Re: High context switches occurring

Thanks for your inputs, Tom. I was going after high concurrent clients,
but should have read this carefully -

-s scaling_factor
this should be used with -i (initialize) option.
number of tuples generated will be multiple of the
scaling factor. For example, -s 100 will imply 10M
(10,000,000) tuples in the accounts table.
default is 1. NOTE: scaling factor should be at least
as large as the largest number of clients you intend
to test; else you'll mostly be measuring update
contention.

I'll rerun the tests.

Thanks,
Anjan

-----Original Message-----
From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
Sent: Tuesday, December 06, 2005 6:45 PM
To: Anjan Dave
Cc: Vivek Khera; Postgresql Performance
Subject: Re: [PERFORM] High context switches occurring

"Anjan Dave" <adave(at)vantage(dot)com> writes:
> -bash-3.00$ time pgbench -c 1000 -t 30 pgbench
> starting vacuum...end.
> transaction type: TPC-B (sort of)
> scaling factor: 1
> number of clients: 1000
> number of transactions per client: 30
> number of transactions actually processed: 30000/30000
> tps = 45.871234 (including connections establishing)
> tps = 46.092629 (excluding connections establishing)

I can hardly think of a worse way to run pgbench :-(. These numbers are
about meaningless, for two reasons:

1. You don't want number of clients (-c) much higher than scaling factor
(-s in the initialization step). The number of rows in the "branches"
table will equal -s, and since every transaction updates one
randomly-chosen "branches" row, you will be measuring mostly row-update
contention overhead if there's more concurrent transactions than there
are rows. In the case -s 1, which is what you've got here, there is no
actual concurrency at all --- all the transactions stack up on the
single branches row.

2. Running a small number of transactions per client means that
startup/shutdown transients overwhelm the steady-state data. You should
probably run at least a thousand transactions per client if you want
repeatable numbers.

Try something like "-s 10 -c 10 -t 3000" to get numbers reflecting test
conditions more like what the TPC council had in mind when they designed
this benchmark. I tend to repeat such a test 3 times to see if the
numbers are repeatable, and quote the middle TPS number as long as
they're not too far apart.

regards, tom lane