hardware-counters

Hardware Performance Counters

Purpose

Guide agents through hardware performance counter analysis: collecting PMU events with perf stat -e, using the PAPI library for portable counter access, interpreting cache miss rates and branch misprediction ratios, computing IPC, and correlating events to source lines with perf annotate.

Triggers

• "How do I measure cache miss rate with perf?"
• "How do I count branch mispredictions?"
• "How do I compute IPC (instructions per clock) with perf?"
• "How do I use the PAPI library for hardware counters?"
• "How do I see which source lines cause the most cache misses?"
• "How do I measure memory bandwidth with performance counters?"

Workflow

1. perf stat — basic counter collection

# Basic hardware event summary
perf stat ./prog

# Output:
#  Performance counter stats for './prog':
#
#      1,234,567,890      instructions
#        456,789,012      cycles
#         12,345,678      cache-misses         #    1.23 % of all cache refs
#         23,456,789      branch-misses        #    2.34 % of all branches
#
#       0.456789012 seconds time elapsed

# Derived metrics (computed from the output)
# IPC = instructions / cycles = 1,234,567,890 / 456,789,012 ≈ 2.70
# CPI = cycles / instructions ≈ 0.37

2. Specifying PMU events with -e

# Specific hardware events
perf stat -e instructions,cycles,cache-misses,branch-misses ./prog

# L1/L2/L3 cache events
perf stat -e \
  L1-dcache-loads,L1-dcache-load-misses,\
  L2-loads,L2-load-misses,\
  LLC-loads,LLC-load-misses \
  ./prog

# Memory bandwidth (Intel)
perf stat -e \
  uncore_imc/cas_count_read/,\
  uncore_imc/cas_count_write/ \
  ./prog

# TLB misses
perf stat -e dTLB-loads,dTLB-load-misses,iTLB-loads,iTLB-load-misses ./prog

# Branch misprediction rate
perf stat -e branches,branch-misses ./prog
# Rate = branch-misses / branches × 100%

# Available events (varies by CPU)
perf list hardware          # generic hardware events
perf list cache             # cache events
perf list pmu               # raw PMU events for your CPU