FW: FW: [Openstreetmap] Recent improvements to the editing applet

Etienne Cherdlu openstreetmap-L at gj0.net
Fri Dec 30 20:21:23 GMT 2005


Lars Aronsson wrote:
> Currently the tiles (both in viewing and editing) are 256 x 128 pixels.  How
> would performance be different if the tiles were, say, 512 x 256 pixels
> instead?  That would cause fewer tiles and fewer calls to the server, but
> every tile would take a little longer to process.

Initially it appears to fetch 30 tiles (6 x 5) even though 12 (4 x 3)
would be sufficient to fill the viewing area.  At least it could fetch
the important 12 first.  And it could even try to fetch all the nodes
and segments first *before* swamping the tile server with 30 requests
for tiles.

When panning it would seem to also be fetching quite a few more tiles
than are needed.  Starting from the left edge of the new area (most of
which will be off the screen).  It would be better if it started at
the edge that was closest to the tiles that have already been
displayed.

The overhead of so many round trips is not going to help performance. 
If the network latency is high then it will be pretty bad.

I'd guess there is some overhead in activating the tile server process
to generate a tile so this will also slow things down.

So, now the server is having to generate 983,040 pixels (256*128*30)
using 30 requests instead of 350,000 pixels (700*500) using 1 request.
 At a minimum it is doing 2.85 times more work than before.  Not good
if I have to wait until its all done before I can do anything.  Even
worse if the server is slow.

Etienne




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