Communication Library Parameters

comm_max_datanode

Parameter description: Specifies the maximum number of DNs supported by the TCP proxy communication library or SCTP communication library.

Type: USERSET

Value range: an integer ranging from 1 to 8192

Default value: actual number of DNs

If the number of DNs is increased, the change takes effect immediately. If the number of DNs is reduced, the cluster needs to be restarted for the change to take effect.

comm_max_stream

Parameter description: Specifies the maximum number of concurrent data streams supported by the TCP proxy communication library or SCTP communication library. The value of this parameter must be greater than: Number of concurrent data streams x Number of operators in each stream x Square of SMP.

Type: POSTMASTER

Value range: an integer ranging from 1 to 60000

Default value: calculated by the following formula: min (query_dop_limit x query_dop_limit x 2 x 20, max_process_memory (bytes) x 0.025/(Maximum number of CNs + Number of current DNs)/260. If the value is less than 1024, 1024 is used. query_dop_limit = Number of CPU cores of a single server/Number of DNs of a single server.

• You are not advised to set this parameter to a large value because this will cause high memory usage (256 bytes x comm_max_stream x comm_max_datanode). If the number of concurrent data streams is large, the query is complex and the smp is large, resulting in insufficient memory.
• 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: 65535

• 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.

comm_max_receiver

Parameter description: Specifies the maximum number of receiving threads for the TCP proxy communication library or SCTP 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 consecutively sent by the TCP proxy communication library or SCTP 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 available for buffering on the TCP proxy communication library or SCTP communication library on a single DN.

Type: POSTMASTER

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, there may be out-of-memory (OOM). If it is too small, the performance of the TCP proxy communication library or SCTP communication library may deteriorate.

comm_memory_pool_percent

Parameter description: Specifies the percentage of the memory pool resources that can be used by the TCP proxy communication library or the SCTP communication library in 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 TCP proxy communication library or SCTP 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 TCP proxy communication library or SCTP 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 statistics mode of the TCP proxy communication library or SCTP 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 the connections between CNs and DNs are logical, with the libcomm component in use.
• 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