Tuning Salsa

LRU Cache

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

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.

See The LRU RFC for more details.

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.

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

See:

• The Intern Queries RFC
• The compiler example, which uses interning.

Granularity of Incrementality

See:

• common patterns: selection and
• The selection example

Cancellation

Queries that are no longer needed due to concurrent writes or changes in dependencies are cancelled by Salsa. Each accesss 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 Opinionated cancellation RFC.
• The tests for cancellation behavior in the Salsa repo.