Performance: stream-head background updates schedule a Task.Run per commit under write load

[AGiorgetti]

Summary

When snapshot support is enabled and background stream-head updates are on, each successful commit calls UpdateStreamHeadInBackgroundThread(...) via Task.Run. This schedules a new thread-pool work item per commit, adding task-scheduling overhead and GC pressure at high write rates.

Why this matters

• Task-scheduling overhead accumulates at high write rates.
• Fire-and-forget work can add GC pressure and increase tail latency.
• Benchmarks show a 2.5× write-time increase with background updates enabled vs disabled.

Baseline (net10, 2026-05-20)

Benchmark	Mean
WriteToStream(CommitsToWrite=1000, DisableSnapshotSupport=False, PersistStreamHeadsOnBackgroundThread=True)	2,631.6 ms
WriteToStream(CommitsToWrite=1000, DisableSnapshotSupport=False, PersistStreamHeadsOnBackgroundThread=False)	1,059.4 ms

Delta: +148% overhead with background updates enabled.

Proposed investigation

• Determine whether the 2.5× overhead is from task-scheduling cost or actual MongoDB write contention from background updates.
• Evaluate batching or debouncing stream-head updates rather than scheduling one Task.Run per commit.
• Consider a bounded channel / dedicated background worker to coalesce stream-head updates.

Reference

See docs/Performance-Investigation.md → Finding #6 for full context.