Optimize Semaphore #10348
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Optimize Semaphore #10348
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Rewrite Semaphore to use a single AtomicLong instead of a Ref[Either[Queue, Long]]. The old implementation used an immutable Scala queue inside a Ref to track waiters. Every acquire/release went through ref.modify, which meant allocating new queue instances and ZIO effects even on the happy path. The new implementation packs permit/waiter state into a single AtomicLong - positive = available permits - negative = waiters are present The new implementation uses a MutableConcurrentQueue for pending waiters instead of copying immutable queues, though this could be changed for a more performant queue later. The new `Waiter` trait is used to represent waiters requiring a single permit and multiple permits. This prevents allocating an AtomicLong per waiter in the most common case. Benchmarks show a 30-100% performance improvement. See benchmarks here: https://jmh.morethan.io/?gists=5889786ef8c51621de513da9ec45091d,1b73e3a975d46b09f92936c30e3b83cb
HollandDM
reviewed
Jan 14, 2026
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| if (waiterPermits <= remaining) { | ||
| // We have enough permits to fulfill this waiter completely | ||
| fulfillWaiter(state, waiterPermits) |
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This can have a race condition if multi releaseUnsafe get triggerred, and only some of them can theoratically fullfill. In this case the left over will basically running into assertions
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/fixes #9093 /claim #9093
Rewrite Semaphore to use a single AtomicLong instead of a Ref[Either[Queue, Long]].
The old implementation used an immutable Scala queue inside a Ref to track waiters. Every acquire/release went through ref.modify, which meant allocating new queue instances and ZIO effects even on the happy path.
The new implementation packs permit/waiter state into a single AtomicLong
The new implementation uses a MutableConcurrentQueue for pending waiters instead of copying immutable queues, though this could be changed for a more performant queue later.
The new
Waitertrait is used to represent waiters requiring a single permit and multiple permits. This prevents allocating an AtomicLong per waiter in the most common case.Benchmarks show a 30-100% performance improvement. See benchmarks here: https://jmh.morethan.io/?gists=5889786ef8c51621de513da9ec45091d,1b73e3a975d46b09f92936c30e3b83cb