Hi Robert,
How should we tackle it, if current
approach is wrong ?
[..]
If you do want to mix the two then you need to ask questions about how
you want to do intersections and how your thread them. Do you do
intersections in the main loop? In a separate thread? Do you run the
intersections multi-threaded? I.e. multiple intersections traversals
at one time? Should the intersection traversal wait to load external
tiles to get the result, or should then just return the intersections
for what is already loaded into memory.
What approach you will want to use will depend upon all of this.
Our case is following: We mostly have to update camera position and some
objects around camera. And ocassionaly we reposition a camera to new
location in the world. We are doing our intersections in update traversal. I
assume its allowed to use IntersectVisitor in update and load new tiles
then. We load many objects in update traversal.
And believe it or not but our scheme was working. Cache kept nodes bit
longer than DatabasePager wanted, but not used tiles were eventually freed.
I know it did work, because we made quick dirty fix by renaming
"NeedToRemove" nodes to empty string name when fetching PagedLODs back to
activePageLOD list from cache (described in Pawel Ksiezopolski post I
mentioned earlier). Unfortuantely this fix was not appropriate as elegant
submission. Sharing loaded PageLOD tiles through cache was actually working
as some preload for DatabasePager. DatabasePager still could load whatever
it wanted.
Second important issue for me is usage of _name in scene graph. I always
expected that _name is reserved for users and its a normal rule in all
Scene
Graph implementations that libraries do not change it. Names are ususlly
used to identify certain portions of models and hook up the code
properly.
Thats something that provide standard linking mechanisms between artists
and
programmers works.
I agree, but... in this instance the DatabasePager's algorithm was
about deleting a subgraph that would no longer have any role to play
in the applications life so the changing of name should never have got
outside that algorithm as the subgraph would be just deleted. So it
is in theory just a black box, how it does it's job shouldn't effect
anything else. Alas in this case it looks like the algorithm in
DatabasePager is flawed.
Please also note that we use cache becuse we otherwise were loading
PageLOD
files twice. Is it reasonable ?.
Using a cache to prevent a subgraph from being deleted defeats the
load balancing that the DatabasePager will be attempting to do, so
it's a dangerous thing to do - it's a recipe for relentless growth in
memory usage.
Well yes, but in our case PagedLOD usage timeframe should be extended to
period when the tiles were used for intersections and cache maybe in bit
hacky way provided this prolonged life time. And use of cache is not direct
cause of leaks becaus osgDB cache is freed when node ref_count reaches zero
so when DatabasePager & IntersectionVisitor stopped using the tile it was
effectively removed from the cache as well.
Memory leaks were the result of extra NeedToRemove nodes that appeared in
activePageLOD lists when some nodes were resurected from Cache because
camera moved over them again. I know its sophistry, but in some way, it was
more effective than native DatabasePager management because it loaded nodes
in no time while DatabasePager would have to load them from disk in this
case ;-).
Even if we skip intersections, other
situations are also possible. For example we may have few highly detailed
special PageLOD tiles with ariports which we want to preload and keep in
memory for whole application runtime. So we modify readFileCallback to
work
for such cache and return these preloaded models each time thery are
requested.
Use of the cache in conjunction with a paged database should be used
very sparingly and for only very specific types of assets. It does
also open the question of how DatabasePager should deal with such
datasets, without lots of reflection on the issue I can't say. I can
say in the design and development of DatabasePager I have made the
assumption that it'd be the master of the PagedLOD's and manage all
reading and expiring, and not have code on the outside managing things
in a parallel.
Well I thought that if PagedLOD class is public and offers public interface
and methods we could use it as long as we do not hit compiler errors.
Perhaps different set of classes should be created for use in more VPB
specific closed way.
It's worth noting that PagedLOD has settings that allow you to control
what happens with expiry - so you can individually switch off the
expiry.
I have not thought about it. I will have to learn what we can do using this
mechanism, perhaps we could do something smarter.
Wojtek
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