Re: Timing overhead and Linux clock sources

On Mon, Aug 27, 2012 at 04:42:34PM -0400, Robert Haas wrote:
> On Mon, Aug 27, 2012 at 1:18 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > He wrote it that way to allow for simpler C code --- he could just start
> > from 31 and keeping skipping entries until he hit a non-zero.
> >
> > My format makes it easy to see which line should have the majority of
> > the entries, e.g. first line should be > 90%. I doubt there are enough
> > people running this cross-version that consistency in output makes any
> > difference between major PG versions.
>
> I don't see why it's better for the first line to have a big number
> than the last line. What difference does it make?

When you are looking at that output, the <1 usec is where you want to
see most of the percentage, and it trails off after that.

Here is an example from the current output format:

Histogram of timing durations:
< usec: count percent
16: 3 0.00007%
8: 563 0.01357%
4: 3241 0.07810%
2: 2990371 72.05956%
1: 1155682 27.84870%

That first line is pretty meaningless. You have to look at the last
line, see that only 27% of <1, then look up to see that 72% is 1<2,
which isn't good. My format shows:

< usec % of total count
1 27.84870 1155682
2 72.05956 2990371
4 0.07810 3241
8 0.01357 563
16 0.00007 3

First line, 27%, that's a problem, look down for more details.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +