[Chameleon] performance of drawmap.php

[bartvde at xs4all.nl]

Hi Paul,

I had a closer look and ofcourse the biggest difference is that Mapserver
WMS client is in between in the case of my drawmap.php call. At least in
my situation (all data is served through WMS). I found that this is the
biggest performance decline, as I did a test on requesting an image from
my application MAP file directly through Mapserver CGI.

This process takes 1300 ms. This means that Mapserver WMS client takes
about 900 ms in the whole process. This is the case when the Mapserver WMS
client has to go through a proxy (I use an external URL in my app MAP
file). If I just use localhost in my application MAP file, it all boils
down to 833 ms (so around 400 ms overhead for the WMS client).

Will it increase my performance and keep my download context functionality
alive if I use the following:

CONNECTION "http://localhost/...."
"wms_onlineresource"	"the external URL which goes through the proxy"

???? Then this would be advisable to use or not?

Concluding, the drawmap process only takes an additional overhead of
around 100 ms.

Your option 1 would not suffice for my applications as mostly I need to
combine layers from different WMS servers. And as we can see now, the
overhead is not that much in drawmap.php.

Any possibility that Mapserver WMS client could get faster? Or is this
maximum performance? Maybe Daniel has some thoughts about this?

BTW: I don't use simultaneous requests in the web stress tool.

Best regards,
Bart

> Bart,
>
> good question :)  I still have it on my list to look at this problem.
> The differences boil down to:
>
> The drawmap method does the following:
>
> 1. start apache process
> 2. load php
> 3. load mapscript
> 4. start session
> 5. load map object
> 6. execute a bunch of php code
> 7. draw map
>
> The mapserv cgi method does the following
>
> 1. start apache process
> 2. start mapserv cgi
> 3. load map object
> 4. draw map
>
> I suspect that the load map and draw map steps are roughly the same
> within a couple of milliseconds.  Starting apache and loading
> mapscript/mapserv should be roughly the same.
>
> So the difference lies in:
>
> 1. loading php
> 2. starting the session
> 3. execute a bunch of php code
>
> The code between loading the map object and actually producing a map
> image adds some overhead (but I would be suprised if it is more than 50
> ms).
>
> Note that if the web stress tool does simulatenous requests, they will
> all stall because php does session locking.  I had originally intended
> to access the session in some read-only mode that would not lock it,
> this is difficult to do because we actually trap drawing errors and put
> them into the session so that the user can see why layers failed to draw.
>
> The reason why we use drawmap is to delay drawing the map until the page
> has started loading.  This increases apparent performance while
> decreasing overall performance by requiring a second process to be
> started to draw the map.  Because of the session locking, the drawmap
> request cannot be performed until after the page has been sent to the
> browser and the main thread has exited.  Drawing the map as part of the
> execution of the application *could* speed up the final product
> significantly, but in general I think it would result in a slower
> feeling application, especially if you are using WMS sources.  At least
> this way, the page loads reasonably quickly and you are left waiting for
> the image.
>
> Alternatives ...
>
> 1. add a method of requesting the map directly by giving the
> mapdhtmlwidget a mapserv cgi and passing it the path to the map file
>
> - pros: this is probably the absolute fastest way to get the image
> without actually drawing it while the page loads
> - cons: this is a security risk because it exposes absolute paths.
> There may be some ways to limit the security risk, esp on linux systems
>        : lose the ability to provide feedback to the user on why map
> draws failed.
>
> 2. add an attribute that would cause the mapdhtmlwidget to render the
> image during execution of the application.  I think most of the code to
> support this is in the mapdhtmlwidget, it would just need a bit of
> tweaking and a new attribute.
> - pros: the overall page load time would probably decrease
>        : would maintain the ability to report errors to the user
> - cons: the responsi
> veness of the application would be worse
>
> 3. write a stripped down drawmap.php, removing some of the extra code
> - pros: maintain existing code in mapdhtmlwidget, maintain
> responsiveness of application
> - cons: minimal speed benefit likely
>
> Any comments, discussion and more ideas welcome ... personally, I like
> option 1 if security is not an issue (or there are work arounds)
>
> Cheers,
>
> Paul
>
> bartvde at xs4all.nl wrote:
>
>> Hi list,
>>
>> I have done a test with the Microsoft Web Stress test tool by requesting
>> an image to drawmap.php, and requesting the same image to Mapserver CGI
>> directly. I only have one visible layer in my application. All tests
>> performed on the server itself (localhost), so no network traffic
>> involved.
>>
>> These are the results:
>>
>> directly to Mapserver WMS:
>> 136 hits in 1 minute: average is 438.86 ms.
>>
>> using drawmap.php:
>> 42 hits in 1 minute: average is 1409.50 ms.
>>
>> Why is there such a big performance loss in this step? This seems to be
>> the largest performance bottleneck in Chameleon.
>>
>> Best regards,
>> Bart
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>>
>
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>   -----------------------------------------------------------------
> |Paul Spencer                           pspencer at dmsolutions.ca   |
> |-----------------------------------------------------------------|
> |Applications & Software Development                              |
> |DM Solutions Group Inc                 http://www.dmsolutions.ca/|
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