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1. gp_connections_per_thread: Sets the number of client connections handled in each thread.
2. gp_connection_send_timeout:  controls the timeout value for sending data to unresponsive Greenplum Database user clients during query processing

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1. gp_connections_per_thread
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[gpadmin@sachi ~]$ gpconfig -s gp_connections_per_thread
Values on all segments are consistent
GUC          : gp_connections_per_thread
Master  value: 64
Segment value: 64
[gpadmin@sachi ~]$ 

gp_connections_per_thread default value was changed from 512 to 64 in GPDB 4.2

A value larger than or equal to the number of primary segments means that each slice in a query plan will get its own thread when dispatching to the segments. A value of 0 indicates that the dispatcher should use a single thread when dispatching all query plan slices to a segment. Lower values will use more threads, which utilizes more resources on the master. Typically, the default does not need to be changed unless there is a known throughput performance problem.
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VALUE RANGE   DEFAULT         SET CLASSIFICATIONS
integer                 64                      master, session, reload
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2. gp_connection_send_timeout
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The Greenplum Database 4.3.4 server configuration parameter gp_connection_send_timeout controls the timeout value for sending data to unresponsive Greenplum Database user clients during query processing. When the timeout is reached, the query is cancelled. There few new Server Configuration Parameters have been introduced in Greenplum Database 4.3.4.
• gp_connection_send_timeout
• gp_create_table_random_default_distribution
• gp_default_storage_options
• pgcrypto.fips
• runaway_detector_activation_percent
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gp_connection_send_timeout parameter sets Timeout for sending data to unresponsive Greenplum Database user clients during query processing. Avalue of 0 disables the timeout, Greenplum Database waits indefinitely for a client. When the timeout is reached, the query is cancelled with this message: Could not send data to client: Connection timed out. 
Value Range                  Default                                      Set Classifications
number of seconds         3600 (1 hour)                             master,system,reload

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