profiling connection overhead

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: profiling connection overhead
Date: 2010-11-24 05:07:27
Message-ID: AANLkTikX872517kk-SkLTgZ=o8rewHGqJQEeb2eH_TPh@mail.gmail.com
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Per previous threats, I spent some time tonight running oprofile
(using the directions Tom Lane was foolish enough to provide me back
in May). I took testlibpq.c and hacked it up to just connect to the
server and then disconnect in a tight loop without doing anything
useful, hoping to measure the overhead of starting up a new
connection. Ha, ha, funny about that:

120899 18.0616 postgres AtProcExit_Buffers
56891 8.4992 libc-2.11.2.so memset
30987 4.6293 libc-2.11.2.so memcpy
26944 4.0253 postgres hash_search_with_hash_value
26554 3.9670 postgres AllocSetAlloc
20407 3.0487 libc-2.11.2.so _int_malloc
17269 2.5799 libc-2.11.2.so fread
13005 1.9429 ld-2.11.2.so do_lookup_x
11850 1.7703 ld-2.11.2.so _dl_fixup
10194 1.5229 libc-2.11.2.so _IO_file_xsgetn

In English: the #1 overhead here is actually something that happens
when processes EXIT, not when they start. Essentially all the time is
in two lines:

56920 6.6006 : for (i = 0; i < NBuffers; i++)
: {
98745 11.4507 : if (PrivateRefCount[i] != 0)

Non-default configs:

max_connections = 100
shared_buffers = 480MB
work_mem = 4MB
maintenance_work_mem = 128MB
checkpoint_segments = 30
random_page_cost = 2.0

Anything we can do about this? That's a lot of overhead, and it'd be
a lot worse on a big machine with 8GB of shared_buffers.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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