vector-index-tuning

name	vector-index-tuning
description	Optimize vector index performance for latency, recall, and memory. Use when tuning HNSW parameters, selecting quantization strategies, or scaling vector search infrastructure.
owner	RStack developed by Richardson Gunde

Vector Index Tuning

Guide to optimizing vector indexes for production performance.

When to Use This Skill

• Tuning HNSW parameters
• Implementing quantization
• Optimizing memory usage
• Reducing search latency
• Balancing recall vs speed
• Scaling to billions of vectors

Core Concepts

1. Index Type Selection

Data Size           Recommended Index
────────────────────────────────────────
< 10K vectors  →    Flat (exact search)
10K - 1M       →    HNSW
1M - 100M      →    HNSW + Quantization
> 100M         →    IVF + PQ or DiskANN

2. HNSW Parameters

Parameter	Default	Effect
M	16	Connections per node, ↑ = better recall, more memory
efConstruction	100	Build quality, ↑ = better index, slower build
efSearch	50	Search quality, ↑ = better recall, slower search

3. Quantization Types

Full Precision (FP32): 4 bytes × dimensions
Half Precision (FP16): 2 bytes × dimensions
INT8 Scalar:           1 byte × dimensions
Product Quantization:  ~32-64 bytes total
Binary:                dimensions/8 bytes

Templates

Template 1: HNSW Parameter Tuning

import numpy as np
from typing import List, Tuple
import time

def benchmark_hnsw_parameters(
    vectors: np.ndarray,
    queries: np.ndarray,
    ground_truth: np.ndarray,
    m_values: List[int] = [8, 16, 32, 64],
    ef_construction_values: List[int] = [64, 128, 256],
    ef_search_values: List[int] = [32, 64, 128, 256]
) -> List[dict]:
    """Benchmark different HNSW configurations."""
    import hnswlib

    results = []
    dim = vectors.shape[1]
    n = vectors.shape[0]

    for m in m_values:
        for ef_construction in ef_construction_values:
            # Build index
            index = hnswlib.Index(space='cosine', dim=dim)
            index.init_index(max_elements=n, M=m, ef_construction=ef_construction)

            build_start = time.time()
            index.add_items(vectors)
            build_time = time.time() - build_start

            # Get memory usage
            memory_bytes = index.element_count * (
                dim * 4 +  # Vector storage
                m * 2 * 4  # Graph edges (approximate)
            )

            for ef_search in ef_search_values:
                index.set_ef(ef_search)

                # Measure search
                search_start = time.time()
                labels, distances = index.knn_query(queries, k=10)
                search_time = time.time() - search_start

                # Calculate recall
                recall = calculate_recall(labels, ground_truth, k=10)

                results.append({
                    "M": m,
                    "ef_construction": ef_construction,
                    "ef_search": ef_search,
                    "build_time_s": build_time,
                    "search_time_ms": search_time * 1000 / len(queries),
                    "recall@10": recall,
                    "memory_mb": memory_bytes / 1024 / 1024
                })

    return results


def calculate_recall(predictions: np.ndarray, ground_truth: np.ndarray, k: int) -> float:
    """Calculate recall@k."""
    correct = 0
    for pred, truth in zip(predictions, ground_truth):
        correct += len(set(pred[:k]) & set(truth[:k]))
    return correct / (len(predictions) * k)


def recommend_hnsw_params(
    num_vectors: int,
    target_recall: float = 0.95,
    max_latency_ms: float = 10,
    available_memory_gb: float = 8
) -> dict:
    """Recommend HNSW parameters based on requirements."""

    # Base recommendations
    if num_vectors < 100_000:
        m = 16
        ef_construction = 100
    elif num_vectors < 1_000_000:
        m = 32
        ef_construction = 200
    else:
        m = 48
        ef_construction = 256

    # Adjust ef_search based on recall target
    if target_recall >= 0.99:
        ef_search = 256
    elif target_recall >= 0.95:
        ef_search = 128
    else:
        ef_search = 64

    return {
        "M": m,
        "ef_construction": ef_construction,
        "ef_search": ef_search,
        "notes": f"Estimated for {num_vectors:,} vectors, {target_recall:.0%} recall"
    }

Template 2: Quantization Strategies

import numpy as np
from typing import Optional

class VectorQuantizer:
    """Quantization strategies for vector compression."""

    @staticmethod
    def scalar_quantize_int8(
        vectors: np.ndarray,
        min_val: Optional[float] = None,
        max_val: Optional[float] = None
    ) -> Tuple[np.ndarray, dict]:
        """Scalar quantization to INT8."""
        if min_val is None:
            min_val = vectors.min()
        if max_val is None:
            max_val = vectors.max()

        # Scale to 0-255 range
        scale = 255.0 / (max_val - min_val)
        quantized = np.clip(
            np.round((vectors - min_val) * scale),
            0, 255
        ).astype(np.uint8)

        params = {"min_val": min_val, "max_val": max_val, "scale": scale}
        return quantized, params

    @staticmethod
    def dequantize_int8(
        quantized: np.ndarray,
        params: dict
    ) -> np.ndarray:
        """Dequantize INT8 vectors."""
        return quantized.astype(np.float32) / params["scale"] + params["min_val"]

    @staticmethod
    def product_quantize(
        vectors: np.ndarray,
        n_subvectors: int = 8,
        n_centroids: int = 256
    ) -> Tuple[np.ndarray, dict]:
        """Product quantization for aggressive compression."""
        from sklearn.cluster import KMeans

        n, dim = vectors.shape
        assert dim % n_subvectors == 0
        subvector_dim = dim // n_subvectors

        codebooks = []
        codes = np.zeros((n, n_subvectors), dtype=np.uint8)

        for i in range(n_subvectors):
            start = i * subvector_dim
            end = (i + 1) * subvector_dim
            subvectors = vectors[:, start:end]

            kmeans = KMeans(n_clusters=n_centroids, random_state=42)
            codes[:, i] = kmeans.fit_predict(subvectors)
            codebooks.append(kmeans.cluster_centers_)

        params = {
            "codebooks": codebooks,
            "n_subvectors": n_subvectors,
            "subvector_dim": subvector_dim
        }
        return codes, params

    @staticmethod
    def binary_quantize(vectors: np.ndarray) -> np.ndarray:
        """Binary quantization (sign of each dimension)."""
        # Convert to binary: positive = 1, negative = 0
        binary = (vectors > 0).astype(np.uint8)

        # Pack bits into bytes
        n, dim = vectors.shape
        packed_dim = (dim + 7) // 8

        packed = np.zeros((n, packed_dim), dtype=np.uint8)
        for i in range(dim):
            byte_idx = i // 8
            bit_idx = i % 8
            packed[:, byte_idx] |= (binary[:, i] << bit_idx)

        return packed


def estimate_memory_usage(
    num_vectors: int,
    dimensions: int,
    quantization: str = "fp32",
    index_type: str = "hnsw",
    hnsw_m: int = 16
) -> dict:
    """Estimate memory usage for different configurations."""

    # Vector storage
    bytes_per_dimension = {
        "fp32": 4,
        "fp16": 2,
        "int8": 1,
        "pq": 0.05,  # Approximate
        "binary": 0.125
    }

    vector_bytes = num_vectors * dimensions * bytes_per_dimension[quantization]

    # Index overhead
    if index_type == "hnsw":
        # Each node has ~M*2 edges, each edge is 4 bytes (int32)
        index_bytes = num_vectors * hnsw_m * 2 * 4
    elif index_type == "ivf":
        # Inverted lists + centroids
        index_bytes = num_vectors * 8 + 65536 * dimensions * 4
    else:
        index_bytes = 0

    total_bytes = vector_bytes + index_bytes

    return {
        "vector_storage_mb": vector_bytes / 1024 / 1024,
        "index_overhead_mb": index_bytes / 1024 / 1024,
        "total_mb": total_bytes / 1024 / 1024,
        "total_gb": total_bytes / 1024 / 1024 / 1024
    }

Template 3: Qdrant Index Configuration

from qdrant_client import QdrantClient
from qdrant_client.http import models

def create_optimized_collection(
    client: QdrantClient,
    collection_name: str,
    vector_size: int,
    num_vectors: int,
    optimize_for: str = "balanced"  # "recall", "speed", "memory"
) -> None:
    """Create collection with optimized settings."""

    # HNSW configuration based on optimization target
    hnsw_configs = {
        "recall": models.HnswConfigDiff(m=32, ef_construct=256),
        "speed": models.HnswConfigDiff(m=16, ef_construct=64),
        "balanced": models.HnswConfigDiff(m=16, ef_construct=128),
        "memory": models.HnswConfigDiff(m=8, ef_construct=64)
    }

    # Quantization configuration
    quantization_configs = {
        "recall": None,  # No quantization for max recall
        "speed": models.ScalarQuantization(
            scalar=models.ScalarQuantizationConfig(
                type=models.ScalarType.INT8,
                quantile=0.99,
                always_ram=True
            )
        ),
        "balanced": models.ScalarQuantization(
            scalar=models.ScalarQuantizationConfig(
                type=models.ScalarType.INT8,
                quantile=0.99,
                always_ram=False
            )
        ),
        "memory": models.ProductQuantization(
            product=models.ProductQuantizationConfig(
                compression=models.CompressionRatio.X16,
                always_ram=False
            )
        )
    }

    # Optimizer configuration
    optimizer_configs = {
        "recall": models.OptimizersConfigDiff(
            indexing_threshold=10000,
            memmap_threshold=50000
        ),
        "speed": models.OptimizersConfigDiff(
            indexing_threshold=5000,
            memmap_threshold=20000
        ),
        "balanced": models.OptimizersConfigDiff(
            indexing_threshold=20000,
            memmap_threshold=50000
        ),
        "memory": models.OptimizersConfigDiff(
            indexing_threshold=50000,
            memmap_threshold=10000  # Use disk sooner
        )
    }

    client.create_collection(
        collection_name=collection_name,
        vectors_config=models.VectorParams(
            size=vector_size,
            distance=models.Distance.COSINE
        ),
        hnsw_config=hnsw_configs[optimize_for],
        quantization_config=quantization_configs[optimize_for],
        optimizers_config=optimizer_configs[optimize_for]
    )


def tune_search_parameters(
    client: QdrantClient,
    collection_name: str,
    target_recall: float = 0.95
) -> dict:
    """Tune search parameters for target recall."""

    # Search parameter recommendations
    if target_recall >= 0.99:
        search_params = models.SearchParams(
            hnsw_ef=256,
            exact=False,
            quantization=models.QuantizationSearchParams(
                ignore=True,  # Don't use quantization for search
                rescore=True
            )
        )
    elif target_recall >= 0.95:
        search_params = models.SearchParams(
            hnsw_ef=128,
            exact=False,
            quantization=models.QuantizationSearchParams(
                ignore=False,
                rescore=True,
                oversampling=2.0
            )
        )
    else:
        search_params = models.SearchParams(
            hnsw_ef=64,
            exact=False,
            quantization=models.QuantizationSearchParams(
                ignore=False,
                rescore=False
            )
        )

    return search_params

Template 4: Performance Monitoring

import time
from dataclasses import dataclass
from typing import List
import numpy as np

@dataclass
class SearchMetrics:
    latency_p50_ms: float
    latency_p95_ms: float
    latency_p99_ms: float
    recall: float
    qps: float


class VectorSearchMonitor:
    """Monitor vector search performance."""

    def __init__(self, ground_truth_fn=None):
        self.latencies = []
        self.recalls = []
        self.ground_truth_fn = ground_truth_fn

    def measure_search(
        self,
        search_fn,
        query_vectors: np.ndarray,
        k: int = 10,
        num_iterations: int = 100
    ) -> SearchMetrics:
        """Benchmark search performance."""
        latencies = []

        for _ in range(num_iterations):
            for query in query_vectors:
                start = time.perf_counter()
                results = search_fn(query, k=k)
                latency = (time.perf_counter() - start) * 1000
                latencies.append(latency)

        latencies = np.array(latencies)
        total_queries = num_iterations * len(query_vectors)
        total_time = sum(latencies) / 1000  # seconds

        return SearchMetrics(
            latency_p50_ms=np.percentile(latencies, 50),
            latency_p95_ms=np.percentile(latencies, 95),
            latency_p99_ms=np.percentile(latencies, 99),
            recall=self._calculate_recall(search_fn, query_vectors, k) if self.ground_truth_fn else 0,
            qps=total_queries / total_time
        )

    def _calculate_recall(self, search_fn, queries: np.ndarray, k: int) -> float:
        """Calculate recall against ground truth."""
        if not self.ground_truth_fn:
            return 0

        correct = 0
        total = 0

        for query in queries:
            predicted = set(search_fn(query, k=k))
            actual = set(self.ground_truth_fn(query, k=k))
            correct += len(predicted & actual)
            total += k

        return correct / total


def profile_index_build(
    build_fn,
    vectors: np.ndarray,
    batch_sizes: List[int] = [1000, 10000, 50000]
) -> dict:
    """Profile index build performance."""
    results = {}

    for batch_size in batch_sizes:
        times = []
        for i in range(0, len(vectors), batch_size):
            batch = vectors[i:i + batch_size]
            start = time.perf_counter()
            build_fn(batch)
            times.append(time.perf_counter() - start)

        results[batch_size] = {
            "avg_batch_time_s": np.mean(times),
            "vectors_per_second": batch_size / np.mean(times)
        }

    return results

Best Practices

Do's

• Benchmark with real queries - Synthetic may not represent production
• Monitor recall continuously - Can degrade with data drift
• Start with defaults - Tune only when needed
• Use quantization - Significant memory savings
• Consider tiered storage - Hot/cold data separation

Don'ts

• Don't over-optimize early - Profile first
• Don't ignore build time - Index updates have cost
• Don't forget reindexing - Plan for maintenance
• Don't skip warming - Cold indexes are slow

Resources

• HNSW Paper
• Faiss Wiki
• ANN Benchmarks

Completion Protocol

STATUS: DONE | DONE_WITH_CONCERNS | BLOCKED | NEEDS_CONTEXT REASON: [1–2 sentences if not DONE] ATTEMPTED: [what was tried, if BLOCKED]