I see on a fairly regularly basis reports that OpenSim supports 45,000 prims versus Second Life®’s 15,000. It’s rubbish; but it comes from a somewhat logical source. The viewer itself will not display more than 45,000 objects in the ‘prims parcel supports’ field. There’s no technical reason for it – it just clamps the value (and I’m not entirely sure why). If OpenSim is set to unlimited prims (or 99,999,999), the viewer will show it as ‘Supports 45,000 Prims’ and not what it really is (’Supports “99,999,999″ prims’).

But, you can go well above that boundary. Some of Shenlei’s Leviathan builds are now breaking the 160,000 primitive count mark (and I have no doubt she intends to push it further!), but those builds are incredibly intensive on other aspects of the system, particularly memory usage. Primitive counts as a resource delimiter have never been accurate as far as underlying consumption goes; how they evolved goes straight back to when SL was still a MMORPG with nifty building tools (circa 2002-2004).

This is evident in SL – certain scripts cause “lag”, popular clubs can block other users from accessing the region (by eating up the whole max avatar count) – neither of these is factored into the current resource limits. This is applicable in OpenSim too – an unscripted region will behave better than a scripted one, an empty region will have uptimes measured in months, a popular one in days (or hours).

So, with this entry I aim to do two things – first dispell the myth that OpenSim supports 45K primitives. That is incorrect – OpenSim supports whatever you tell it to handle, whether it behaves is up to the underlying consumption required for your build, and what you are hosting it on. And second – clarify where the limits really are, and how you can optimise them.

With SimHost, we needed to give a number to our potential customers that is indicative of their usage, in a manner that can be understood like prim counts, but reflects the actual capacity used. We decided to go with RAM usage – this is because memory is the primary requirement of the OpenSim software. The amount of memory a region needs is pretty much directly proportional to all the other requirements (scripts need a roughly equal amount of memory as CPU, so do avatars, prims, etc.). There are other limits too – network bandwidth, processor usage, etc. All of these can become a bottleneck depending on the design of a region.

To give you a rough estimate of capacity-by-memory, one of our heavier customers has a 10,000 prim sim which hosts weekly meetings; it’s somewhat scripted – memory use for this region is between 405MB (Resident) and 1070MB (Total). Each avatar to the region adds between 20 and 50mb to the “resident” figure (and when occupied, some of the paged memory moves into the resident as it is accessed). If you use this as an example – 1024MB of resident memory should get you a “standard region equivilent”; if you want to start pushing on it further, then you might want to allocate 2GB dedicated to the region.

Network bandwidth is directly tied to # of avatars plus, # of primitives plus, # and size of textures. You can drop your bandwidth requirements fairly dramatically simply by building more efficiently, encouraging texture re-use, optimising your textures, etc. Sculpties actually work to your benefit here – since they can replace many prims with just one; and that one is ‘instanced’ – so that every copy you use, is only downloaded once by the viewer.

Processor usage is generally not a problem; to avoid any issues – giving a region a dedicated core will let it do it’s own thing. Scripts are about the only thing that can really push this figure (and physics to a much lesser degree). With recent updates to OpenSim enabling much larger concurrencies – processor usage is beggining to appear; but an average user will often struggle to push an average CPU core usage of more than 10%.

So, next time you see the claim that ‘OpenSim supports 45,000 prims’ (as I often do) – think of it not as a hard limit, or even a ballpark figure that is remotely accurate. OpenSim will try serve whatever you tell it to — but whether it does so successfully is more likely to be up to other factors relating to the underlying hardware; than the software itself.