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Re: LWLock contention: I think I understand the problem

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Fredrik Estreen <estreen(at)algonet(dot)se>
Cc: Tatsuo Ishii <t-ishii(at)sra(dot)co(dot)jp>, pgman(at)candle(dot)pha(dot)pa(dot)us, pgsql-hackers(at)postgresql(dot)org, jwbaker(at)acm(dot)org
Subject: Re: LWLock contention: I think I understand the problem
Date: 2002-01-04 01:02:54
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Lists: pgsql-hackerspgsql-odbc
Fredrik Estreen <estreen(at)algonet(dot)se> writes:
> Here are some results for Linux 2.2 on a Dual PentiumPro 200MHz, SCSI 
> disks and way too litte RAM (just 128MB).

Many thanks for the additional datapoints!  I converted the data into
a plot (attached) to make it easier to interpret.

> I observed the loadavg. with the three different 7.2 versions and 50
> clients, without patch the load stayed low (2-3), with patch no1 very
> high (12-14) and with patch no2 between the two others (6-8).

That makes sense.  The first patch would release more processes than
it probably should, which would translate to more processes in the
kernel's run queue = higher load average.  This would only make a
difference if the additional processes were not able to get the lock
when they finally get a chance to run; which would happen sometimes
but not always.  So the small improvement for patch2 is pretty much
what I would've expected.

> I could run benchmarks on 7.1 if that would be interesting.

Yes, if you have the time to run the same test conditions on 7.1, it
would be good.

Also, per recent discussions, it would probably be better to try to keep
the total number of transactions the same for all runs (maybe about
10000 transactions total, so -t would vary between 10000 and 200 as
-c ranges from 1 to 50).

			regards, tom lane

Attachment: bench.gif
Description: image/gif (4.9 KB) (inlined above)

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