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part of Epic https://github.com/neondatabase/neon/issues/7386 # Motivation The materialized page cache adds complexity to the code base, which increases the maintenance burden and risk for subtle and hard to reproduce bugs such as #8050. Further, the best hit rate that we currently achieve in production is ca 1% of materialized page cache lookups for `task_kind=PageRequestHandler`. Other task kinds have hit rates <0.2%. Last, caching page images in Pageserver rewards under-sized caches in Computes because reading from Pageserver's materialized page cache over the network is often sufficiently fast (low hundreds of microseconds). Such Computes should upscale their local caches to fit their working set, rather than repeatedly requesting the same page from Pageserver. Some more discussion and context in internal thread https://neondb.slack.com/archives/C033RQ5SPDH/p1718714037708459 # Changes This PR removes the materialized page cache code & metrics. The infrastructure for different key kinds in `PageCache` is left in place, even though the "Immutable" key kind is the only remaining one. This can be further simplified in a future commit. Some tests started failing because their total runtime was dependent on high materialized page cache hit rates. This test makes them fixed-runtime or raises pytest timeouts: * test_local_file_cache_unlink * test_physical_replication * test_pg_regress # Performance I focussed on ensuring that this PR will not result in a performance regression in prod. * **getpage** requests: our production metrics have shown the materialized page cache to be irrelevant (low hit rate). Also, Pageserver is the wrong place to cache page images, it should happen in compute. * **ingest** (`task_kind=WalReceiverConnectionHandler`): prod metrics show 0 percent hit rate, so, removing will not be a regression. * **get_lsn_by_timestamp**: important API for branch creation, used by control pane. The clog pages that this code uses are not materialize-page-cached because they're not 8k. No risk of introducing a regression here. We will watch the various nightly benchmarks closely for more results before shipping to prod.
8 lines
113 B
Markdown
8 lines
113 B
Markdown
# Page cache
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TODO:
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- shared across tenants
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- store pages from layer files
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- store pages from "in-memory layer"
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