Communication Library Parameters

comm_max_datanode

Parameter description: Specifies the maximum number of DNs supported by the communication library.

Type: USERSET

Value range: an integer ranging from 1 to 8192

Default value: actual number of DNs

Increasing this parameter value takes effect immediately, while decreasing the value takes effect after the cluster is restarted.

comm_max_stream

Parameter description: maximum number of logical connection data structures cached in the communication library.

Type: SIGHUP

Value range: an integer ranging from 1 to 65535

Default value: 1024

If the value of comm_max_datanode is small, the process memory is sufficient. In this case, you can increase the value of comm_max_stream.

max_stream_pool

Parameter description: Specifies the maximum number of stream threads that can be contained in a stream thread pool. This feature is supported in 8.1.2 or later.

Type: SUSET

Value range: an integer ranging from –1 to INT_MAX. The values –1 and 0 indicate that the stream thread pool is disabled.

Default value:

• The formula for a new cluster is max_stream_pool=MIN(max_connections, max_process_memory/16/5MB, 1024).
• The formula for a cluster upgraded from versions earlier than is max_stream_pool = MIN(max_connections, max_process_memory/16/5MB, 1024, value of the old cluster). During the upgrade, the settings for the new cluster are forcibly used, but the old value is used if it is smaller.

• The number of stream threads in a thread pool can be reduced in real time. If the value of this parameter is increased, the number of stream threads is increased to meet the service requirements.
• Generally, you are advised not to change the value of this parameter because the stream thread pool supports the automatic cleanup function.
• If too many idle stream threads occupy the memory, you can decrease the value of this parameter to save the memory.

enable_stream_sync_quit

Parameter description: whether the stream threads exit synchronously when the stream plan ends. This parameter is supported only by clusters of version 8.2.1.300 or later.

Type: USERSET

Value range: Boolean

• on indicates that threads in the stream thread group exit after the steam plan ends.
• off indicates that stream threads exit directly after the stream plan ends without waiting for the threads in the stream thread group to exit.

Default value: off

enable_connect_standby

Parameter description: Sets the connection between a CN and a standby DN. This parameter is supported only by clusters of version 8.3.0 or later.

Type: USERSET

Value range: Boolean

• on indicates that the CN connects to the standby server.
• off indicates that the CN connects to the primary DN.

Default value: off

• You are not advised to use this parameter in routine services. This parameter applies only to O&M operations. You are not advised to use the gs_guc tool for global settings. Otherwise, problems such as data inconsistency and result set errors may occur.
• Enabling this parameter for a session with temporary tables will delete the temporary table data on DNs and prevent further actions on those tables.

comm_max_receiver

Parameter description: Specifies the number of internal receiving threads of the communication library.

Type: POSTMASTER

Value range: an integer ranging from 1 to 50

Default value: 4

comm_quota_size

Parameter description: Specifies the maximum size of packets that can be continuously sent by the communication library. When you use a 1GE NIC, a small value ranging from 20 KB to 40 KB is recommended.

Type: USERSET

Value range: an integer ranging from 0 to 102400. The default unit is KB. The value 0 indicates that the quota mechanism is not used.

Default value: 1 MB

comm_usable_memory

Parameter description: Specifies the maximum memory that can be used by the communication library cache on a single DN.

Type: SIGHUP

Value range: an integer ranging from 1 to 256. The default unit is KB. The minimum size cannot be less than 1 GB for installation.

Default value: max_process_memory/8

This parameter must be specifically set based on environment memory and the deployment method. If it is too large, out-of-memory (OOM) may occur. If it is too small, the performance of the communication library may deteriorate.

comm_memory_pool_percent

Parameter description: Specifies the percentage of the memory pool resources that can be used by the communication library on a DN. This parameter is used to adaptively reserve memory used by the communication libraries.

Type: POSTMASTER

Value range: an integer ranging from 0 to 100

Default value: 0

If the memory used by the communication library is small, set this parameter to a small value. Otherwise, set it to a large value.

comm_client_bind

Parameter description: Specifies whether to bind the client of the communication library to a specified IP address when the client initiates a connection.

Type: USERSET

Value range: Boolean

• on indicates that the client is bound to a specified IP address.
• off indicates that the client is not bound to any IP addresses.

If multiple IP addresses of a node in a cluster are on the same communication network segment, set this parameter to on. In this case, the client is bound to the IP address specified by listen_addresses. The concurrency performance of a cluster depends on the number of random ports because a port can be used only by one client at a time.

Default value: off

comm_no_delay

Parameter description: Specifies whether to use the NO_DELAY attribute of the communication library connection. Restart the cluster for the setting to take effect.

Type: USERSET

Value range: Boolean

Default value: off

If packet loss occurs because a large number of packets are received per second, set this parameter to off to reduce the total number of packets.

comm_debug_mode

Parameter description: Specifies the debug mode of the communication library, that is, whether to print logs about the communication layer. The setting is effective at the session layer.

When the switch is set to on, the number of printed logs is huge, adding extra overhead and reducing database performance. Therefore, set the switch to on only in the debug mode.

Type: USERSET

Value range: Boolean

• on indicates the detailed debug log of the communication library is printed.
• off indicates the detailed debug log of the communication library is not printed.

Default value: off

comm_ackchk_time

Parameter description: Specifies the duration after which the communication library server automatically triggers ACK when no data package is received.

Type: USERSET

Value range: an integer ranging from 0 to 20000. The unit is millisecond (ms). 0 indicates that automatic ACK triggering is disabled.

Default value: 2000

comm_timer_mode

Parameter description: Specifies the timer mode of the communication library, that is, whether to print timer logs in each phase of the communication layer. The setting is effective at the session layer.

When the switch is set to on, the number of printed logs is huge, adding extra overhead and reducing database performance. Therefore, set the switch to on only in the debug mode.

Type: USERSET

Value range: Boolean

• on indicates the detailed timer log of the communication library is printed.
• off indicates the detailed timer log of the communication library is not printed.

Default value: off

comm_stat_mode

Parameter description: Specifies the stat mode of the communication library, that is, whether to print statistics about the communication layer. The setting is effective at the session layer.

When the switch is set to on, the number of printed logs is huge, adding extra overhead and reducing database performance. Therefore, set the switch to on only in the debug mode.

Type: USERSET

Value range: Boolean

• on indicates the statistics log of the communication library is printed.
• off indicates the statistics log of the communication library is not printed.

Default value: off

enable_stateless_pooler_reuse

Parameter description: Specifies whether to enable the pooler reuse mode. The setting takes effect after the cluster is restarted.

Type: POSTMASTER

Value range: Boolean

• on or true indicates that the pooler reuse mode is enabled.
• off or false indicates that the pooler reuse mode is disabled.
  

  Set this parameter to the same value for CNs and DNs. If enable_stateless_pooler_reuse is set to off for CNs and set to on for DNs, the cluster communication fails. Restart the cluster to make the setting take effect.

Default value: off

comm_cn_dn_logic_conn

Parameter description: Specifies a switch for logical connections between CNs and DNs. The parameter setting takes effect only after the cluster is restarted.

Type: POSTMASTER

Value range: Boolean

• on or true indicates that CNs and DNs are logically connected and the libcomm component is used.
• off or false indicates that the connections between CNs and DNs are physical, with the libpq component in use.
  

  If comm_cn_dn_logic_conn is set to off for CNs and set to on for DNs, cluster communication will fail. You are advised to set this parameter to the same value for all CNs and DNs. Restart the cluster to make the setting take effect.

Default value: off

client_connection_check_interval

Parameter description: Specifies the interval for checking the client connection status. This parameter is supported by clusters of version 8.2.0 or later.

Type: USERSET

Value range: an integer ranging from 0 to INT_MAX. The unit is ms. The value 0 indicates that the client connection status is not checked.

Default value: 10000

During a long query executed in a session where a client (such as gsql, JDBC, or ODBC) directly connects to the CN,

• The CN checks the client connection status at the interval specified by client_connection_check_interval. If it detects that the client has been disconnected from the CN, the server terminates the long query and releases related resources to avoid waste of cluster resources.
• The DN checks its connection to the CN at the interval specified by client_connection_check_interval. If the DN detects that it has been disconnected from the CN, it terminates the long query and releases related resources to avoid waste of cluster resources.

conn_recycle_timeout

Parameter description: the interval for reclaiming idle connections between a CN and other nodes to the connection pool. This parameter is supported only by clusters of version 8.2.1 or later.

Type: USERSET

Value range: an integer ranging from 0 to 3600, in second (s). 0 indicates that the function of reclaiming idle connections is disabled.

Default value: 30