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neon/pageserver
Christian Schwarz f5b9af6ac7 page cache: improve eviction-related metrics (#6077)
These changes help with identifying thrashing.

The existing `pageserver_page_cache_find_victim_iters_total` is already
useful, but, it doesn't tell us how many individual find_victim() calls
are happening, only how many clock-LRU steps happened in the entire
system,
without info about whether we needed to actually evict other data vs
just scan for a long time, e.g., because the cache is large.

The changes in this PR allows us to
1. count each possible outcome separately, esp evictions
2. compute mean iterations/outcome

I don't think anyone except me was paying close attention to
`pageserver_page_cache_find_victim_iters_total` before, so,
I think the slight behavior change of also counting iterations
for the 'iters exceeded' case is fine.

refs https://github.com/neondatabase/cloud/issues/8351
refs https://github.com/neondatabase/neon/issues/5479
2023-12-08 15:27:21 +00:00
..
2023-11-28 12:50:53 -05:00