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Re: 8.3devel slower than 8.2 under read-only load

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Guillaume Smet" <guillaume(dot)smet(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: 8.3devel slower than 8.2 under read-only load
Date: 2007-11-21 23:59:03
Message-ID: 21985.1195689543@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
"Guillaume Smet" <guillaume(dot)smet(at)gmail(dot)com> writes:
> I took a couple of very simple read only queries executed in the pages
> to create a pgbench script and I have the following results:

Hmm ... I can reproduce a consistent difference of about three percent
between 8.2 and HEAD.  Using pgbench's built-in SELECT-only transaction
after "pgbench -i -s 10 bench", I get

HEAD:

$ time pgbench -n -S -c 10 -t 100000 bench
transaction type: SELECT only
scaling factor: 10
number of clients: 10
number of transactions per client: 100000
number of transactions actually processed: 1000000/1000000
tps = 9399.185809 (including connections establishing)
tps = 9402.305058 (excluding connections establishing)

real    1m46.402s
user    0m19.889s
sys     0m23.497s

8.2:

$ time pgbench -n -S -c 10 -t 100000 bench82
transaction type: SELECT only
scaling factor: 10
number of clients: 10
number of transactions per client: 100000
number of transactions actually processed: 1000000/1000000
tps = 9729.892729 (including connections establishing)
tps = 9732.769774 (excluding connections establishing)

real    1m42.785s
user    0m19.250s
sys     0m23.646s

Vanilla build (--enable-debug but not much else), C locale, SQL_ASCII
encoding, dual Xeon/EMT on Fedora Core 6.  Configuration parameters
are all defaults in both cases, except I had fsync off, which shouldn't
matter anyway in a read-only test.

The weird thing is that after a couple of hours of poking at it with
oprofile and other sharp objects, I have no idea *why* it's slower.
oprofile shows just about the same relative percentages for all the
hot-spot functions in the backend.  strace shows that there's no
particular increase in kernel calls (indeed, HEAD seems to use
significantly fewer semops/selects, indicating that we had some
success in reducing contention).  It's not that autovacuum is now
on by default --- turning it off made no particular difference.
It's not that stats collection is now on by default --- ditto.
Slowing down the walwriter and bgwriter doesn't help either.
It's not pgbench itself --- I get about the same results if I use
8.2 pgbench with HEAD or vice versa.

The best theory I can come up with is that all the new stuff added
to the backend (the executable is about 12% larger than in 8.2)
has resulted in some generalized slowdown just because the code is
larger.  But most of the added code isn't getting exercised by this
test, so in theory the code bloat shouldn't be hurting us either.

Weird.  Given that it's only a couple percent I'm not gonna panic
about it, but I would like to know where the time is going ...

			regards, tom lane

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Subject: Re: 8.3devel slower than 8.2 under read-only load
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