[OpenLayers-Users] Performance Issue with multiple layers

[Stephen Woodbridge]

Eric Lemoine wrote:
> On 8/6/07, Marc Desharnais <marc.desharnais at ec.gc.ca> wrote:
>> Hi all,
>>
>> I don't know if it's normal, but my page using 7 layers is really slow.
>> I have to mention that the layers are not all activated. Only 2 of them
>> are shown of on the map.
>>
>> Initial loading takes up to 30 seconds. Zooming to a particular area :
>> 10 seconds.
>> And the CPU consumption of my workstation jumps up to 80% when loading
>> times. Is this normal?
>> Logically the workstation should not work really hard since it is
>> querying some PNGs... Am I wrong?
>>
>> No server is on my workstation. The server and all the data used in the
>> layers is on LAN.
>>
>> 6 layers are colored points and one is a shapefile converted to postgis.
>>
>> What is slowing down the process?
> 
> Hi Marc,
> 
> Seven layers is a lot of layers, and is probably what kills you here.
> Can't you merge your layers to reduce the number of layers in
> OpenLayers?

Marc,

Depending on the type of layer you are using, this might make 7 image 
requests if you are not using tiles and if these are all going back to 
the same server then they will pretty much get serialized unless you 
play some DNS tricks to give the server multiple CNAMES.

If you are using tiles, you might be loading 9-25 tiles for each layer 
times 7 layers and the same issues I mentioned above would apply.

This is why Mark has suggested combining layers into a single request if 
your user model will allow for that.

You can use Foxfire and FireBug to monitor the time to load the various 
images and this will give you a better idea of where the time is getting 
spent. You should also watch the server and see if it is the bottle neck 
and how many requests is is getting and processing.

Hope this explains your issue a little better.

-Steve W