MySQL supports creation and management of resource groups, and permits assigning threads running within the server to particular groups so that threads execute according to the resources available to the group. Group attributes enable control over its resources, to enable or restrict resource consumption by threads in the group. DBAs can modify these attributes as appropriate for different workloads.
Currently, CPU time is a manageable resource, represented by the concept of “virtual CPU” as a term that includes CPU cores, hyperthreads, hardware threads, and so forth. The server determines at startup how many virtual CPUs are available, and database administrators with appropriate privileges can associate these CPUs with resource groups and assign threads to groups.
For example, to manage execution of batch jobs that need not
execute with high priority, a DBA can create a
Batch resource group, and adjust its priority
up or down depending on how busy the server is. (Perhaps batch
jobs assigned to the group should run at lower priority during
the day and at higher priority during the night.) The DBA can
also adjust the set of CPUs available to the group. Groups can
be enabled or disabled to control whether threads are assignable
The following sections describe aspects of resource group use in MySQL:
On some platforms or MySQL server configurations, resource groups are unavailable or have limitations. In particular, Linux systems might require a manual step for some installation methods. For details, see Resource Group Restrictions.
These capabilities provide the SQL interface for resource group management in MySQL:
SQL statements enable creating, altering, and dropping resource groups, and enable assigning threads to resource groups. An optimizer hint enables assigning individual statements to resource groups.
Resource group privileges provide control over which users can perform resource group operations.
Status variables provide execution counts for each management SQL statement.
Resource groups have attributes that define the group. All attributes can be set at group creation time. Some attributes are fixed at creation time; others can be modified any time thereafter.
These attributes are defined at resource group creation time and cannot be modified:
Each group has a name. Resource group names are identifiers like table and column names, and need not be quoted in SQL statements unless they contain special characters or are reserved words. Group names are not case sensitive and may be up to 64 characters long.
Each group has a type, which is either
USER. The resource group type affects the range of priority values assignable to the group, as described later. This attribute together with the differences in permitted priorities enables system threads to be identified so as to protect them from contention for CPU resources against user threads.
System and user threads correspond to background and foreground threads as listed in the Performance Schema
These attributes are defined at resource group creation time and can be modified any time thereafter:
The CPU affinity is the set of virtual CPUs the resource group can use. An affinity can be any nonempty subset of the available CPUs. If a group has no affinity, it can use all available CPUs.
The thread priority is the execution priority for threads assigned to the resource group. Priority values range from -20 (highest priority) to 19 (lowest priority). The default priority is 0, for both system and user groups.
System groups are permitted a higher priority than user groups, ensuring that user threads never have a higher priority than system threads:
For system resource groups, the permitted priority range is -20 to 0.
For user resource groups, the permitted priority range is 0 to 19.
Each group can be enabled or disabled, affording administrators control over thread assignment. Threads can be assigned only to enabled groups.
By default, there is one system group and one user group,
USR_default, respectively. These default
groups cannot be dropped and their attributes cannot be
modified. Each default group has no CPU affinity and priority
Newly created system and user threads are assigned to the
USR_default groups, respectively.
For user-defined resource groups, all attributes are assigned at group creation time. After a group has been created, its attributes can be modified, with the exception of the name and type attributes.
To create and manage user-defined resource groups, use these SQL statements:
Those statements require the
To manage resource group assignments, use these capabilities:
Resource group definitions are stored in the
resource_groups data dictionary table so
that groups persist across server restarts. Because
resource_groups is part of the data
dictionary, it is not directly accessible by users. Resource
group information is available using
which is implemented as a view on the data dictionary table.
See Section 24.20, “The INFORMATION_SCHEMA RESOURCE_GROUPS Table”.
RESOURCE_GROUPS table has
these rows describing the default groups:
mysql> SELECT * FROM INFORMATION_SCHEMA.RESOURCE_GROUPS\G *************************** 1. row *************************** RESOURCE_GROUP_NAME: USR_default RESOURCE_GROUP_TYPE: USER RESOURCE_GROUP_ENABLED: 1 VCPU_IDS: 0-3 THREAD_PRIORITY: 0 *************************** 2. row *************************** RESOURCE_GROUP_NAME: SYS_default RESOURCE_GROUP_TYPE: SYSTEM RESOURCE_GROUP_ENABLED: 1 VCPU_IDS: 0-3 THREAD_PRIORITY: 0
THREAD_PRIORITY values are 0,
indicating the default priority. The
VCPU_IDS values show a range comprising all
available CPUs. For the default groups, the displayed value
varies depending on the system on which the MySQL server runs.
Earlier discussion mentioned a scenario involving a resource
Batch to manage execution of
batch jobs that need not execute with high priority. To create
such a group, use a statement similar to this:
CREATE RESOURCE GROUP Batch TYPE = USER VCPU = 2-3 -- assumes a system with at least 4 CPUs THREAD_PRIORITY = 10;
To verify whether the resource group was created as expected,
mysql> SELECT * FROM INFORMATION_SCHEMA.RESOURCE_GROUPS WHERE RESOURCE_GROUP_NAME = 'Batch'\G *************************** 1. row *************************** RESOURCE_GROUP_NAME: Batch RESOURCE_GROUP_TYPE: USER RESOURCE_GROUP_ENABLED: 1 VCPU_IDS: 2-3 THREAD_PRIORITY: 10
THREAD_PRIORITY value is 0 rather
than 10, check Resource Group Restrictions,
to see whether your platform or system configuration limits
the resource group capability.
To assign a thread to the
Batch group, do
SET RESOURCE GROUP Batch FOR thread_id;
Thereafter, statements in the named thread execute with
Batch group resources.
If a session's own current thread should be in the
Batch group, execute this statement within
SET RESOURCE GROUP Batch;
Thereafter, statements in the session execute with
Batch group resources.
To execute a single statement using the
Batch group, use the
RESOURCE_GROUP optimizer hint:
INSERT /*+ RESOURCE_GROUP(Batch) */ INTO t2 VALUES(2);
Threads assigned to the
Batch group execute
with its resources, which can be modified as desired:
For times when the system is highly loaded, decrease the number of CPUs assigned to the group, lower its priority, or (as shown) both:
ALTER RESOURCE GROUP Batch VCPU = 3 THREAD_PRIORITY = 19;
For times when the system is lightly loaded, increase the number of CPUs assigned to the group, raise its priority, or (as shown) both:
ALTER RESOURCE GROUP Batch VCPU = 0-3 THREAD_PRIORITY = 0;
Resource group management is local to the server on which it
occurs. Resource group SQL statements and modifications to the
resource_groups data dictionary table are
not written to the binary log and are not replicated.
On some platforms or MySQL server configurations, resource groups are unavailable or have limitations:
Resource groups are unavailable if the thread pool plugin is installed.
Resource groups are unavailable on macOS, which provides no API for binding CPUs to a thread.
On FreeBSD and Solaris, resource group thread priorities are ignored. (Effectively, all threads run at priority 0.) Attempts to change priorities result in a warning:
mysql> ALTER RESOURCE GROUP abc THREAD_PRIORITY = 10; Query OK, 0 rows affected, 1 warning (0.18 sec) mysql> SHOW WARNINGS; +---------+------+-------------------------------------------------------------+ | Level | Code | Message | +---------+------+-------------------------------------------------------------+ | Warning | 4560 | Attribute thread_priority is ignored (using default value). | +---------+------+-------------------------------------------------------------+
On Linux, resource groups thread priorities are ignored unless the
CAP_SYS_NICEcapability is set. MySQL package installers for Linux systems should set this capability. For installation using a compressed tar file binary distribution or from source, the
CAP_SYS_NICEcapability can be set manually using the setcap command, specifying the path name to the mysqld executable (this requires sudo access). You can check the capabilities using getcap. For example:
shell> sudo setcap cap_sys_nice+ep ./bin/mysqld shell> getcap ./bin/mysqld ./bin/mysqld = cap_sys_nice+epImportant
If manual use of setcap is required, it must be performed after each reinstall.
On Windows, threads run at one of five thread priority levels. The resource group thread priority range of -20 to 19 maps onto those levels as indicated in the following table.
Table 8.3 Resource Group Thread Priority on Windows
Priority Range Windows Priority Level -20 to -10
-9 to -1
1 to 10
11 to 19