jit_profiling_support

Definition PG 18 manual

If LLVM has the required functionality, emit the data needed to allow perf to profile functions generated by JIT. This writes out files to ~/.debug/jit/; the user is responsible for performing cleanup when desired. The default setting is off. Only superusers and users with the appropriate SET privilege can change this parameter at session start, and it cannot be changed at all within a session.

How it works

jit_profiling_support emits registration data so Linux perf can attribute samples to JIT-generated functions when LLVM support is available. Files are written under the server user's ~/.debug/jit directory.

The value is fixed at backend start and does not enable JIT. It only affects queries that generate code after crossing normal JIT gates.

PostgreSQL does not clean the profiling files automatically. Long-lived or busy systems can accumulate sensitive artifacts and consume disk. Its superuser-backend context fixes the value during connection startup and restricts who may set it.

Operational considerations

Leaving jit_profiling_support enabled after the bounded diagnostic or recovery task.

Running the experiment on the only copy of production data.

Underestimating log, core-file, temporary-file, WAL, CPU, or connection-slot amplification.

Treating a server that merely starts or completes a query as proof that data and behavior are correct.

Workload guidance

OLAP: Long analytical runs can amplify jit_profiling_support's debug overhead and artifacts. Prefer standard EXPLAIN and statistics first, and isolate any developer experiment from normal users.

OLTP: Do not tune production OLTP with jit_profiling_support. Enable it only for a bounded reproduction with an owner, log/disk budget, rollback condition, and evidence-capture plan; restore the default immediately afterward.

SMALL: Keep jit_profiling_support at its upstream default. A small host has less spare CPU, disk, connection, and log capacity for developer instrumentation.