What do the HNSW parameters M, ef_construction, and ef_search control?

M controls the maximum number of connections per node, typically 16 to 64, where higher M improves recall but increases memory and index build time. ef_construction controls how many candidate neighbors are considered when building each node's connections, with higher values producing a better graph at the cost of longer indexing. ef_search is the query-time parameter that sets how many candidates the search keeps in its priority queue, directly trading latency for recall. You can tune ef_search per query without rebuilding the index.