[mapserver-dev] Speed in accessing World .wld files varies across disk systems

Chris Galli (XC Skies) cgalli at xcskies.com
Fri Oct 17 18:55:18 EDT 2008

Thanks for the post Paul. I can clearly see what you've pointed out so I 
can chuck that theory out the window. I'll dig deeper into the 
intricacies of my different disks again and see what I can shake out.


Paul Ramsey wrote:
> Here's the function in question:
> http://trac.osgeo.org/mapserver/browser/trunk/mapserver/mapraster.c#L342
> As you can see, it doesn't do a directory search, though it does work
> its way through a number of possible extension options. Note that
> "wld" is the *first* option though, so that's not your problem.
> P.
> On Fri, Oct 17, 2008 at 3:29 PM, Chris Galli <cgalli at xcskies.com> wrote:
>> Hi Everyone,
>> I know the above statement seems like it deserves an obvious answer, so
>> first let me say that I understand the complexities of disk implementations
>> enough to realize that speed depends on a tremendous amount of factors and
>> so cannot be easily discussed in terms of absolutes when comparing different
>> disk systems. With that said, however, I'm seeing behaviour that leads me to
>> believe the discovery process for .wld files can be improved in mapserv.
>> I've tested with V 4.10 and 5.2 and they produce identical results.
>> Here's the crux:
>> When rendering raster images (say png files) which use .wld world files via
>> the cgi interface, I get wildly different response times on different linux
>> systems. After a lengthy discovery process of why this was, I have come to
>> the conclusion that mapserv is probably not targeting wld files directly on
>> the file system, and instead looking for matching wld files for raster
>> images by using some type of 'wild card' or other inefficient scan of the
>> file's current directory.
>> For example, if I place a single raster png file called world.png with a
>> world.wld in an empty directory and turn on mapserver debug, response times
>> seem reasonable. As I increase the amount of files within the directory, the
>> mapserv raster rendering becomes increasingly slower (asking for a single
>> 256x256 tile from a 1MB png file). When I perform the same test on another
>> system, I barely see a slowdown in performance. Why? Because one disk system
>> is much more robust with directory caching and disk-to-memory hardware. Fair
>> enough. But when I run the same tests on tiff files, both systems produce
>> identical results to within a few milliseconds. This implies that wld files
>> are likely not being targeted efficiently.
>> In addition to the above, I have some custom python code that accesses the
>> exact same png raster files and servers them up to the exact extents and
>> tile size as does mapserv using the GD libs.  And that code was actually
>> returning tiles faster on the system which mapserv was running so poorly. My
>> code expects a file to exist and so does not need to 'discover' it, making
>> the process much more efficient.
>> Does anyone know or suspect that the above is true? If so, how does one go
>> about providing more details and elevating this to a potential
>> change/enhancement?
>> Thanks!
>> -Chris
>> --
>> View this message in context: http://www.nabble.com/Speed-in-accessing-World-.wld-files-varies-across-disk-systems-tp20042027p20042027.html
>> Sent from the Mapserver - Dev mailing list archive at Nabble.com.
>> _______________________________________________
>> mapserver-dev mailing list
>> mapserver-dev at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/mapserver-dev

More information about the mapserver-dev mailing list