OpenSearch Microbenchmark Suite

This directory contains the microbenchmark suite of OpenSearch. It relies on JMH.

Purpose

Microbenchmarks are intended to spot performance regressions in performance-critical components.

The microbenchmark suite is also handy for ad-hoc microbenchmarks but please remove them again before merging your PR.

Getting Started

Just run gradlew -p benchmarks run from the project root directory. It will build all microbenchmarks, execute them and print the result.

Running Microbenchmarks

Running via an IDE is not supported as the results are meaningless because we have no control over the JVM running the benchmarks.

If you want to run a specific benchmark class like, say, MemoryStatsBenchmark, you can use --args:

gradlew -p benchmarks run --args ' MemoryStatsBenchmark'

Everything in the ' gets sent on the command line to JMH. The leading inside the 's is important. Without it parameters are sometimes sent to gradle.

Adding Microbenchmarks

Before adding a new microbenchmark, make yourself familiar with the JMH API. You can check our existing microbenchmarks and also the JMH samples.

In contrast to tests, the actual name of the benchmark class is not relevant to JMH. However, stick to the naming convention and end the class name of a benchmark with Benchmark. To have JMH execute a benchmark, annotate the respective methods with @Benchmark.

Tips and Best Practices

To get realistic results, you should exercise care when running benchmarks. Here are a few tips:

Do

• Ensure that the system executing your microbenchmarks has as little load as possible. Shutdown every process that can cause unnecessary runtime jitter. Watch the Error column in the benchmark results to see the run-to-run variance.
• Ensure to run enough warmup iterations to get the benchmark into a stable state. If you are unsure, don't change the defaults.
• Avoid CPU migrations by pinning your benchmarks to specific CPU cores. On Linux you can use taskset.
• Fix the CPU frequency to avoid Turbo Boost from kicking in and skewing your results. On Linux you can use cpufreq-set and the performance CPU governor.
• Vary the problem input size with @Param.
• Use the integrated profilers in JMH to dig deeper if benchmark results to not match your hypotheses:
  • Add -prof gc to the options to check whether the garbage collector runs during a microbenchmarks and skews your results. If so, try to force a GC between runs (-gc true) but watch out for the caveats.
  • Add -prof perf or -prof perfasm (both only available on Linux) to see hotspots.
• Have your benchmarks peer-reviewed.

Don't

• Blindly believe the numbers that your microbenchmark produces but verify them by measuring e.g. with -prof perfasm.
• Run more threads than your number of CPU cores (in case you run multi-threaded microbenchmarks).
• Look only at the Score column and ignore Error. Instead take countermeasures to keep Error low / variance explainable.

Disassembling

Disassembling is fun! Maybe not always useful, but always fun! Generally, you'll want to install perf and FCML's hsdis. perf is generally available via apg-get install perf or pacman -S perf. FCML is a little more involved. This worked on 2020-08-01:

wget https://github.com/swojtasiak/fcml-lib/releases/download/v1.2.2/fcml-1.2.2.tar.gz
tar xf fcml*
cd fcml*
./configure
make
cd example/hsdis
make
sudo cp .libs/libhsdis.so.0.0.0 /usr/lib/jvm/java-14-adoptopenjdk/lib/hsdis-amd64.so

If you want to disassemble a single method do something like this:

gradlew -p benchmarks run --args ' MemoryStatsBenchmark -jvmArgs "-XX:+UnlockDiagnosticVMOptions -XX:CompileCommand=print,*.yourMethodName -XX:PrintAssemblyOptions=intel"

If you want perf to find the hot methods for you then do add -prof:perfasm.