Topic 6 notes — buffer pool & memory management

Predictions (fill BEFORE running)

• pool_vs_mmap p50: mmap ___ ns vs pool ___ ns (who wins the median and why?)
• pool_vs_mmap p99.9: mmap ___ vs pool ___ (who wins the tail?)
• CLOCK hit rate vs strict LRU on Zipf(0.99), 16× universe: gap of ___ %
• strict LRU ns/access vs CLOCK: ___× slower

pool_vs_mmap results

(paste; run warm and note whether the file fit the OS page cache)

Pool hit rate: ___ %. Explanation of the tail difference:

eviction bench results

Verdict — is strict LRU’s hit-rate edge worth its per-hit cost here?

Buffer pool build log (src/buffer_pool.rs)

• Where the WAL rule hooks in (which write, which LSN check):
• What a background writer would take off the miss path:
• What a 6-page scan did to usage counts (hot_page_survives_scan_pressure):

Reading-guide questions

postgres bufmgr (reading-postgres-bufmgr.md)

1. 18 refcount bits vs 4 usage bits — which cap fails gracefully:
2. Who unpins after a crashed query:
3. Buffer rings (admission) vs cooling stage (eviction) — what each misses:
4. What O_DIRECT buys/costs given double buffering:

DuckDB buffer manager (reading-duckdb-buffer.md)

1. Why weak_ptr in eviction nodes:
2. Worst-case dead-node ratio; when CLOCK’s fixed array is strictly better:
3. Throw-on-pressure vs error-when-all-pinned — capstone choice:

LeanStore (reading-leanstore.md)

1. One-parent constraint walkthrough; are FalkorDB matrix blocks a tree or DAG:
2. Why eviction must be bottom-up:
3. Random-cooling hit-rate loss vs LRU (estimate, then measure):
4. What survives in vmcache, what dies:

mmap paper (reading-mmap-paper.md)

1. Which topic-5 crash_test would fail under mmap and why:
2. Why eviction (not fault-in) triggers TLB shootdowns:
3. Reconciling the paper with LMDB’s wins:
4. vmcache vs the four problems — solved vs softened:

LeanStore/vmcache papers (reading-leanstore-paper.md)

1. Classic-pool overhead as arithmetic (topic-0 numbers):
2. Why cooling is a FIFO, not a stack:
3. vmcache state word vs postgres packed state — what colocation buys:
4. Which design is admissible for matrix-tile DAGs:

redis zmalloc (reading-redis-zmalloc.md)

1. Cost of a shared fetch_add per malloc at 8 threads:
2. Key eviction vs page eviction — why the comparison is unfair:
3. My pool vs turso’s page_cache.rs — three design diffs:

M6 log (capstone)

• Buffer pool under the persistent backends
• Per-backend pools vs shared pool + MemoryTag accounting — decision + why:
• mmap write-back unpredictability reproduced (numbers):