[pgpool-hackers: 4461] Eliminate unnecessary memory allocation

[Tatsuo Ishii]

Currently when Pgpool-II runs in extended query mode in streaming
replication (and logical replication mode), it adds info on target
PostgreSQL node ids and query context etc., to the "pending messages
list" when parse, bin, describe, execute and close message are
arrived. Later on, PostgreSQL sends respond message like parse
complete message to Pgpool-II. The info in the list is used to decide
from which PostgreSQL node to be read.

When pending messages are created, Pgpool-II does like:

pmsg = pool_pending_message_create();	/* create a pending message */
pool_pending_message_dest_set(pmsg, query_context)	/* set PostgreSQL node ids to be sent */
pool_pending_message_query_set(pmsg, query_context);	/* add query context */
pool_pending_message_add(pmsg);		/* add the pending message to the list */
pool_pending_message_free_pending_message(pmsg);	/* free memory allocated by pool_pending_message_create();

The reason why pool_pending_message_free_pending_message(pmsg) is
called here is, pool_pending_message_add() creates a copy of the
pending message then add it to the list. Thus pmsg is not necessary
any more. But is it really needed? I think we can eliminate copying
the pending message and instead add the pending message to the list
directory. Since the pending message is frequently created and
discarded while processing extended query protocol, this should reduce
memory footprint and CPU cycle.

Attached is the patch to implement this. After applying the patch I
observe slight performance enhancement (less than 1%).
(I ran pgbench: pgbench -M extended -S -p 11000 -T 300 -c 4 test)

I am not sure if this is just a noise or not. But I am sure the patch
reduce memory usage and CPU cycle.

Comments/suggestions are welcome.

Best reagards,
--
Tatsuo Ishii
SRA OSS LLC
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp
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