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Multiprocessor performance

From: "Valentin Puente" <vpuente(at)atc(dot)unican(dot)es>
To: <pgsql-hackers(at)postgresql(dot)org>
Subject: Multiprocessor performance
Date: 2001-06-05 12:39:55
Message-ID: 00c601c0edbc$a023a8e0$c6b990c1@breca (view raw or flat)
Thread:
Lists: pgsql-hackers
Hi all,

I'm not a postgres hacker, but I' think that you must be the most
appropriate person to give me a pointer about this question.... sorry for
any possible mistake.

Now I'm trying to use postgresql plus the pgbench like a
first test to stress the interconnection system in a parallel machine. I
know that tpc-b is just a toy (no too much real... but before to do
something more complex like tpc-c y want to see the postgres behavior).

Ok...well I'm running this benchmarks in different SMP machines (SGI with 4
to 8 processors and the results are odd). The best performance is achieved
with just one backend (1 client). When I try to run more clients the tps
falls quickly.

In all cases I see that when I increase the number of clients the total CPU
usage falls. With one client I can see a 100% usage (after a warm-up to get
all data from disk - I'm running without fsync and with a large shared
buffer).My systems have a lot of memory then this is normal. But when I try
with more clients each CPU usage falls between 40% for 2 clients to 10% to 8
clients. I assume the access to the shared memory through critical regions
(lock-unlock) must be one reason... but this is too much. I've heard that
locks in postgress are at page level instead tuple level. I'm wrong?.

Some suggestion about this?.

Thanks in advance for your support.

--vpuente


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