[OSM-talk] Tile refresh on openstreetmap.org

Tom Hughes tom at compton.nu
Sun Mar 2 10:16:43 UTC 2014

On 02/03/14 10:03, Christoph Hormann wrote:
> On Sunday 02 March 2014, Tom Hughes wrote:
>>> The load from the re-rendering job is adjustable. Of course, if you
>>> go for a low load, then the re-rendering job will take a lot
>>> longer.
>> Not really - the load we are talking about is not the pre-rendering
>> of low zoom, rather it is the load from normal tile requests
>> triggering re-rendering of high zoom tiles.
> Well - in principle i think re-rendering of very old tiles (like >2
> weeks age) at the low zooms should take precedence over style induced
> updates at the high zooms.  Not sure how feasible it is to implement
> this.

They do. The algorithm goes something like this:

* Update stylesheet, remembering time
* Render z0 to z10 in the background
* Mark planet as dirty, using saved time
* Start rendering z11 to z12 in the background

So only at the third step will user requests starting triggering a 
rerender for tiles which are older then than the stylesheet.

At that point z0 to z10 are already done (only takes a few hours).

The rendering of z11 to z12 does overlap, but we normally arrange for 
that to happen overnight so that it can be done before load picks up in 
the morning.

Both background renders are limited to one quarter of the available 
render threads, and will suspend themselves if the load gets too high.

Note that nothing in z0 to z12 is marked dirty as a result of changes to 
the data, so they only re-render when the style changes, or once a month 
as a background job.


Tom Hughes (tom at compton.nu)

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