[osg-users] How do PagedLOD children expire?

Donald Tidrow donald.r.tidrow at saic.com
Mon Sep 10 17:12:49 PDT 2007

Paul Martz wrote:
> Hi Robert -- My understanding of the DatabasePager is that PagedLOD 
> children that are not visible in the current view volume should 
> eventually "expire", which I assume means be unloaded from memory. OSG 
> would then have to reload such children from disk if they ever came back 
> within range. However, I am not seeing a decrease in process size when 
> PagedLOD children are not visible. Please let me know if I've somehow 
> misunderstood the intent of the DatabasePager and PagedLOD.
Even if the PagedLOD children are getting freed properly, I'm not sure 
that the process will actually release the associated memory back to the 
  OS.  A lot depends on how the heap memory manager deals with freed 
memory - most cases I've seen the memory manager assumes that you'll 
need that memory again soon and keeps it as part of the overall process 
memory footprint.  Even so, the OS will often swap out those pages if 
they're not actively being used and the physical memory is needed by 
other processes.

If you can, check to see how much of the program's memory is actually 
'resident' in system RAM vs. the 'virtual' memory total - I know that on 
Linux/Unix the 'top' command will show this, not sure about Windows though.

> If I understand correctly, the default expiry delay is 10 seconds, so I 
> should see children being deleted after they are moved outside the view 
> volume for 10 consecutive seconds.
> FYI, I'm attempting to cook my own paged database for a tiled OpenFlight 
> terrain database. I'm doing this with a top-level Group node with 
> several PagedLOD child nodes, one for each tile in the database. Each 
> PagedLOD has its own explicit center and radius based on the tile 
> location and size. So, if you think the issue might be misconfiguration 
> of the PagedLOD nodes, then I'd say this is possible, though I can't see 
> what I might be doing wrong from looking at the PagedLOD and LOD headers.
> I'm using osgViewer, so DatabasePager config is handled automatically.
> Thanks for any insight,
> Paul Martz
> *Skew Matrix Software LLC*
> http://www.skew-matrix.com <http://www.skew-matrix.com/>
> 303 859 9466
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|  Don Tidrow                            |
|  Visualization Software Engineer       |
|  SAIC                                  |
|  ph: 703-253-1094                      |

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