import SelfManaged from '@site/docs/_snippets/_self_managed_only_no_roadmap.md';

Profile guided optimization

Profile-Guided Optimization (PGO) is a compiler optimization technique where a program is optimized based on the runtime profile.

According to the tests, PGO helps with achieving better performance for ClickHouse. According to the tests, we see improvements up to 15% in QPS on the ClickBench test suite. The more detailed results are available here. The performance benefits depend on your typical workload - you can get better or worse results.

More information about PGO in ClickHouse you can read in the corresponding GitHub issue.

How to build ClickHouse with PGO? {#how-to-build-clickhouse-with-pgo}

There are two major kinds of PGO: Instrumentation and Sampling (also known as AutoFDO). In this guide is described the Instrumentation PGO with ClickHouse.

Build ClickHouse in Instrumented mode. In Clang it can be done via passing -fprofile-generate option to CXXFLAGS.
Run instrumented ClickHouse on a sample workload. Here you need to use your usual workload. One of the approaches could be using ClickBench as a sample workload. ClickHouse in the instrumentation mode could work slowly so be ready for that and do not run instrumented ClickHouse in performance-critical environments.
Recompile ClickHouse once again with -fprofile-use compiler flags and profiles that are collected from the previous step.

A more detailed guide on how to apply PGO is in the Clang documentation.

If you are going to collect a sample workload directly from a production environment, we recommend trying to use Sampling PGO.