[postgis-devel] Caching Double-based Boxes

[Mark Cave-Ayland]

On 26/11/11 20:44, maplabs at light42.com wrote:

> Hi All-
>
>    I have devised a second set of tests, based on the first run and some
> suggestions. There are three sets of results for each of the source
> builds, float bbox and double bbox. The condensed records are here:
>
>      http://download.osgeo.org/postgis/postgis_dbl_boxes_test_results22.csv
>
> Notes:
>
> * there is one test result in one run that was lost..
> * I have not analyzed these numbers.. I expect people will want to do
> that for themselves. Barring an undiscovered problem in the setup, there
> are very large differences in run times
> for the double-boxes branch, but almost entirely consistant run times
> for the float-trunk
> branch.
>
> * The third run in each case was after a database restart, in order,
> from top to bottom. The isolated numbers in the columns are 'cold' bbox
> hits.. much much larger than
> running them a second time, so I noted them as an aside

Hi Brian,

Thanks for spending time on devising and running the tests. I must admit 
I am surprised by the amount of slowdown that you experienced during 
your test run as there are a couple of reasons why things should, in 
theory, be the same or faster:

1) Doubles will give 8 byte alignments on 64-bit architectures (so that 
should align these values automatically within the structs which as we 
know gives us a good speed increase)

2) If we're using doubles, then this should be the most optimal path for 
hardware assisted IEEE calculations (or at least, it should equivalent 
and not slower)

I'm wondering if perhaps something else is going on here, such as we're 
now doing extra internal copying or similar? Is there any chance of 
generating a gcc profile from both tests so we can find out exactly 
where the time is going?

I'm not sure why but something about this intuitively feels wrong in 
terms of the amount of slowdown related to the change in work done here...


ATB,

Mark.

-- 
Mark Cave-Ayland - Senior Technical Architect
PostgreSQL - PostGIS
Sirius Corporation plc - control through freedom
http://www.siriusit.co.uk
t: +44 870 608 0063

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