MoFlo Memory Optimization

When the default moflo memory settings stop being enough — past ~100k entries, or when p95 search latency climbs — these are the levers.

HNSW Parameters

HNSW has three knobs. They trade build time, query time, memory, and recall.

import { HNSWIndex } from 'moflo/dist/src/cli/memory/index.js';

const index = new HNSWIndex({
  dimensions: 1536,        // must match your embedding model
  maxElements: 1_000_000,  // pre-allocated capacity
  M: 16,                   // graph connectivity (default 16)
  efConstruction: 200,     // build-time search width (default 200)
  metric: 'cosine',        // 'cosine' | 'l2' | 'ip'
});

Knob	Higher	Lower	When to change
M	better recall, more RAM (~2×M pointers per point)	less RAM, worse recall	Bump to 32–64 if recall@10 < 0.95; drop to 8 if memory-bound
efConstruction	better index quality, slower build	faster build, worse queries	200–400 is sweet spot; only lower in test fixtures
ef (search-time, passed to search())	better recall, slower queries	faster queries, worse recall	Start at 2×k, raise until recall plateaus

Rule of thumb: M and efConstruction are set once. ef is the runtime dial.

Quantization

moflo memory supports scalar quantization (Float32 → Int8) for a ~4× memory reduction with a ~1-2% recall hit. Turn it on when the index doesn't fit comfortably in RAM.

const index = new HNSWIndex({
  dimensions: 1536,
  maxElements: 5_000_000,
  quantization: {
    enabled: true,
    type: 'scalar',   // scalar (Int8) is the supported path
    rebuildThreshold: 10_000,
  },
});

Measure recall before/after on your own query distribution — public benchmarks don't predict your domain.

Batch Operations

Single-entry writes pay the HNSW insert cost per call. For bulk ingest, batch:

const entries: Array<[string, Float32Array]> = buildCorpus();

// Parallelise at the adapter level; don't await sequentially.
await Promise.all(
  entries.map(([id, vec]) => index.addPoint(id, vec))
);

// Or via MCP for moflo-native batch into .swarm/memory.db:
await mcp.memory_store(/* … */);  // upsert: true + Promise.all is fine

For >10k entries, prefer bin/build-embeddings.mjs / bin/index-all.mjs — they stream in batches with a progress bar and skip unchanged chunks via a hash file.

Caching

MofloDbAdapter has a built-in LRU cache (default 10k entries, 5-min TTL):

import { MofloDbAdapter } from 'moflo/dist/src/cli/memory/index.js';

const store = new MofloDbAdapter({
  cacheEnabled: true,
  cacheSize: 50_000,      // scale with working set, not total corpus
  cacheTtl: 10 * 60_000,  // 10 minutes
});

Cache hits on exact keys bypass HNSW entirely. If your workload is read-heavy and hits a narrow keyspace, this is the cheapest win.

Measuring

npx vitest bench src/cli/memory/benchmarks/vector-search.bench.ts

The bench prints linear vs HNSW times for 1k and 10k vectors. Run it before and after any parameter change — "it felt faster" is not a benchmark.

For production memory stats:

const stats = await mcp.memory_stats({});
// { entryCount, indexSize, cacheHitRate, avgSearchMs, … }

Common Bottlenecks

Symptom	Likely cause	Fix
Cold-start of 5s on first search	HNSW loading from disk	Share a single instance via beforeAll in tests; keep the adapter resident in long-running processes
Search latency climbs linearly with limit	Over-fetching and re-ranking on the hot path	Lower limit; raise threshold to prune
Inserts slow past ~100k entries	maxElements too close to entry count → reallocation	Set maxElements to 2× expected corpus
High RSS on a small corpus	Vector dimension mismatch with index	Confirm dimensions matches embedder output (OpenAI = 1536, local models vary)

Anti-Patterns

• Don't rebuild the index on every test. Use a module-level singleton + beforeAll. HNSW cold-boot is ~5s.
• Don't raise ef globally. Raise it on the specific queries that need recall. Default is fine for 90% of calls.
• Don't quantize a small corpus. Below ~500k vectors the RAM saving doesn't justify the recall cost.
• Don't measure in dev mode. The memory stack behaves differently under NODE_ENV=production; benches should match the target.

See Also

• memory-patterns skill — API usage and namespace design
• vector-search skill — RAG-specific patterns on top of the optimized index
• src/cli/memory/benchmarks/ — runnable benches for every knob above