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## Problem Follows: https://github.com/neondatabase/neon/pull/7182 - Sufficient concurrent writes could OOM a pageserver from the size of indices on all the InMemoryLayer instances. - Enforcement of checkpoint_period only happened if there were some writes. Closes: https://github.com/neondatabase/neon/issues/6916 ## Summary of changes - Add `ephemeral_bytes_per_memory_kb` config property. This controls the ratio of ephemeral layer capacity to memory capacity. The weird unit is to enable making the ratio less than 1:1 (set this property to 1024 to use 1MB of ephemeral layers for every 1MB of RAM, set it smaller to get a fraction). - Implement background layer rolling checks in Timeline::compaction_iteration -- this ensures we apply layer rolling policy in the absence of writes. - During background checks, if the total ephemeral layer size has exceeded the limit, then roll layers whose size is greater than the mean size of all ephemeral layers. - Remove the tick() path from walreceiver: it isn't needed any more now that we do equivalent checks from compaction_iteration. - Add tests for the above. --------- Co-authored-by: Arpad Müller <arpad-m@users.noreply.github.com>