perf(ingester): Convert postings cache from FIFO to LRU eviction#7510
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The expanded postings cache previously used FIFO eviction, which evicts the oldest entry regardless of access frequency. Under memory pressure with high series churn, actively-queried entries get evicted simply because they were inserted earliest, causing repeated expensive recomputations. LRU eviction moves entries to the back of the list on every access, ensuring frequently-queried entries (e.g., ruler queries running every 30s) stay cached even when the cache is full. Only entries that haven't been accessed recently get evicted. Implementation: add a map[string]*list.Element for O(1) MoveToBack on cache hit. The eviction logic (shouldEvictHead/evictHead) remains unchanged — it still evicts from the front, but now the front contains the least-recently-used entry instead of the oldest-inserted entry. Signed-off-by: Alan Protasio <approtas@amazon.com>
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Signed-off-by: Alan Protasio <approtas@amazon.com>
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What this PR does:
The expanded postings cache previously used FIFO eviction, which evicts the oldest entry regardless of access frequency. Under memory pressure with high series churn, actively-queried entries get evicted simply because they were inserted earliest, causing repeated expensive recomputations.
LRU eviction moves entries to the back of the list on every access, ensuring frequently-queried entries (e.g., ruler queries running every 30s) stay cached even when the cache is full. Only entries that haven't been accessed recently get evicted.
Implementation: add a map[string]*list.Element for O(1) MoveToBack on cache hit. The eviction logic (shouldEvictHead/evictHead) remains unchanged — it still evicts from the front, but now the front contains the least-recently-used entry instead of the oldest-inserted entry.
Which issue(s) this PR fixes:
Fixes #
Checklist
CHANGELOG.mdupdated - the order of entries should be[CHANGE],[FEATURE],[ENHANCEMENT],[BUGFIX]docs/configuration/v1-guarantees.mdupdated if this PR introduces experimental flags