Re: pgbench - allow to specify scale as a size

From: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: pgbench - allow to specify scale as a size
Date: 2018-02-17 17:33:12
Message-ID: alpine.DEB.2.20.1802171823290.20813@lancre
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Hello Tom,

>> Here is a attempt at extending --scale so that it can be given a size.
> I do not actually find this to be a good idea. It's going to be
> platform-dependent, or not very accurate, or both, and thereby
> contribute to confusion by making results less reproducible.

I have often wanted to have such an option for testing, with criterion
like "within shared_buffers", "within memory", "twice the available
memory", to look for behavioral changes in some performance tests.

I you want reproducible (for some definition of reproducible) and
accurate, you can always use scale with a number. The report provides the
actual scale used anyway, so providing the size is just a convenience for
the initialization phase. I agree that it cannot be really exact.

Would it be more acceptable with some clear(er)/explicit caveat?

> Plus, what do we do if the backend changes table representation in
> some way that invalidates Kaarel's formula altogether?

Then the formula (a simple linear regression, really) should have to be

> More confusion would be inevitable.

There is no much confusion when the "scale" is reported. As for confusion,
a performance tests is influenced by dozen of parameters anyway.

Now if you do not want such a feature, you can mark it as rejected, and we
will keep on trying to guess or look for the formula till the end of


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