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