Understanding Performance using Query Profile

To understand the detailed performance characteristics for a query, issue the PROFILE command in impala-shell immediately after executing a query. This low-level information includes physical details about memory, CPU, I/O, and network usage, and thus is only available after the query is actually run.

The PROFILE command, available in impala-shell, produces a detailed low-level report showing how a query was executed. Unlike the EXPLAIN plan, this information is only available after the query has finished. It shows physical details such as the number of bytes read, maximum memory usage, and so on for each node. You can use this information to determine if the query is I/O-bound or CPU-bound, whether some network condition is imposing a bottleneck, whether a slowdown is affecting some nodes but not others, and to check that recommended configuration settings such as short-circuit local reads are in effect.

By default, time values in the profile output reflect the wall-clock time taken by an operation. For values denoting system time or user time, the measurement unit is reflected in the metric name, such as ScannerThreadsSysTime or ScannerThreadsUserTime. For example, a multi-threaded I/O operation might show a small figure for wall-clock time, while the corresponding system time is larger, representing the sum of the CPU time taken by each thread. Or a wall-clock time figure might be larger because it counts time spent waiting, while the corresponding system and user time figures only measure the time while the operation is actively using CPU cycles.

The EXPLAIN plan is also printed at the beginning of the query profile report, for convenience in examining both the logical and physical aspects of the query side-by-side. The EXPLAIN_LEVEL query option also controls the verbosity of the EXPLAIN output printed by the PROFILE command.

The Per Node Profiles section in the profile output includes the following metrics that can be controlled by the RESOURCE_TRACE_RATIO query option.

• CpuIoWaitPercentage
• CpuSysPercentage
• CpuUserPercentage
• HostDiskReadThroughput: All data read by the host as part of the execution of this query (spilling), by the HDFS data node, and by other processes running on the same system.
• HostDiskWriteThroughput: All data written by the host as part of the execution of this query (spilling), by the HDFS data node, and by other processes running on the same system.
• HostNetworkRx: All data received by the host as part of the execution of this query, other queries, and other processes running on the same system.
• HostNetworkTx: All data transmitted by the host as part of the execution of this query, other queries, and other processes running on the same system.

PROFILE [ALL | LATEST | ORIGINAL]