscan(56) in mcsched.c
rdmsr: 177788861 at start_shscan(56) in mcsched.c
rdmsr: 186589920 at start_shscan(56) in mcsched.c
rdmsr: 195331675 at start_shscan(56) in mcsched.c
rdmsr: 204166715 at start_shscan(56) in mcsched.c
rdmsr: 213045449 at start_shscan(56) in mcsched.c
rdmsr: 221942627 at start_shscan(56) in mcsched.c
rdmsr: 231073520 at start_shscan(56) in mcsched.c