[mapserver-users] expected speeds.

[pcreso at pcreso.com]

Hi Sam,

You have no mentioned whether or not you created spatial indices on your geometries. Such indexes won't help much when plotting the entire dataset, but will dramatically improve plotting (retrieving) subsets of it.

I suggest you use the PostGIS function ST_SimplifyPreserveTopology() to populate an extra geometry column in each table, which should have significantly fewer vertices. Then in your mapfile, use scales dependent layers, so that when zoomed out you use the simplified column, and at larger scales you use the full resolution column. You can assign these to the same group, so they are treated as a single layer from the user's perspective, but mapserver retrieves and renders data at a detail (& hence volume) appropriate for the current scale automatically. When you zoom out, you seldom need the full resolution data, so simplified versions are fine, and much faster.

This is the vector data equivalent of raster layers at different resolutions to support zoom layering (or scale based layers).

If you have the cpu & memory, PostGIS can be fast enough to have the simplified layer generated as a view on the full dataset, or as a query in the mapfile data definition sql, but given your specs, a separate column will probably give better results.


Cheers,

   Brent Wood 


--- On Thu, 5/7/09, Robert Sanson <SansonR at asurequality.com> wrote:

> From: Robert Sanson <SansonR at asurequality.com>
> Subject: Re: [mapserver-users] expected speeds.
> To: "Sam Ingarfield" <sjfingarfield at gmail.com>, mapserver-users at lists.osgeo.org
> Date: Thursday, May 7, 2009, 8:48 AM
> 
>  
>  
>  
> Hi Sam
>  
> I would try turning the Coastline layer into a
> Shapefile and serving it from the Mapserver machine in a
> local directory. Postgis is probably overkill given the
> layer is probably static (unlikely to need changing).
>  
> regards,
>  
> Robert
> 
> >>> Sam Ingarfield <sjfingarfield at gmail.com>
> 7/05/2009 2:27 a.m. >>>
> Hi;
> 
> I've just been through a bit of an optimisation process
> on my server and was wondering whether the speeds that I am
> obtaining are somewhat inline with what could be expected
> with the hardware/datasets that are in use.
> 
> Firstly: Hardware.
> 2.2Ghz Xeon (Single Core, but with hyperthreading. not that
> hyperthreading helps much.)
> 1.2Gb Memory coupled with over 2Gb of swap (was filling up
> memory and original swap realllly fast so swap was
> expanded)
> Data being served off an array of three ultra320 15k scsi
> drives in a raid-5 config.
> 
> Software.
> OS: Debian
> Mapserver: Nighly build as of 6/5/09, in fastcgi mode
> Data source: Postgis database
> 
> Data:
> Coastline Layer (9 Polygons with a stupid number of
> vertices, this is a known slow point due to toast table
> problems. or at least we think it is)
> Islands Layer (~9000 polygons, only ~4mb)
> Internal Waterbodies layer (~11000 polygons, plus ~1.2m
> lines, 800mb all up)
> 
> Performance:
> On a moderate zoom level, about 2s per 256x256 tile.
> On more zoomed out levels, process times out before
> completing. and yes, some generalisation needs to take place
> at further out zoomage but thats not sorted yet.
> If i drop the coastline layer out (the toast problem one),
> speed ups to like 80-100ms/tile
> 
> 
> So, watchathink?
> 
> 
> 
> 
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