Introduction to Perf
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Perf is a Linux profiler tool that uses performance monitoring hardware to count various events like CPU cycles, instructions, and cache misses. It can count events for a single thread, entire process, specific CPUs, or system-wide. Perf stat is used to count events during process execution, while perf record collects profiling data in a file for later analysis with perf report.
Introduction to Perf
- 2. What is Perf? • Perf is a profiler tool for Linux 2.6+ based systems that abstracts away CPU hardware differences in Linux performance measurements and presents a simple command line interface. • Perf is based on the perf_events interface exported by recent versions of the Linux kernel.
- 3. Events • software events - pure kernel counters • context-switches • hardware events - Performance Monitoring Unit (PMU) • measure micro-architectural events such as the number of cycles, instructions retired, L1 cache misses and so on • hardware cache events - events provided by the CPU • tracepoint events - kernel ftrace infrastructure
- 4. perf stat • For any of the supported events, perf can keep a running count during process execution. • Events are designated using their symbolic names followed by optional unit masks and modifiers. • perf stat -e cycles <command> • perf stat -e cycles:u <command> • perf stat -e cycles,instructions,cache-misses <command>
- 5. Modifiers
- 6. Multiplexing and Scaling Events • If there are more events than counters, the kernel uses time multiplexing (switch frequency = HZ, generally 100 or 1000) to give each event a chance to access the monitoring hardware. • Multiplexing only applies to PMU events. • At the end of the run, the tool scales the count based on total time enabled vs time running. • final_count = raw_count * time_enabled/time_running
- 7. • The perf tool can be used to count events on a per- thread, per-process, per-cpu or system-wide basis.
- 8. • per-thread • the counter only monitors the execution of a designated thread. • When the thread is scheduled out, monitoring stops. • By default, perf stat counts in per-thread mode. • Attaching to a running kernel thread • perf stat -e cycles -t <thread-id>
- 9. • per-process • all threads of the process are monitored • Counts and samples are aggregated at the process level. • The perf_events interface allows for automatic inheritance on fork() and pthread_create(). • Attaching to a running process • perf stat -e cycles -p <pid>
- 10. • per-cpu • all threads running on the designated processors are monitored. • perf stat -e cycles:u,instructions:u -a <command> • perf stat -e cycles:u,instructions:u -a -C 0,2-3 <command>
- 11. perf record • collect profiles on per-thread, per-process and per- cpu basis • This generates an output file called perf.data.
- 12. Event-Based Sampling • By default, perf record uses the cycles event as the sampling event. • The perf_events interface allows two modes to express the sampling period: • the number of occurrences of the event (period) • perf record -e retired_instructions:u -c 2000 <command> • the average rate of samples/sec (frequency) • The perf tool defaults to the average rate. It is set to 1000Hz, or 1000 samples/sec. • perf record -e instructions:u -F 250 <command>
- 13. perf report • Samples collected by perf record are saved into a binary file called, by default, perf.data. The perf report command reads this file and generates a concise execution profile.