[pgpool-hackers: 408] Adding listen_backlog parameter

[Tatsuo Ishii]

Hi pgpool hackers,

I would like to add new parameter "listen_backlog". This essentially
same as Apache's ListenBacklog parameter:

The second parameter of listen(2) system call.

As you might already know, pgpool accepts concurrent connections up to
num_init_children. Exceeding connections will be queued in kernel's
backlog queue. If the queue is long enough to accept a new connection,
the request from the client will be almost immediately accepted by
pgpool once existing client disconnects to pgpool. However, if the
queue is overflowed, the client might retry after 3 seconds or so, and
user thinks that pgpool's response is too bad. Or disconnected by
pgpool with "Connection to database "test" failed: could not connect
to server: Connection timed out".

The solution is, making the backlog parameter large enough for such
possible load. Currently the backlog is calculated by
num_init_children * 2. But this may not be large enough for certain
load. For the case, the new proposed parameter can set the listen()'s
backlog parameter longer than num_init_children * 2.

Here is an experiment I did to illustrate the effect of the
parameter(script attached).

I started pgpool with num_init_children = 2, and connected to it by
using 128 forked pgbench.
Each pgbench executes 100 read only queries.

	 pgbench -C -c 1 -n -S test;date)&

The pgpool server is a Linux box running kernel 3.4.68.

Stock pgpool-II 3.3.1, which set the backlog parameter to 4, took 1
minute and 7 seconds. And some requests failed with "Connection to
database "test" failed" error.

On the other hand if I set the backlog parameter to 1024, it took only
15 seconds without any error.

Opinions?
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese: http://www.sraoss.co.jp
-------------- next part --------------
export LANG=C
export PGPORT=11002
export PGHOST=133.137.177.158
cnt=128
while :
do
(date;pgbench -C -c 1 -n -S test;date)&
cnt=`expr $cnt - 1`
if [ $cnt -le 0 ];then
	break;
fi
done
wait