Re: gaussian distribution pgbench

On Sun, Jul 13, 2014 at 2:27 AM, Mitsumasa KONDO
<kondo(dot)mitsumasa(at)gmail(dot)com> wrote:
> I still agree with Fabien-san. I cannot understand why our logical proposal
> isn't accepted...

Well, I think the feedback has been pretty clear, honestly. Here's
what I'm unhappy about: I can't understand what these options are
actually doing.

And this isn't helping me a bit:

> [nttcom(at)localhost postgresql]$ contrib/pgbench/pgbench --exponential=10
> starting vacuum...end.
> transaction type: Exponential distribution TPC-B (sort of)
> scaling factor: 1
> exponential threshold: 10.00000
>
> decile percents: 63.2% 23.3% 8.6% 3.1% 1.2% 0.4% 0.2% 0.1% 0.0% 0.0%
> highest/lowest percent of the range: 9.5% 0.0%

I don't have a clue what that means. None.

Here is an example of an explanation that would make sense to me.
This is not the actual behavior of your patch, I'm quite sure, so this
is just an example of the *kind* of explanation that I think is
needed:

The --exponential option causes pgbench to select lower-numbered
account IDs exponentially more frequently than higher-numbered account
IDs. The argument to --exponential controls the strength of the
preference for lower-numbered account IDs, with a smaller value
indicating a stronger preference. Specifically, it is the percentage
of the total number of account IDs which will receive half the total
accesses. For example, with --exponential=10, half the accesses will
be to the smallest 10 percent of the account IDs; half the remaining
accesses will be to the next-smallest 10 percent of account IDs, and
so on. --exponential=50 therefore represents a completely flat
distribution; larger values are not allowed.

--
Robert Haas
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