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@momjian.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@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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