salsa/book/src/tuning.md

# Tuning Salsa

## LRU Cache

You can specify an LRU cache size for any non-input query:

```rs
let lru_capacity: usize = 128;
base_db::ParseQuery.in_db_mut(self).set_lru_capacity(lru_capacity);
```

The default is `0`, which disables LRU-caching entirely.

Note that there is no garbage collection for keys and
results of old queries, so LRU caches are currently the
only knob available for avoiding unbounded memory usage
for long-running apps built on Salsa.

## Intern Queries

Intern queries can make key lookup cheaper, save memory, and
avoid the need for [`Arc`](https://doc.rust-lang.org/std/sync/struct.Arc.html).

Interning is especially useful for queries that involve nested,
tree-like data structures.

See:
- The [`compiler` example](https://github.com/salsa-rs/salsa/blob/master/examples/compiler/main.rs),
which uses interning.

## Granularity of Incrementality

See:
- [common patterns: selection](./common_patterns/selection.md) and
- The [`selection` example](https://github.com/salsa-rs/salsa/blob/master/examples/selection/main.rs)

## Cancellation

Queries that are no longer needed due to concurrent writes or changes in dependencies are cancelled
by Salsa. Each access of an intermediate query is a potential cancellation point. Cancellation is
implemented via panicking, and Salsa internals are intended to be panic-safe.

If you have a query that contains a long loop which does not execute any intermediate queries,
salsa won't be able to cancel it automatically. You may wish to check for cancellation yourself
by invoking `db.unwind_if_cancelled()`.

For more details on cancellation, see the tests for cancellation behavior in the Salsa repo.