# Concurrent Execution

Doing work in parallel. pperl ships two distinct concurrency stories
and they answer different questions:

- **Auto-parallelization** — `pperl`'s runtime detects parallelisable
  patterns in ordinary Perl and spreads them across cores without
  any user-level threading. This is the preferred answer for
  CPU-bound loops over large data.
- **Classical ithreads (partial support)** — Perl 5's `threads` and
  `threads::shared` model. pperl's support here is deliberately
  limited; many programs that historically used ithreads are better
  rewritten against the auto-parallel path.

Start with the [decision chapter](alternatives) if you have an
existing threaded program and are deciding how to port it; start
with [parallel](parallel) if you are writing new code and want to
understand when pperl speeds your loop up.

```{toctree}
:maxdepth: 2

parallel
threads
```