Evaluate Performance of Current Operations
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The following sections describe techniques for evaluating operational performance.
Use the Database Profiler to Evaluate Operations Against the Database
MongoDB provides a database profiler that shows performance characteristics of each operation against the database. Use the profiler to locate any queries or write operations that are running slow. You can use this information, for example, to determine what indexes to create.
For more information, see Database Profiling.
For MongoDB 3.6 deployments, starting in version 3.6.11, secondary members of a replica set now log oplog entries that take longer than the slow operation threshold to apply. These slow oplog messages are logged for the secondaries in the diagnostic log under the REPL component with the text applied op: <oplog entry> took <num>ms. These slow oplog entries depend only on the slow operation threshold. They do not depend on the log levels (either at the system or component level), or the profiling level, or the slow operation sample rate. The profiler does not capture slow oplog entries.
Use db.currentOp() to Evaluate mongod Operations
The db.currentOp() method reports on current operations running on a mongod instance.
Use explain to Evaluate Query Performance
The cursor.explain() and db.collection.explain() methods return information on a query execution, such as the index MongoDB selected to fulfill the query and execution statistics. You can run the methods in queryPlanner mode, executionStats mode, or allPlansExecution mode to control the amount of information returned.
Example
To use cursor.explain() on a query for documents matching the expression { a: 1 }, in the collection named records, use an operation that resembles the following in the mongo shell:
db.records.find( { a: 1 } ).explain("executionStats")
For more information, see Explain Results, cursor.explain(), db.collection.explain(), and Analyze Query Performance.