[osg-users] How can we have a single VRAM footprint over multiple contexts/windows/views/camera in a multi-thread safe way?

[OpenSceneGraph Users]

Hi Vaillancourt,

The OSG does not provide thread safe sharing of contexts, and OpenGL itself
doesn't provide it either, so even if we attempted to mutex lock all access
to GL objects that multi-threaded nature of graphics would step around
locks we attempted to add.  If you want to share contexts then you'll need
to use single threading.

With the VSG Vulkan objects are per logical device so you can share objects
on the same hardware in a more user controlled way, one still has to take
care because the objects aren't generally safe to read/write to one has to
use explicit synchronization when managing objects to avoid threading
collisions.  The VSG and Vulkan have waaay lower CPU overhead so
multi-threading is actually far less critical for performance,
paradoxically as Vulkan is so much better at handling threading!

Robert.

On Wed, 30 Sep 2020 at 14:05, OpenSceneGraph Users <
osg-users at lists.openscenegraph.org> wrote:

> Greetings.
>
> Using OSG 3.6.3, we have experimented with the 'osgwindows' example,
> loading a test model and observing the VRAM usage.
>
> We saw that having two graphics context, set to each camera of the two
> windows (as in the example) has twice the VRAM footprint usage compared to
> using a single graphics context, camera and window.
>
> Is there a multi-thread safe way to only have a single VRAM footprint
> usage, using two graphics contexts/cameras/windows ?
>
> We have tried "sharing" the first graphics context to the
> 'traits->sharedContext' of the second window.
>
> This seems to have given us the the desired effect of single VRAM
> footprint usage, but any multi-threading model seems to be unstable, not
> functional.
>
> Thank you!
>
> - Vaillancourt
>
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