Time Dilation / Sim FPS puzzle

Ideally, a sim's Sim FPS should be 45, and it's Time Dilation should be 1.0. 45 frames per second = almost exactly 22ms per frame. When the sim's Total Frame Time = 22ms, the sim is good but there is no spare time capacity. When the the Total Frame Time is less than 22ms, the sim is good and it has some spare time capacity.

I'm watching a sim that has a Total Frame Time of between 18ms and 22ms (as normal, each frame time is different) so, since it has some spare time capacity, it's Time Dilation should be 1.0, and its Sim FPS should be 45, more or less constantly. But it isn't.

The Sim FPS has occasional 1 frame peaks at 45 but is usually 43 to 44, and the Time Dilation never hits 1.0. Apart from the occasional negative spike (trough), it's 0.97 to 0.99.

Can anyone shed any light on this puzzle?

I will confess I don't understand the frame time stuff completely, but I've been watching sim FPS and time dilation for a long time - I always keep my stats bar open and I have a hud which changes colour based on the sim performance. FPS in the 43-45 range are all very acceptable and time dilation over 0.95 is also very good. Neither will be at perfect 45 / 1.0 all the time; there are always going to be minor fluctuations.

Stuff like Avatars TPing in or out of the sim for instance, will caues a momentary blip as their data / attachments / scripts are loaded by the sim, and at the same time the sim is feeding them textures and object data. Other things in the sim can cause fluctuations even when there's no avatars present - physical objects moving around, rezzers doing their thing...

Personally, I don't worry unless the sim is showing FPS / Time dilation below 40 / 0.9 respectively. And even those troughs are going to happen - it's when it dips and stays low for more than a second or two, that you really start to notice it.

-Atashi

If your Total Frame Time is less than 22ms, then your Time Dilation should be 1.0 and Sim FPS should be 45, all other things being equal.

What other things are there? Remember, your region is being simulated on a server that it shares with three other regions. If those other regions are especially laggy, it can affect the performance of your land.

Also, there can be other processes running on your server, unrelated to any region. I've seen TD spikes down to 0.2, in a region where the Total Frame Time was less than 6ms. I asked a Linden to investigate, and he found a "rogue process" running on the server. It's like if you're trying to play Second Life, except you've got WoW running in the background. When he killed the process, my TD went to 1.0 and stayed there.

Yes, all that is quite normal, Atasha. The puzzle is that the Total Frame Time is consistently less than the time needed to maintain 45 FPS and 1.0 Time Dilation.

You might have the answer there, Rifkin - the other sims on the server.

[added]
I'll settle for that

Generally speaking, I like the "other processes" explanation a lot better than the "other sims on server" explanation; I don't think one sim's processes ever get to run on another sim's CPU, except for the OpenSpace sims. (I suppose, though, theoretically, the sims could start to compete for memory bus bandwidth or some other shared resource.)

In case anybody who's not already familiar wants to decrypt the stats bar, this from the wiki is a reasonable introduction: https://wiki.secondlife.com/wiki/Statistics_Bar_Guide .

hmm... To be honest, I really don't know how SL's multi-processors are integrated into a box. I don't imagine that a processor with some spare time takes some of the load from another that's being overworked, but I also don't think that there are 4 autonomous computer systems in each box. I do think, as you suggested, that there are shared resources in each box.

I've just remembered something that I was told a long time ago, by an IBM employee who's job is in SL - that sims in a box can cause lag in the other sims in the same box.

On the whole, I prefer the 'other sims' idea to the rogue process one.