perf: de-parallelize upscale ancestor lookup to avoid thread-pool starvation - #264
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perf: de-parallelize upscale ancestor lookup to avoid thread-pool starvation#264shimoncohen wants to merge 1 commit into
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…rvation InternalGetLastExistingTile fetched all ancestor coords via an unbounded Parallel.ForEachAsync and blocked on .Result. When called from inside the service-level Parallel.ForEach merge loop this nests parallelism and, via the sync-over-async .Result, blocks an outer worker thread while inner Task.Run items compete for the same thread pool -> starvation under load. The inner GetTile calls also serialize on the source connection, so the concurrency bought little. Replace with a sequential highest-zoom-first loop that short-circuits on the first existing tile. Same result (closest ancestor), fewer GetTile calls, no nesting, no sync-over-async. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Problem
InternalGetLastExistingTile(Data.cs) fetched all ancestor coords for upscale via an unboundedParallel.ForEachAsyncand then blocked on.Result. When invoked from inside the service-levelParallel.ForEachmerge loop (see PR #260), this nests parallelism, and the sync-over-async.Resultblocks an outer worker thread while innerTask.Runitems compete for the same thread pool → thread-pool starvation under load. The innerGetTilecalls also serialize on the source connection, so the concurrency bought little.This addresses @asafmas-rnd's "check that Parallel doesn't call other parallel's" comment on #260 — the inner parallel is pre-existing (from #119, 2023); #260's outer parallel is what turns it into a nest.
Fix
Sequential, highest-zoom-first loop that short-circuits on the first existing tile.
coordsListis already ordered high→low zoom, so the first hit is the closest ancestor — identical result, fewerGetTilecalls, no nesting, no sync-over-async.Verification
System.Collections.Concurrent🤖 Generated with Claude Code