How to Keep Physics Engine Load Low

What is ODE?

Today most OpenSim installations use Open Dynamics Engine “ODE” as physics engine. It is delivered as integrated part of current OpenSim versions. It is used to simulate a (more or less) realistic physical environment in the virtual world. The physical behavior of physical objects, like some vehicle types, and avatars is calculated by ODE.

Under certain circumstances the physics engine can be very busy and that physics engine load can be experiences as server lag. This article tries to describe how to identify physics engine lag and how to improve your regions to cause less such lag.

Additionally to the server lag you experience much physics engine activity might cause that OpenSim consumes more and more memory over time, due to memory leaks. This may cause lower stability and more frequent restarts of your OpenSim installation.

How to identify physics engine lag?

Enter “show stats” on your OpenSim console window. After that you will see some output like this:

ASSET STATISTICS
Asset cache contains        0 assets
Latest asset request time after cache miss: 0s
Blocked client requests for missing textures: 0
Asset service request failures: 0

CONNECTION STATISTICS
Abnormal client thread terminations: 0

INVENTORY STATISTICS
Initial inventory caching failures: 0

FRAME STATISTICS
Dilatn  SimFPS  PhyFPS  AgntUp  RootAg  ChldAg  Prims   AtvPrm  AtvScr  ScrLPS
  1.00      55    45.5     0.0       0       0    3516       0     419      14

PktsIn  PktOut  PendDl  PendUl  UnackB  TotlFt  NetFt   PhysFt  OthrFt  AgntFt  ImgsFt
    27      10       0       0       0     5.3     0.0     4.3     0.0     0.0     0.0

MEMORY STATISTICS
Allocated to OpenSim : 152 MB

Have a look at the value shown under the label PhysFt. This number is the so called Physics Frame Time. If that value is high, above 20 or even higher,  your physics engine is quite busy. Best run “show stats” multiple times and check if you always get more or less the same PhysFt values or if you see peaks of physical engine activity.

If you see such peaks, even if nobody is currently visiting your region (use “show users” command on the OpenSim console to check that), then you probably have moving objects causing physics engine lag. Otherwise it is possible that moving avatars cause such lag, for example by walking on prim surfaces or by colliding with other objects or avatars.

The PhyFPS value shows the Physics Engine Frame Rate. Values above 45 fps are good. Usually high PhysFt values cause low PhyFPS frame rate values.

You can also display the Physics Engine Frame Rate in the viewer. Select the menu item “View > Statistics Bar”. The Statistics windows opens and shows the Physics FPS value under the label Simulator. If you cannot see it, simply click on the label Simulator.

Reasons for physics engine lag

In the following situations the physics engine has to do calculations:

• Movements of physical objects
  • unscripted physical objects (i.e. a ball)
  • scripted physical objects (i.e. vehicles implemented as physical objects)
• Movements of avatars
  • avatar walking over terrain
  • avatar walking over a prim surface (normal prims or sculpties)
• Collisions between non phantom objects and avatars
  • avatars colliding with each other
  • non phantom objects colliding
  • avatar colliding with a non phantom object

Many physical movements require the detection of collisions. Collisions are detected based on so called Bounding Boxes, rectangular boxes using the X, Y and Z axis and that contain the whole object, avatar or segment of terrain.

You can toggle displaying these internal Bounding Boxes in the viewer using the following menu item in the Advanced menu (which can you open using ctrl alt D):  Advanced > Rendering > Info Displays > BBoxes

How to reduce physics engine lag?

To reduce physics engine lag you should do the following:

• Try to avoid physical objects (i.e. use vehicles that are not physical objects)
• Never create huge physical objects, because they easily cause many collision events
• Make objects phantom if possible (collision detection ignores phantom objects)
• Make moving/rotating objects phantom
• Use llTargetOmega and llSetTextureAnim for viewer side rotations/animations
• Do not use hollow prims with the intention to let other objects or avatars move inside
• Do not use too complex, rough sculpty prims and terrains as walking surface for avatars

The general rule is to try to avoid collisions of Bounding Boxes as much as possible to reduce physics engine load.

By the way, that is also the reason why avatar attachments are always phantom objects.

One of my renters could reduce Physics Engine Frame Rate values from about 50 with peaks up to 150 to much lower values of about 4. The main reason was a hollow prim with a scripted vehicle constantly moving inside. This did cause physics engine lag. But a bigger issue was the constantly growing memory consumption (memory leak) and thus more frequent region restarts than necessary.

Appendix

This is a short description of the more cryptic labels of the “show stats” console output:

• Dilatn time dilation
• SimFPS sim FPS
• PhyFPS physics FPS
• AgntUp # of agent updates
• RootAg # of root agents
• ChldAg # of child agents
• Prims # of total prims
• AtvPrm # of active prims
• AtvScr # of active scripts
• ScrLPS # of script lines per second
• PktsIn # of in packets per second
• PktOut # of out packets per second
• PendDl # of pending downloads
• PendUl # of pending uploads
• UnackB # of unacknowledged bytes
• TotlFt total frame time
• NetFt net frame time
• PhysFt physics frame time
• OthrFt other frame time
• AgntFt agent frame time
• ImgsFt image frame time

Snoopy Pfeffer
Founder and CEO of Dreamland Metaverse

snoopy.pfeffer@yahoo.com
http://www.3dmetaverse.com/

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