How to max out CPU cores on Elasticsearch cluster

Question

How many shards and replicas do I have to set to use every cpu core (I want 100% load, fastest query results) in my cluster?

I want to use Elasticsearch for aggregations. I read that Elasticsearch uses multiple cpu cores, but found no exact details about cpu cores regarding sharding and replicas.

My observations are, that a single shard does not use more than 1 core/thread at query time (considerung there is only one query at a time). With replicas, the query of a 1-shard index are not faster, since Elasticsearch does not seem to use the other nodes to distribute the load on a shard.

My questions (one query at a time):

• A shard does not use more than one cpu core?
• Shards must always be scanned completely, replicas cannot be used to divide intra-shard load between nodes?
• The formular for best performance is SUM(CPU_CORES per node) * PRIMARY_SHARDS?

Answer 1

When doing an operation (indexing, searching, bulk indexing etc) a shard on a node uses one thread of execution, meaning one CPU core.

If you have one query running at a given moment, that will use one CPU core per shard. For example, a three node cluster with a single index that has 6 primary shards and one replica, will have in total 12 shards, 4 shards on each node.

If there is only one query running on the cluster, for that index, ES will query all the 6 shards of the index (no matter if they are primaries or replicas) and each node will use between 0 and 4 CPU cores for the job, because the round-robin algorithm used by ES to choose which copy of a shard performs the search can choose no shards on one node or maximum 4 shards on one node.