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Vlad Lazar
0e490f3be7
## Problem Previously, we couldn't read from an in-memory layer while a batch was being written to it. Vice-versa, we couldn't write to it while there was an on-going read. ## Summary of Changes The goal of this change is to improve concurrency. Writes happened through a &mut self method so the enforcement was at the type system level. We attempt to improve by: 1. Adding interior mutability to EphemeralLayer. This involves wrapping the buffered writer in a read-write lock. 2. Minimise the time that the read lock is held for. Only hold the read lock while reading from the buffers (recently flushed or pending flush). If we need to read from the file, drop the lock and allow IO to be concurrent. The new benchmark variants with concurrent reads improve between 70 to 200 percent (against main). Benchmark results are in this [commit](https://github.com/neondatabase/neon/pull/12151/commits/891f094ce6fe6b9fdde7abd8183d59f8698804e5). ## Future Changes We can push the interior mutability into the buffered writer. The mutable tail goes under a read lock, the flushed part goes into an ArcSwap and then we can read from anything that is flushed _without_ any locking.