Eviction

The eviction option on #[salsa::tracked] bounds the number of memoized values retained for that function. The policy defaults to SIEVE. To use LRU instead, set policy = lru.

SIEVE is recommended because its lock-free cache-hit path supports higher throughput under concurrency without serializing on the eviction policy. The SIEVE paper also found miss ratios comparable to or better than more complex eviction policies. SIEVE can favor repeatedly accessed entries over one-hit wonders, but tracks approximate rather than exact recency and may inspect several residents when choosing a victim. Results remain workload-dependent.

LRU maintains exact recency. Salsa's current implementation guards that order with an exclusive mutex. Every query access, including a cache hit, takes the mutex and updates the order, so concurrent hits to otherwise independent entries serialize.

A configured function generates function_name::set_eviction_capacity(&mut db, capacity) to adjust its capacity at runtime.

Eviction drops values from older memos to conserve memory, but retains their dependency information so Salsa can still determine whether dependent values may have changed. See cache eviction for configuration examples.