[pgrouting-users] Speed when using big maps

[Philipp Hamm]

Thanks Steve!
But I still have some questions:

>> To understand this I have some questions about how pgRouting works:
>> Does pgRouting reloads the road data on every request and does the 
>> slow calculation results only from fetching the graph?
>> If so, is it possible to load the graph into the memory and reuse it 
>> for further requests?
>
>No, not currently because there is no place to load the graph into memory in the database that is persistent.

What do you mean? Is there not enough space on the RAM for the whole road network?
And does "not currently" mean that someone is working on that? ;) Or are there any ideas and hints to achieve that?
Because the most recent posting I've found about that is this one: http://lists.osgeo.org/pipermail/pgrouting-users/2012-December/001379.html


>> As I want to execute a lot of kDijkstras consecutively I guess only 
>> bounding boxes won't make it fast enough.
>
>If you need many routes from a single start point, then you should look at pgr_kdijkstra().

Yes, this is what I am going to do. But in fact I want to calculate a many-to-many matrix. 
So I have to run the one-to-many Dijkstra n times, which would cause a huge waste of time when rebuilding the graph n times

Regards, 
Philipp



-----Ursprüngliche Nachricht-----
Von: pgrouting-users-bounces at lists.osgeo.org [mailto:pgrouting-users-bounces at lists.osgeo.org] Im Auftrag von Stephen Woodbridge
Gesendet: Donnerstag, 24. Oktober 2013 15:04
An: pgrouting-users at lists.osgeo.org
Betreff: Re: [pgrouting-users] Speed when using big maps

On 10/24/2013 6:16 AM, Philipp Hamm wrote:
> Hi everyone,
>
> I've just started testing pgRouting on a big network (Germany) and 
> want to use the whole Europe map later, but it's pretty slow.
> I takes around 2,5 min to calculate one route and it makes no 
> difference if the route distance is 7km or 700km.
>
> Is there anything I can do to speed it up?
> I have already indices to 'source' and 'target' and id is primary key, 
> but I don't know how to use bounding boxes.

You need to create a spatial index:

create index myroads_gidx on myroads using gist (the_geom);

> Furthermore I've noticed: when selecting only a part of the graph 
> (keeping in mind that the area of the dijkstra expansion is loaded) it 
> gets much faster.
> Is this similar to what the bounding box would do?

Then only load the part of the roads needed to solve the graph. The more roads you load the long it takes to load the data and to solve the problem. Also astar and trsp are both faster then Dijkstra.

> To understand this I have some questions about how pgRouting works:
> Does pgRouting reloads the road data on every request and does the 
> slow calculation results only from fetching the graph?
> If so, is it possible to load the graph into the memory and reuse it 
> for further requests?

No, not currently because there is no place to load the graph into memory in the database that is persistent.

> As I want to execute a lot of kDijkstras consecutively I guess only 
> bounding boxes won't make it fast enough.

If you need many routes from a single start point, then you should look at pgr_kdijkstra().

If your routes are long and you want to use highways, then you can load data like a barbell using three bbox queries and union the results together. So, query 1, gets all roads for some radius around the start point, query2, gets all roads for some radius around the end point, and query 3, gets all the highways only in the bounding box between the start and end points.

-Steve

> Thanks in advance!
>
>
>
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> Pgrouting-users at lists.osgeo.org
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