[GraphHopper] Understanding the impact of prepare.chShortcuts=fastest

[Peter K]

Hi Ludwig,

> Is the impact of these settings bigger for larger areas (longer routes)
> and less important for areas like individual cities with shorter routes?

Exaclty. Test if you have enough memory for the longest possible route.
Also there is a problem on master if there are disconnected areas - see
issue 105. Or throw away unlikely points in your algorithm (queue + map)
if memory is too low, at a later stage graphhopper can do this
automatically, but not yet implemented.

To further increase speed on Android
 * you can try AStarBidirection and tune the approximation - the lower
the faster but the probability increases to create non-optimal routes.
Reducing this should also lower the memory requirements.
 * for small areas you can try in-memory instead of mmap graph (e.g. via
hopper.setInMemory(true, true)). But of course this would require you
even more RAM.

The good thing about none-CH is that you can offer your users a
customizable profile which they are able to tune differently for every
query. For CH you need to know those details upfront.

Regards,
Peter.

> To prepare routing files for more than one vehicle type at once, I
> have to comment out
>  
> #prepare.chShortcuts=fastest
>
> and then also in the Java code add
>
> setCHShortcuts(false, false);
>
> This setting for the generator creates significantly smaller routing
> files, which is a good thing, but it disables the use of the
> contraction hierarchies, which should slow run-time route generation
> down.
>
> I have not actually measured the slow-down, but on the Android devices
> that I tested it still feels pretty fast, certainly fast enough,
> particularly when considering that the files get much smaller.
>
> Are there any worst case scenarios that I should be aware of? Are
> there any other trade-offs (such as larger memory use, inability to
> find certain routes)? Is the impact of these settings bigger for
> larger areas (longer routes) and less important for areas like
> individual cities with shorter routes?
>
> Ludwig
>
>
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