MEM_LIMIT Query Option

When resource management is not enabled, defines the maximum amount of memory a query can allocate on each node. Therefore, the total memory that can be used by a query is the MEM_LIMIT times the number of nodes.

There are two levels of memory limit for Impala. The -mem_limit startup option sets an overall limit for the impalad process (which handles multiple queries concurrently). That limit is typically expressed in terms of a percentage of the RAM available on the host, such as -mem_limit=70%. The MEM_LIMIT query option, which you set through impala-shell or the SET statement in a JDBC or ODBC application, applies to each individual query. The MEM_LIMIT query option is usually expressed as a fixed size such as 10gb, and must always be less than the impalad memory limit.

If query processing exceeds the specified memory limit on any node, either the per-query limit or the impalad limit, Impala cancels the query automatically. Memory limits are checked periodically during query processing, so the actual memory in use might briefly exceed the limit without the query being cancelled.

When resource management is enabled in CDH 5, the mechanism for this option changes. If set, it overrides the automatic memory estimate from Impala. Impala requests this amount of memory from YARN on each node, and the query does not proceed until that much memory is available. The actual memory used by the query could be lower, since some queries use much less memory than others. With resource management, the MEM_LIMIT setting acts both as a hard limit on the amount of memory a query can use on any node (enforced by YARN) and a guarantee that that much memory will be available on each node while the query is being executed. When resource management is enabled but no MEM_LIMIT setting is specified, Impala estimates the amount of memory needed on each node for each query, requests that much memory from YARN before starting the query, and then internally sets the MEM_LIMIT on each node to the requested amount of memory during the query. Thus, if the query takes more memory than was originally estimated, Impala detects that the MEM_LIMIT is exceeded and cancels the query itself.

Type: numeric

Units: A numeric argument represents memory size in bytes; you can also use a suffix of m or mb for megabytes, or more commonly g or gb for gigabytes. If you specify a value with unrecognized formats, subsequent queries fail with an error.

Default: 0 (unlimited)

Usage notes:

The MEM_LIMIT setting is primarily useful in a high-concurrency setting, or on a cluster with a workload shared between Impala and other data processing components. You can prevent any query from accidentally using much more memory than expected, which could negatively impact other Impala queries.

Use the output of the SUMMARY command in impala-shell to get a report of memory used for each phase of your most heavyweight queries on each node, and then set a MEM_LIMIT somewhat higher than that. See Using the SUMMARY Report for Performance Tuning for usage information about the SUMMARY command.

Examples:

The following examples show how to set the MEM_LIMIT query option using a fixed number of bytes, or suffixes representing gigabytes or megabytes.

[localhost:21000] > set mem_limit=3000000000;
MEM_LIMIT set to 3000000000
[localhost:21000] > select 5;
Query: select 5
+---+
| 5 |
+---+
| 5 |
+---+

[localhost:21000] > set mem_limit=3g;
MEM_LIMIT set to 3g
[localhost:21000] > select 5;
Query: select 5
+---+
| 5 |
+---+
| 5 |
+---+

[localhost:21000] > set mem_limit=3gb;
MEM_LIMIT set to 3gb
[localhost:21000] > select 5;
+---+
| 5 |
+---+
| 5 |
+---+

[localhost:21000] > set mem_limit=3m;
MEM_LIMIT set to 3m
[localhost:21000] > select 5;
+---+
| 5 |
+---+
| 5 |
+---+
[localhost:21000] > set mem_limit=3mb;
MEM_LIMIT set to 3mb
[localhost:21000] > select 5;
+---+
| 5 |
+---+

The following examples show how unrecognized MEM_LIMIT values lead to errors for subsequent queries.

[localhost:21000] > set mem_limit=3tb;
MEM_LIMIT set to 3tb
[localhost:21000] > select 5;
ERROR: Failed to parse query memory limit from '3tb'.

[localhost:21000] > set mem_limit=xyz;
MEM_LIMIT set to xyz
[localhost:21000] > select 5;
Query: select 5
ERROR: Failed to parse query memory limit from 'xyz'.

The following examples shows the automatic query cancellation when the MEM_LIMIT value is exceeded on any host involved in the Impala query. First it runs a successful query and checks the largest amount of memory used on any node for any stage of the query. Then it sets an artificially low MEM_LIMIT setting so that the same query cannot run.

[localhost:21000] > select count(*) from customer;
Query: select count(*) from customer
+----------+
| count(*) |
+----------+
| 150000   |
+----------+

[localhost:21000] > select count(distinct c_name) from customer;
Query: select count(distinct c_name) from customer
+------------------------+
| count(distinct c_name) |
+------------------------+
| 150000                 |
+------------------------+

[localhost:21000] > summary;
+--------------+--------+----------+----------+---------+------------+----------+---------------+---------------+
| Operator     | #Hosts | Avg Time | Max Time | #Rows   | Est. #Rows | Peak Mem | Est. Peak Mem | Detail        |
+--------------+--------+----------+----------+---------+------------+----------+---------------+---------------+
| 06:AGGREGATE | 1      | 230.00ms | 230.00ms | 1       | 1          | 16.00 KB | -1 B          | FINALIZE      |
| 05:EXCHANGE  | 1      | 43.44us  | 43.44us  | 1       | 1          | 0 B      | -1 B          | UNPARTITIONED |
| 02:AGGREGATE | 1      | 227.14ms | 227.14ms | 1       | 1          | 12.00 KB | 10.00 MB      |               |
| 04:AGGREGATE | 1      | 126.27ms | 126.27ms | 150.00K | 150.00K    | 15.17 MB | 10.00 MB      |               |
| 03:EXCHANGE  | 1      | 44.07ms  | 44.07ms  | 150.00K | 150.00K    | 0 B      | 0 B           | HASH(c_name)  |
| 01:AGGREGATE | 1      | 361.94ms | 361.94ms | 150.00K | 150.00K    | 23.04 MB | 10.00 MB      |               |
| 00:SCAN HDFS | 1      | 43.64ms  | 43.64ms  | 150.00K | 150.00K    | 24.19 MB | 64.00 MB      | tpch.customer |
+--------------+--------+----------+----------+---------+------------+----------+---------------+---------------+

[localhost:21000] > set mem_limit=15mb;
MEM_LIMIT set to 15mb
[localhost:21000] > select count(distinct c_name) from customer;
Query: select count(distinct c_name) from customer
ERROR:
Memory limit exceeded
Query did not have enough memory to get the minimum required buffers in the block manager.