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Re: LWLock contention: I think I understand the problem

From: Hannu Krosing <hannu(at)tm(dot)ee>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: LWLock contention: I think I understand the problem
Date: 2002-01-04 11:45:43
Message-ID: 3C3595E7.B454CB0@tm.ee (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-odbc
Tom Lane wrote:
> 
> Hannu Krosing <hannu(at)tm(dot)ee> writes:
> > Tom Lane wrote:
> >> Unfortunately, at low scaling factors pgbench is guaranteed to look
> >> horrible because of contention for the "branches" rows.
> >>
> > Not really! See graph in my previous post - the database size affects
> > performance much more !
> 
> But the way that pgbench is currently set up, you can't really tell the
> difference between database size effects and contention effects, because
> you can't vary one while holding the other constant.

What I meant was that a small -s (lot of contention and small database) 
runs much faster than  tham big -s (low contention and big database)

> I based my comments on having done profiles that show most of the CPU
> time going into attempts to acquire row locks for updates and/or
> checking of dead tuples in _bt_check_unique.  So at least in the
> conditions I was using (single CPU) I think those are the bottlenecks.
> I don't have any profiles for SMP machines, yet.

You have good theoretical grounds for your claim - it just does not fit 
with real-world tests. It may be due to contention in some other places 
but not on the branches table (i.e small scale factor)

--------------
Hannu


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