FAQ : HDM, huge, mega, giant prims

UPDATE : The FAQ is now inside my HDM prims pack available in-world and at :
http://shop.onrez.com/item/342721
http://www.slexchange.com/modules.php?name=Marketplace&file=item&ItemID=349565

These are informative posts posted by Lindens :
/8/c0/208385/2.html#post1668370
/8/c0/208385/2.html#post1668493
/8/c0/208385/2.html#post1668918

Related thread : The Big Prim Problem /13/b7/216541/1.html

Good info, Simil, especially the link to Zero's explanation of the sim communication problems.

I second the motion to sticky this thread.

I'm not sure I get what Zero was saying.. If I go near a sim border, I can see what's over the line and even stuff a fair amount over the line, megaprim or not. Where's the extra cost come in?

Not that I'm disputing Zero - far from it. I'm just trying to understand so I can yell at neighbors without sounding like a total bozo.

I don't quite get it either. I keep my draw distance at 512, and I can generally see objects 2 sims away with no problem. The only ones I don't see within the 512 meter radius are the ones that that are small enough to culled by LOD. Those appear and disappear as my av moves closer and further away. Large objects are always present.

So what is this alleged "upped communication cost"? Clearly it's got nothing to do with communication of visual geometry information to avatars. It must be something else. What, I don't know.

I would really like a clearer version of Zero's explanation as well, as the owner of the estate I'm on has put a megaprim over the entire sim at 700+m.

Now I see weird oscillations in performance, even when this class 5 sim is completely empty, and if it's related I would like to be able to clearly explain it to him.

I also see weird sky rendering quite often.

Mmm... I'd read what Zero was saying a bit more carefully.

Sims simply don't communicate multiple sim-distances away right now.

So rather than make a lot of lag to be seen, a 1km prim would simply not be seen at all if you were too far away.

No visibility, no lag from the communications cost of seeing it either.

So, Desmond, may I see if I understand you? You are saying that Simil is misreading the conversation below, that Zero is not saying that sim-size megaprims cause lag NOW. He is saying that if they implemented changes to make them fully supported (i.e. render them correctly from adjacent sims), THEN there would be lag? Is that what you meant?

Nika

13:24] Zero Linden: Tao, realize that they cause big issues with sim to sim communication - and can't work over two 512m
[13:24] Tao Takashi: what sort of issues actually? physics engine or something more?
[13:25] Simil Miles: why not over 512 ? i have several over 512
[13:25] Zero Linden: well - sims have to communicate objects that are near borders
[13:25] Zero Linden: the bigger the prims - the more distance over which the sims have to know
...
[13:27] Zero Linden: so - not more than 512 because then a prim has to be communicated to a sim more than one sim neighboring
[13:27] Zero Linden: and we have no infrastructure for that - nor do we want to put one it
[13:27] Qarl Linden: sculpted mega prims are as supported as regular mega prims.
[13:27] Zero Linden: that really ups communication costs
[13:27] Zero Linden: read: "supported in so far as mega prims are supported"
[13:27] Qarl Linden: yes.
[13:28] Qarl Linden: which is "not at all"?
[13:28] Qarl Linden:
[13:28] Khamon Fate: See this is why we like Zero. He can explain in one sentence why large prims cause adverse effects without droning on about technicalities and vaporous timelines. We Love You Zero.
[13:28] Zero Linden: and - then there the community effects of objects on parcels and how far they can overlapp your neighbors
[13:28] Tao Takashi: I wonder what's different if I build a huge tower with 100 single prims or 1 megaprim
[13:28] Zero Linden: awwwww
[13:28] Zha Ewry: The commo coast
[13:28] Zha Ewry: cost
[13:28] Zha Ewry: te 100 prim tower oesn't make the sim talk 2 sims away
[13:29] Zero Linden: Tao - you could certainly make a case that there are a class of mega prims that don't
[13:29] Zero Linden: cause the problems we can have with mega prims
[13:29] Zero Linden: but that means much the code has to have more complex checks than "not bigger than 10m"
[13:29] Which Linden: Megaprims might be supported if we did some sort of subdivision, but then you might as well just link a buncha smaller prims
[13:29] Wyn Galbraith: So mega prims, or wild prims that are not bigger than a sim and are centered in the sim, those are less likely to cause problems?
...
[13:30] Zero Linden: So yes, possible, but a fair bit of work in code for not a huge benefit (unless you're the one paying the tier bill, of course! )
[13:30] Which Linden: Ha, true

Hm.. You sure about that? The one thing I _did_ get from Zero's blog was that megaprims can really crank up communications costs..

Or am I not understanding what you're saying either?

What's your definition of "communicate", Desmond? As I said earlier, all you need to do is set your draw distance to 512 and you can see stuff over 2 sims away. I would call that "multiple sim distances", wouldn't you?

If anyone's interested, I just tried a quick experiment using RenderFarClip in the debug settings. I cranked my draw distance up from 512 to 4096 (the length of 16 sims). While the amount of visible land increased dramatically (for about 5 minutes anway, right up until the viewer crashed) the amount of visible prims increased only slightly. Standing in Indigo and looking West at 512 draw, I can see everything in Crimson, Chatreuse, Mocha, and Mauve, along with a little bit of what's in Bisque, Green, Aqua, and Blue. With the increase to 4096, just a few more objects appear in Aqua, Green, and Bisque. And now that those are cached, they persist even when I drop the draw back to 512.

So, is 2.5 sims perhaps the "communication limit"? If so, was Zero perhaps talking about a megaprim larger than 2.5 sims possibly forcing an increase to that limit? Or is visual data not the type of "communication" in question. I'm still just as confused as before.

I'm myself not sure what was meant. I sent an IM to Zero for more details.

You can see objects in multiple sims because you connect to multiple sims in order to see them. The sims need to communicate with *one another* though, for the purpose of things like physics simulation. Amirite?

512 meters???????? I have 5 FPS when I put 512 meters in the draw distance. How do you do to don't get lag?

That makes sense, Day. However, since megaprims cause physics problems even in a single sim, it still doesn't seem quite to jive with what Zero was saying. I guess maybe to overlying point I'm taking from all this is that sims communicate with each other differently than they communicate with avatars. That's interesting in and of itself, but I'm still curious what aspect(s) of that communication are amplified by megaprims.

Can you or anyone show me in-world an HDM prim causing physic problems ?

I've never personally tried it, but I've had people tell me they've crashed sims pretty quickly with physical megaprims. From the sound of it, it doesn't take much. If I had an island of my own, I'd experiment, but unfortunately I don't.


The answer is high end hardware. Unfortunately, good performance in SL is expensive. Here's a brief summary of my system specs that matter for SL speed, if you're curious:

2.6 GHz Core2 Extreme (quad core)
2GB 800 MHz DDR2 RAM
Dual GeForce 8800 GTX each with 768 MB DDR3 VRAM
5 WD Raptor 150GB 10,000 RPM hard drives, one dedicated exclusively for cache
EVGA nVidia 680i SLI motherboard

On private islands, I usually get anywhere from 75-115 FPS. On the mainland, it's more like 20-35. That's with every graphics option turned on and maxed, plus 16XQ anti-aliasing.

If I turn on SLI, I get about a 20-40% boost in FPS, but I run dual monitors so I don't normally use SLI. It's plenty fast enough without it. The two video cards are mainly there just to keep SL and Maya as separated as possible since I almost always have SL, Maya, and Photoshop running at the same time.

Anyway, let's not sidetrack this thread too much, If you want to continue the hardware/performance discussion, let's start a new thread for that.

There are plenty of empty islands to test that. If it can be proved I'll add it to the FAQ.
I previously wrote somewhere on the forums that I let a 256m physical box drop on the ground and the simulator's FPS didn't move a bit.

I feel so inadequate now!

Chosen, couldn't you try out destructive megaprims on the Crash Me sim?

hmm.. I have been using Mega prims for a about a year.. and I know for a fact that other builders use them.. alot of this information is good but some of it is wrong.

Want proof.. go to ben and jerrys.. and check out the ocean..

End of line.

(sorry this is a kinda sore spot with me.. alot of people think they know what is going on but dont.. or havnt tried to use them.. I would put this guide at about 75% true) there are more mega prims that I know of also.. that havnt been tourted.. one that comes to mind is the 50x10x.01 one.

I like to do things right.

I went to check the ocean at Ben and jerry, it's the 65535, 65535, 0.01 which I already listed.
Where is the 50, 10, 0.01 that you mention ? send me a copy so I can verify.

What is the 25% wrong made of ? Give me a list.
Claims without proof are worthless.

I think Zero is refering to the edge zone that is copied from one sim to the next. Which objects get copied is done from the prim position, so anything with a megaprim could be outside the edge zone but still visible in it.

So to properly support mega prims, the size of the edge zone would need increasing. With sim sized mega prims, that means the edges would have to be increased to half a sim. With traffic going both ways, that's a full sim of data per sim edge. At the moment I think the edge zone is 10m, so 80m of prim data is transfered over the 4 edges. So it's over 12 times more data to properly support sim sized megaprims.

With a draw distance of 512m you won't see some of the problems with megaprims. My old portable only had 64Mb graphics memory so 64m was my usual draw distance. On sims with mega prim floors, I wouldn't see the floor at the edges of the sim, only when I was nearer the middle.

I have 20x20x 0.1 and 20x20x 0.2 (my favorite ones).

Well then send me a copy of each

Very interesting thread, with lots of useful information.

'20x20x0.1' and '20x20x 0.2' would be useful additions indeed; at the moment I have only the standard '20x20x 0.5'

Am also interested in how one could torture a 100 x 100 x 100 sphere into, say, 80 x 80 x 80 as, if I've understood the posts below correctly, the implication is that this is possible. Would be grateful for further info on this...

Thanks!

You can divide XY by 2 or 4 and Z by 50 maximum.
That's how you get 20, 20, 0.1 and 20, 20, 0.2 from 20, 20, 0.5.