impala set设置MEM_LIMIT

The MEM_LIMIT query option defines the maximum amount of memory a query can allocate on each node. 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 process memory limit can be expressed either as a percentage of RAM available to the process such as ‑‑mem_limit=70% or as a fixed amount of memory, such as 100gb. The memory available to the process is based on the host's physical memory and, since Impala 3.2, memory limits from Linux Control Groups. E.g. if an impalad process is running in a Docker container on a host with 100GB of memory, the memory available is 100GB or the Docker container's memory limit, whichever is less.

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 approaches the specified memory limit on any node, either the per-query limit or the impalad limit, then the SQL operations will start to reduce their memory consumption, for example by writing the temporary data to disk (known as spilling to disk). The result is a query that completes successfully, rather than failing with an out-of-memory error. The tradeoff is decreased performance due to the extra disk I/O to write the temporary data and read it back in. The slowdown could potentially be significant. Thus, while this feature improves reliability, you should optimize your queries, system parameters, and hardware configuration to make this spilling a rare occurrence.

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 for production workloads. Impala's Admission Controller can be configured to automatically assign memory limits to queries and limit memory consumption of resource pools. See Concurrent Queries and Admission Control and Memory Limits and Admission Control for more information on configuring the resource usage through admission control.

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.

直接实战

我们查个sql

select count(1)
from odserpjdata_kd.gl_code_combinations  gcc ,
odserpjdata_kd.gl_balances  gb 
where gb.code_combination_id =gcc.code_combination_id 

 set mem_limit=74mb;

select count(1)
from odserpjdata_kd.gl_code_combinations  gcc ,
odserpjdata_kd.gl_balances  gb 
where gb.code_combination_id =gcc.code_combination_id 

 峰值内存由683.4降低为419，看的出来还是有点效果的。但是可以看到查询的时间也多了一点。 Impala内存不够、查询超出内存的原因以及优化方式_impala内存参数设置_Kevin_鹿的博客-CSDN博客