17.00 - Example: Workload Group Throttles Limit Concurrency after Demotions - Teradata Workload Management

Teradata Vantageā„¢ - Workload Management User Guide

prodname
Advanced SQL Engine
Teradata Workload Management
vrm_release
17.00
created_date
June 2020
category
Administration
User Guide
featnum
B035-1197-170K

When TASM demotes requests to a lower-priority workload, the number of active requests can temporarily exceed workload throttle limits. However, workload group throttles can limit the number of active requests for an application, despite the unpredictable effect of workload demotions. The following examples describe the effect of demoted requests with different throttle types.

Demotions with Workload Throttles

In this example, you want to limit the concurrent requests from an application to 9. Two longer-running requests in the Medium workload cause an exception, and TASM demotes them from the Medium workload to the lower-priority Long workload. Both workloads have throttle limits, but if the Long workload throttle is already at the limit, it will exceed the limit temporarily when the demoted requests join the workload. Demoted requests are never delayed because they have already started to run. Also, after TASM demotes the 2 Medium requests, the Medium workload is below the throttle limit, so 2 new requests start running in that workload. Under these conditions, the total application requests can exceed what you intended.

Workload Throttle Request Count Throttle Request Count If TASM Demotes Two Medium Requests
Medium 6 6
Long 3 5
Total Active 9 11

Demotions with Workload Group Throttles

If there are many demotions in a short time and new requests replace the demoted requests in the higher-level workload, the total application requests continue to increase. In this situation, group throttles can keep the application request total below the limit.

In this example, a group throttle that combines Medium and Long is limited to 9 requests at a time. Although Medium loses 2 requests due to demotions, it cannot release 2 requests from the delay queue because of a group throttle. The group throttle is already at the limit of 9, so until 2 more requests affected by the group throttle complete, Medium stays below the throttle limit.

Workload Throttle Request Count Throttle Request Count If Two Medium Requests Are Demoted Throttle Request Count If One Long Request Completes After the Medium Requests Are Demoted
Medium 6 4 5
Long 3 5 4
Group Throttle 9 9 9
Total Active 9 9 9