Use Resource Control to Achieve Resource Isolation

As a cluster administrator, you can use the resource control feature to create resource groups, set read and write quotas for resource groups, and bind users to those groups. This allows the TiDB layer to control the flow of user read and write requests based on the quotas set for the resource groups, and allows the TiKV layer to schedule the requests based on the priority mapped to the read and write quota. By doing this, you can ensure resource isolation for your applications and meet quality of service (QoS) requirements.

The TiDB resource control feature provides two layers of resource management capabilities: the flow control capability at the TiDB layer and the priority scheduling capability at the TiKV layer. The two capabilities can be enabled separately or simultaneously. See the Parameters for resource control for details.

• TiDB flow control: TiDB flow control uses the token bucket algorithm. If there are not enough tokens in a bucket, and the resource group does not specify the BURSTABLE option, the requests to the resource group will wait for the token bucket to backfill the tokens and retry. The retry might fail due to timeout.

• TiKV scheduling: if resource-control.enabled is enabled, TiKV uses the value of RU_PER_SEC of each resource group to determine the priority of the read and write requests for each resource group. Based on the priorities, the storage layer uses the priority queue to schedule and process requests.

Scenarios for resource control

The introduction of the resource control feature is a milestone for TiDB. It can divide a distributed database cluster into multiple logical units. Even if an individual unit overuses resources, it does not crowd out the resources needed by other units.

With this feature, you can:

• Combine multiple small and medium-sized applications from different systems into a single TiDB cluster. When the workload of an application grows larger, it does not affect the normal operation of other applications. When the system workload is low, busy applications can still be allocated the required system resources even if they exceed the set read and write quotas, so as to achieve the maximum utilization of resources.
• Choose to combine all test environments into a single TiDB cluster, or group the batch tasks that consume more resources into a single resource group. It can improve hardware utilization and reduce operating costs while ensuring that critical applications can always get the necessary resources.

In addition, the rational use of the resource control feature can reduce the number of clusters, ease the difficulty of operation and maintenance, and save management costs.

What is Request Unit (RU)

Request Unit (RU) is a unified abstraction unit in TiDB for system resources, which currently includes CPU, IOPS, and IO bandwidth metrics. It is used to indicate the amount of resources consumed by a single request to the database. The number of RUs consumed by a request depends on a variety of factors, such as the type of operations, and the amount of data being queried or modified. Currently, the RU contains consumption statistics for the resources in the following table:

Parameters for resource control

The resource control feature introduces two new global variables.

• TiDB: you can use the tidb_enable_resource_control system variable to control whether to enable flow control for resource groups.

• TiKV: you can use the resource-control.enabled parameter to control whether to use request scheduling based on resource groups.

The results of the combinations of these two parameters are shown in the following table.

For more information about the resource control mechanism and parameters, see RFC: Global Resource Control in TiDB.

How to use resource control

To create, modify, or delete a resource group, you need to have the SUPER or RESOURCE_GROUP_ADMIN privilege.

You can create a resource group in the cluster by using CREATE RESOURCE GROUP, and then bind users to a specific resource group by using CREATE USER or ALTER USER.

For an existing resource group, you can modify the RU_PER_SEC option (the rate of RU backfilling per second) of the resource group by using ALTER RESOURCE GROUP. The changes to the resource group take effect immediately.

You can delete a resource group by using DROP RESOURCE GROUP.

Step 1. Enable the resource control feature

Enable the resource control feature.

SET GLOBAL tidb_enable_resource_control = 'ON';

Set the TiKV resource-control.enabled parameter to true.

Step 2. Create a resource group, and then bind users to it

The following is an example of how to create a resource group and bind users to it.

1. Create a resource group rg1. The RU backfill rate is 500 RUs per second and allows applications in this resource group to overrun resources.

   CREATE RESOURCE GROUP IF NOT EXISTS rg1 RU_PER_SEC = 500 BURSTABLE;
   

2. Create a resource group rg2. The RU backfill rate is 600 RUs per second and does not allow applications in this resource group to overrun resources.

   CREATE RESOURCE GROUP IF NOT EXISTS rg2 RU_PER_SEC = 600;
   

3. Bind users usr1 and usr2 to resource groups rg1 and rg2 respectively.

   ALTER USER usr1 RESOURCE GROUP rg1;
   

   ALTER USER usr2 RESOURCE GROUP rg2;
   

After you complete the above operations of creating resource groups and binding users, the resource consumption of newly created sessions will be controlled by the specified quota. If the system workload is relatively high and there is no spare capacity, the resource consumption rate of usr2 will be strictly controlled not to exceed the quota. Because usr1 is bound by rg1 with BURSTABLE configured, the consumption rate of usr1 is allowed to exceed the quota.

If there are too many requests that result in insufficient resources for the resource group, the client's requests will wait. If the wait time is too long, the requests will report an error.

Monitoring metrics and charts

TiDB regularly collects runtime information about resource control and provides visual charts of the metrics in Grafana's TiDB > Resource Control dashboard. The metrics are detailed in the Resource Control section of TiDB Important Monitoring Metrics.

TiKV also records the request QPS from different resource groups. For more details, see TiKV Monitoring Metrics Detail.

Tool compatibility

The resource control feature is still in its experimental stage and does not impact the regular usage of data import, export, and other replication tools. BR, TiDB Lightning, and TiCDC do not currently support processing DDL operations related to resource control, and their resource consumption is not limited by resource control.

Limitations

Currently, the resource control feature has the following limitations:

• This feature only supports flow control and scheduling of read and write requests initiated by foreground clients. It does not support flow control and scheduling of background tasks such as DDL operations and auto analyze.
• Resource control incurs additional scheduling overhead. Therefore, there might be a slight performance degradation when this feature is enabled.

See also

• CREATE RESOURCE GROUP
• ALTER RESOURCE GROUP
• DROP RESOURCE GROUP
• RESOURCE GROUP RFC