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

[Chris Galli]

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

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