Configuring Admission Control

Impala includes features that balance and maximize resources in your Apache Hadoop cluster. This topic describes how you can improve efficiency of your a Apache Hadoop cluster using those features.

The configuration options for admission control range from the simple (a single resource pool with a single set of options) to the complex (multiple resource pools with different options, each pool handling queries for a different set of users and groups).

Parent topic: Resource Management

Configuring Admission Control in Command Line Interface

To configure admission control, use a combination of startup options for the Impala daemon and edit or create the configuration files fair-scheduler.xml and llama-site.xml.

For a straightforward configuration using a single resource pool named default, you can specify configuration options on the command line and skip the fair-scheduler.xml and llama-site.xml configuration files.

For an advanced configuration with multiple resource pools using different settings:
  1. Set up the fair-scheduler.xml and llama-site.xml configuration files manually.
  2. Provide the paths to each one using the impalad command-line options, ‑‑fair_scheduler_allocation_path and ‑‑llama_site_path respectively.

The Impala admission control feature uses the Fair Scheduler configuration settings to determine how to map users and groups to different resource pools. For example, you might set up different resource pools with separate memory limits, and maximum number of concurrent and queued queries, for different categories of users within your organization. For details about all the Fair Scheduler configuration settings, see the Apache wiki.

The Impala admission control feature uses a small subset of possible settings from the llama-site.xml configuration file: 

llama.am.throttling.maximum.placed.reservations.queue_name
llama.am.throttling.maximum.queued.reservations.queue_name
impala.admission-control.pool-default-query-options.queue_name
impala.admission-control.pool-queue-timeout-ms.queue_name

The impala.admission-control.pool-queue-timeout-ms setting specifies the timeout value for this pool in milliseconds.

Theimpala.admission-control.pool-default-query-options settings designates the default query options for all queries that run in this pool. Its argument value is a comma-delimited string of 'key=value' pairs, 'key1=val1,key2=val2, ...'. For example, this is where you might set a default memory limit for all queries in the pool, using an argument such as MEM_LIMIT=5G.

The impala.admission-control.* configuration settings are available in Impala 2.5 and higher.

Example of Admission Control Configuration

Here are sample fair-scheduler.xml and llama-site.xml files that define resource pools root.default, root.development, and root.production. These files define resource pools for Impala admission control and are separate from the similar fair-scheduler.xmlthat defines resource pools for YARN.

fair-scheduler.xml:

Although Impala does not use the vcores value, you must still specify it to satisfy YARN requirements for the file contents.

Each <aclSubmitApps> tag (other than the one for root) contains a comma-separated list of users, then a space, then a comma-separated list of groups; these are the users and groups allowed to submit Impala statements to the corresponding resource pool.

If you leave the <aclSubmitApps> element empty for a pool, nobody can submit directly to that pool; child pools can specify their own <aclSubmitApps> values to authorize users and groups to submit to those pools. 

<allocations>

    <queue name="root">
        <aclSubmitApps> </aclSubmitApps>
        <queue name="default">
            <maxResources>50000 mb, 0 vcores</maxResources>
            <aclSubmitApps>*</aclSubmitApps>
        </queue>
        <queue name="development">
            <maxResources>200000 mb, 0 vcores</maxResources>
            <aclSubmitApps>user1,user2 dev,ops,admin</aclSubmitApps>
        </queue>
        <queue name="production">
            <maxResources>1000000 mb, 0 vcores</maxResources>
            <aclSubmitApps> ops,admin</aclSubmitApps>
        </queue>
    </queue>
    <queuePlacementPolicy>
        <rule name="specified" create="false"/>
        <rule name="default" />
    </queuePlacementPolicy>
</allocations>

llama-site.xml: 


<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <property>
    <name>llama.am.throttling.maximum.placed.reservations.root.default</name>
    <value>10</value>
  </property>
  <property>
    <name>llama.am.throttling.maximum.queued.reservations.root.default</name>
    <value>50</value>
  </property>
  <property>
    <name>impala.admission-control.pool-default-query-options.root.default</name>
    <value>mem_limit=128m,query_timeout_s=20,max_io_buffers=10</value>
  </property>
  <property>
    <name>impala.admission-control.pool-queue-timeout-ms.root.default</name>
    <value>30000</value>
  </property>
  <property>
    <name>impala.admission-control.max-query-mem-limit.root.default.regularPool</name>
    <value>1610612736</value><!--1.5GB-->
  </property>
  <property>
    <name>impala.admission-control.min-query-mem-limit.root.default.regularPool</name>
    <value>52428800</value><!--50MB-->
  </property>
  <property>
    <name>impala.admission-control.clamp-mem-limit-query-option.root.default.regularPool</name>
    <value>true</value>
  </property>
  <property>
    <name>impala.admission-control.max-query-cpu-core-per-node-limit.root.default.regularPool</name>
    <value>8</value>
  </property>
  <property>
    <name>impala.admission-control.max-query-cpu-core-coordinator-limit.root.default.regularPool</name>
    <value>8</value>
  </property>
</configuration>

Configuring Cluster-wide Admission Control

Important: These settings only apply if you enable admission control but leave dynamic resource pools disabled. In Impala 2.5 and higher, we recommend that you set up dynamic resource pools and customize the settings for each pool as described in Configuring Admission Control in Command Line Interface.

The following Impala configuration options let you adjust the settings of the admission control feature. When supplying the options on the impalad command line, prepend the option name with --.

queue_wait_timeout_ms
Purpose: Maximum amount of time (in milliseconds) that a request waits to be admitted before timing out.

Type: int64

Default: 60000

default_pool_max_requests
Purpose: Maximum number of concurrent outstanding requests allowed to run before incoming requests are queued. Because this limit applies cluster-wide, but each Impala node makes independent decisions to run queries immediately or queue them, it is a soft limit; the overall number of concurrent queries might be slightly higher during times of heavy load. A negative value indicates no limit. Ignored if fair_scheduler_config_path and llama_site_path are set.

Type: int64

Default: -1, meaning unlimited (prior to Impala 2.5 the default was 200)

default_pool_max_queued
Purpose: Maximum number of requests allowed to be queued before rejecting requests. Because this limit applies cluster-wide, but each Impala node makes independent decisions to run queries immediately or queue them, it is a soft limit; the overall number of queued queries might be slightly higher during times of heavy load. A negative value or 0 indicates requests are always rejected once the maximum concurrent requests are executing. Ignored if fair_scheduler_config_path and llama_site_path are set.

Type: int64

Default: unlimited

default_pool_mem_limit
Purpose: Maximum amount of memory (across the entire cluster) that all outstanding requests in this pool can use before new requests to this pool are queued. Specified in bytes, megabytes, or gigabytes by a number followed by the suffix b (optional), m, or g, either uppercase or lowercase. You can specify floating-point values for megabytes and gigabytes, to represent fractional numbers such as 1.5. You can also specify it as a percentage of the physical memory by specifying the suffix %. 0 or no setting indicates no limit. Defaults to bytes if no unit is given. Because this limit applies cluster-wide, but each Impala node makes independent decisions to run queries immediately or queue them, it is a soft limit; the overall memory used by concurrent queries might be slightly higher during times of heavy load. Ignored if fair_scheduler_config_path and llama_site_path are set.
Note: Impala relies on the statistics produced by the COMPUTE STATS statement to estimate memory usage for each query. See COMPUTE STATS Statement for guidelines about how and when to use this statement.

Type: string

Default: "" (empty string, meaning unlimited)

disable_pool_max_requests
Purpose: Disables all per-pool limits on the maximum number of running requests.

Type: Boolean

Default: false

disable_pool_mem_limits
Purpose: Disables all per-pool mem limits.

Type: Boolean

Default: false

fair_scheduler_allocation_path
Purpose: Path to the fair scheduler allocation file (fair-scheduler.xml).

Type: string

Default: "" (empty string)

Usage notes: Admission control only uses a small subset of the settings that can go in this file, as described below. For details about all the Fair Scheduler configuration settings, see the Apache wiki.

llama_site_path
Purpose: Path to the configuration file used by admission control (llama-site.xml). If set, fair_scheduler_allocation_path must also be set.

Type: string

Default: "" (empty string)

Usage notes: Admission control only uses a few of the settings that can go in this file, as described below.