为何Zen5 CPU上PERF_COUNT_HW_REF_CPU_CYCLES方差远高于PERF_COUNT_HW_CPU_CYCLES？

我理解PERF_COUNT_HW_REF_CPU_CYCLES是恒定速率的计数器，不受CPU频率缩放影响；而PERF_COUNT_HW_CPU_CYCLES会受频率缩放影响。原本以为消除频率缩放影响能降低计数方差，但在Zen5 CPU上却出现了方差反而升高的情况！

Zen5 CPU测试结果 #

$ echo; for cycle_type in cycles ref-cycles; do echo -n "$cycle_type stddev: "; for i in {1..100}; do perf stat -e $cycle_type true 2>&1 | grep -E "(cycles|ref-cycles)" | grep -v "seconds"; done | awk '{gsub(/,/,""); print $1}' | awk '{sum+=$1; sumsq+=$1*$1; n++} END {mean=sum/n; print sqrt(sumsq/n - mean*mean)}'; done

cycles stddev: 146434
ref-cycles stddev: 353483

Haswell服务器测试结果（符合预期） #

$ echo; for cycle_type in cycles ref-cycles; do echo -n "$cycle_type stddev: "; for i in {1..100}; do perf stat -e $cycle_type true 2>&1 | grep -E "(cycles|ref-cycles)" | grep -v "seconds"; done | awk '{gsub(/,/,""); print $1}' | awk '{sum+=$1; sumsq+=$1*$1; n++} END {mean=sum/n; print sqrt(sumsq/n - mean*mean)}'; done

cycles stddev: 64606.4
ref-cycles stddev: 46084.4

硬件与软件信息 #

• Zen5 CPU型号：AMD Ryzen AI 9 HX 370 w/ Radeon 890M
• Haswell CPU型号：Intel(R) Xeon(R) CPU E5-2698 v3 @ 2.30GHz
• Perf版本：6.14.2

补充说明 #

Peter Cordes指出原基准测试可能因主要测量进程启动开销而存在误导性，但即使改用perf_event_open配置计数器，再通过rdpmc在进程内对简单微基准测试采样，仍能观察到相同现象。

内容的提问来源于stack exchange，提问作者Joseph Garvin