Geography Reference
In-Depth Information
render a 2048x2048 image at 4 bytes per pixel, or a 8x8 meta-tile of standard 256x256 pixel
tiles.
Table 3 shows the maximum gain that can be achieved by the use of metatiling techniques.
This maximum gain occurs when the whole metatile is used to cache new tiles that were not
yet cached. While this is the case when automatically seeding tiles in sequential order with
non-overlapping metatiles from an empty cache or in the early startup of the service, it would
be useful to evaluate metatiling in the most general scenario where the cache is partially filled.
In that case, each metatile is likely to add redundant information, since it is probable that some
of the tiles encompassed by the metatile were already cached, thus reducing the effective gain
of this method.
The performance of metatiling during dynamic cache population with users' requests has
been evaluated using the
WMSCWrapper
tile cache, described in Section 4. Simulations were
driven by trace files from the public WMS-C tiled web map service of Cartociudad. Being
traces recorded in a real, working system, these logs represent a more realistic pattern of user
behavior than a synthetic pattern. The CORINE WMS service was used as remote backend.
100
90
80
70
60
sin metatiling
metatile centrado
metatile min. corr.
without metatiling
centered metatile
arbitrary metatile
50
40
30
20
10
0
B=0
B=1
B=2
B=3
Buffer size
Figure 12.
Cache-hit ratios obtained for different buffer sizes and metatile configurations.
A total of 1.000.000 requests were made to the cache. The experiment was repeated for
different metatile configurations. For each configuration, the cache-hit ratio and the number of
cached tiles after task completion have been collected, starting from an empty cache. Results
are shown in Figure 12 and Figure 13.
As can be shown, both the cache-hit ratio and the number of cached tiles grow with the buffer
size. For a fixed buffer size, both metatiling strategies (centered and minimum-correlation)
obtain similar results. However, the number of cached objects is significantly improved with
the minimum-correlation configuration. The improvement increases with the metatile size.
Thus, the advantage achieved with the minimum-correlation metatile configuration is that,
maintaining the cache misses, and therefore maintaining the number of requests to the remote
