I have been exploring the capabilities of RenderMan for a few weeks now at the hand of the sample scenes available on the Pixar forum. Managed to get a grip on most of the features and are quite impressed so far especially with the way it handles displacements.
Next step was to see how well Reyes handles high poly counts using particle instancing. I created a rib archive node and had it point to an existing archive of a high-poly clump of grass created from PaintFx. I added the archive node as the instanced object in the particle instancer, emitted 10k particles and hit render.

5 minutes into the render (Intermediate preset) and my machine ran out of memory.

I tried two ways of creating the archive, first using the "Create node and write archive" option and then with export as "Rib Archive". Both ways blew the memory limit. I even tried the "Allow Object Instancing" on the archive node to see if that helps.

I must be doing something stupid; either in the way I create the rib archive or in the way I instance the archives. (Mental ray merrily rendered out 50k MR proxy instances of the same clump of grass.)

Any ideas / guidance?