Greenplum Workload Management

posted Sep 12, 2012, 11:48 AM by Sachchida Ojha
The purpose of workload management is to limit the number of active queries in the system at any given time in order to avoid exhausting system resources such as memory, CPU, and disk I/O. This is accomplished in Greenplum Database with role-based resource queues. A resource queue has attributes that limit the size and/or total number of queries that can be executed by the users (or roles) in that queue. Also, you can assign a priority level that controls the relative share of available CPU used by queries associated with the resource queue. By assigning all database roles to the appropriate resource queue, administrators can control concurrent user queries and prevent the system from being overloaded.

How Resource Queues Work in Greenplum Database

Resource scheduling is enabled by default when you install Greenplum Database. All database roles must be assigned to a resource queue. If an administrator creates a role without explicitly assigning it to a resource queue, the role is assigned to the default resource queue, pg_default.

Greenplum recommends that administrators create resource queues for the various types of workloads in their organization. For example, you may have resource queues for power users, web users, and management reports. You would then set limits on the resource queue based your estimate of how resource-intensive the queries associated with that workload are likely to be. Currently, the configurable limits on a queue include:

•Active statement count. The maximum number of statements that can run concurrently.
•Active statement memory. The total amount of memory that all queries submitted through this queue can consume.
•Active statement priority. This value defines a queue’s priority relative to other queues in terms of available CPU resources.
•Active statement cost. This value is compared with the cost estimated by the query planner, measured in units of disk page fetches.

After resource queues are created, database roles (users) are then assigned to the appropriate resource queue. A resource queue can have multiple roles, but a role can have only one assigned resource queue.

How Resource Queue Limits Work

At runtime, when the user submits a query for execution, that query is evaluated against the resource queue’s limits. If the query does not cause the queue to exceed its resource limits, then that query will run immediately. If the query causes the queue to exceed its limits (for example, if the maximum number of active statement slots are currently in use), then the query must wait until queue resources are free before it can run. Queries are evaluated on a first in, first out basis. If query prioritization is enabled, the active workload on the system is periodically assessed and processing resources are reallocated according to query priority Roles with the SUPERUSER attribute are always exempt from resource queue limits. Superuser queries are always allowed to run immediately regardless of the limits of their assigned resource queue.

How Memory Limits Work

Setting a memory limit on a resource queue sets the maximum amount of memory that all queries submitted through the queue can consume on a segment host. The amount of memory allotted to a particular query is based on the queue memory limit divided by the active statement limit (Greenplum recommends that memory limits be used in conjunction with statement-based queues rather than cost-based queues). For example, if a queue has a memory limit of 2000MB and an active statement limit of 10, each query submitted through the queue is allotted 200MB of memory by default. The default memory allotment can be overridden on a per-query basis using the statement_mem server configuration parameter (up to the queue memory limit). Once a query has started executing, it holds its allotted memory in the queue until it completes (even if during execution it actually consumes less than its allotted amount of memory).

How Priorities Work

Resource limits on active statement count, memory and query cost are admission limits, which determine whether a query is admitted into the group of actively running statements, or whether it is queued with other waiting statements. After a query becomes active, it must share available CPU resources as determined by the priority settings for its resource queue. When a statement from a high-priority queue enters the group of actively running statements, it may claim a significant share of the available CPU, reducing the share allotted to already-running statements.
The comparative size or complexity of the queries does not affect the allotment of CPU. If a simple, low-cost query is running simultaneously with a large, complex query, and their priority settings are the same, they will be allotted the same share of available CPU resources. When a new query becomes active, the exact percentage shares of CPU will be recalculated, but queries of equal priority will still have equal amounts of CPU allotted.
For example, an administrator creates three resource queues: adhoc for ongoing queries submitted by business analysts, reporting for scheduled reporting jobs, and executive for queries submitted by executive user roles. The administrator wants to ensure that scheduled reporting jobs are not heavily affected by unpredictable resource demands from ad-hoc analyst queries. Also, the administrator wants to make sure that queries submitted by executive roles are allotted a significant share of CPU. Accordingly, the resource queue priorities are set as shown:
•adhoc — Low priority
•reporting — High priority
•executive — Maximum priority