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As seen with a recent incident, eviction tasks can cause pageserver-wide permit starvation on the background task semaphore when synthetic size calculation takes a long time for a tenant that has more than our permit number of timelines or multiple tenants that have slow synthetic size and total number of timelines exceeds the permits. Metric links can be found in the internal [slack thread]. As a solution, release the permit while waiting for the state guarding the synthetic size calculation. This will most likely hurt the eviction task eviction performance, but that does not matter because we are hoping to get away from it using OnlyImitiate policy anyway and rely solely on disk usage-based eviction. [slack thread]: https://neondb.slack.com/archives/C06UEMLK7FE/p1713810505587809?thread_ts=1713468604.508969&cid=C06UEMLK7FE