Global SysCache Parameters

enable_global_syscache

Parameter description: Specifies whether to enable the global system cache function. This parameter must be enabled for the multi-tenant database feature (enable_mtd).

Parameter type: Boolean

Unit: None

Value range:

• on indicates that the global system cache function is enabled.
• off indicates that the global system cache function is disabled.

Default value: on

Setting method: This is a POSTMASTER parameter. Set it based on instructions provided in Table 1.

Setting suggestion: Retain the default value. Enabling the global system cache function can reduce the system cache memory usage and improve the concurrent expansion capability. If this parameter is enabled, global_syscache_threshold must be used together. In this case, the thread pool mode is recommended. After this parameter is enabled, you are advised to set wal_level of the standby node to hot_standby or higher level if you need to access the standby node.

Risks and impacts of improper settings: Change the parameter value after fully understanding the parameter meaning and verifying it through testing.

global_syscache_threshold

Parameter description: Specifies the maximum memory usage of the global system cache. To use this parameter, you need to enable the enable_global_syscache parameter. This parameter can be set at the PDB level.

Parameter type: Integer

Unit: KB

Value range: 16384 to 1073741824

Default value: 163840 (196-core CPU/1536 GB memory, 128-core CPU/1024 GB memory, 104-core CPU/1024 GB memory, 96-core CPU/1024 GB memory, 96-core CPU/768 GB memory, 80-core CPU/640 GB memory, 64-core CPU/512 GB memory, 60-core CPU/480 GB memory, 32-core CPU/256 GB memory, 16-core CPU/128 GB memory, 8-core CPU/64 GB memory, 4-core CPU/32 GB memory); 65536 (4-core CPU/16 GB memory). In the PDB scenario, if this parameter is not set, the default value 64MB is used.

Setting method: This is a SIGHUP parameter. Set it based on instructions provided in Table 1. For example, if this parameter is set to 163840 without a unit, it indicates 163840 KB. If this parameter is set to 160MB, it indicates 160 MB. The unit must be KB, MB, or GB if required.

Setting suggestion: Set it to the smaller value of the number of hot databases and the number of threads multiplied by the memory size allocated to each database, that is, global_syscache_threshold = min(count(hot dbs),count(threads)) x memofdb.

The number of hot databases is the number of databases that are frequently accessed. In thread pool mode, the number of threads is the sum of the number of threads in the thread pool and the number of background threads. In non-thread pool mode, the number of hot databases is used.

memofdb indicates the average memory allocated to each database. The background noise memory of each database is 2 MB. Each time a table or index is added, 11 KB memory is added.

Risks and impacts of improper settings: If the value is too small, memory is frequently evicted, and a large number of memory fragments cannot be recycled. As a result, memory control fails. When setting this parameter at the PDB level, do not set this parameter to a large value. Otherwise, other tenants cannot use the reserved memory. You are advised to set this parameter based on the specifications.