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?
- Build ClickHouse in Instrumented mode. In Clang it can be done via passing
- 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-usecompiler 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.