LSL Script Efficiency

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What is Efficiency

Efficiency is how much resource a particular script uses to accomplish a goal. This often goes hand-in-hand with speed of execution, but not always.

Things to watch out for, in rough order of importance:

  • Using a lot of scripts to accomplish something. See Efficient Design.
  • llSleep - This function uses up 0.2 to 0.3 ms per frame while the script sleeps, timers are much better. This is really an accounting error that should be fixed in SL Server 1.27 (see transcript at 17:44)
  • Short frequency timers (<5 sec)
  • Listen - especially to the 0 channel
  • Changing textures often (Others have to download them)
  • Inappropriate use of server/physical rotations instead of omega
  • Event handlers which are not needed (touch, collision, ...)
  • Large amount of email or IM
  • Inefficient algorithms (like linear search)
  • Busy (polling) loops

Efficient Design

Each idle script in an object adds 0.001 to 0.003 milliseconds per frame of script time usage. Fewer scripts is better.

The bottom line is, if you find yourself tempted to put a script in each prim, stop and think about how you could do it differently with fewer scripts. There is almost always an alternative. It is up to you to decide whether the alternative is a viable one for your application.


There are many ways to speed up scripts, such as using ++a instead of a++, however, most of these micro optimizations might not hold true in the future.

How Fast Does That Code Run

The following code will get the time for each function call in millis.

Please first propose changes to the discussion tab. Please do not change the code here without discussion, as any small change could reduce the accuracy of measurements. Please note this code and its comments should exactly match the code and comments presented by the Efficiency Tester page.

Thanks to Xaviar Czervik for the original code, thanks to Strife Onizuka for tuning the code to produce more accurate measurements, and thanks to the other contributors listed in the history of this article.

<lsl> //IMPORTANT: Only perform tests in an empty region. // To reduce contamination and be sure to wearing no attachments. // Preferably do tests in a private sim with one on it. // Don't move while performing the test. // There is a margin of error so run the tests multiple times to determine it.

integer time() { // count milliseconds since the day began

   string stamp = llGetTimestamp(); // "YYYY-MM-DDThh:mm:ss.ff..fZ"
   return (integer) llGetSubString(stamp, 11, 12) * 3600000 + // hh
          (integer) llGetSubString(stamp, 14, 15) * 60000 +  // mm
          llRound((float)llGetSubString(stamp, 17, -2) * 1000000.0)/1000; // ss.ff..f


default {

 state_entry() {
   llOwnerSay((string) llGetFreeMemory());
   //test variables
   float counter;
   //framework variables
   float i = 0;
   float j = 0;
   float max = 10000; // 2ms of work takes 20 seconds to repeat 10,000 times, plus overhead
   float t0 = time();
   do {
     counter += 1;
   }while (++i < max);
   float t1 = time();
   do ; while (++j < max);
   float t2 = time();//remove the time required by the framework
   float elapsed = ((t1 - t0) - (t2 - t1))/max;
   llOwnerSay("The function in the loop took a total of " + (string)elapsed + " milliseconds.");

} </lsl>


The following data was collected using the above function in a recently deployed empty (though not private) sim, with only that one script running in a HUD attachment. 20 tests of each were run and averaged.

  ++a:     0.173780 ms    std. dev.:  0.003393 ms
  a += 1:  0.181720 ms    std. dev.:  0.013267 ms
  a++:     0.243500 ms    std. dev.:  0.013816 ms

Thus a++ takes 40% longer to execute than ++a (rough estimate).

While the following data is correct, the function above was not used. A slightly less optimized version written by Xaviar Czervik was used. If someone could please repeat the tests with the function above.

  ++a:     0.364700 millis
  a += 1:  0.346900 millis
  a++:     0.413700 millis

Testing the same function in for loops:

  ++a:     0.358370 millis
  a += 1:  0.351200 millis
  a++:     0.424600 millis

llOwnerSay v. llSay v. llShout v. llWhisper (Channel 0 where applies):

  llOwnerSay(): 4.359000 millis
  llWhisper():  5.201000 millis
  llSay():      5.226000 millis
  llShout():   14.877000 millis

llOwnerSay v. llSay v. llShout v. llWhisper (Re-tested under Mono as of 04/02/09. Channel 1 for regionsay, listener set up nearby):

  llOwnerSay():   3.607200 millis
  llWhisper():    4.871200 millis
  llSay():        3.975200 millis
  llShout():     10.931200 millis
  llRegionSay():  0.312000 millis

Different Channels (llSay() Used for all):

  -100000000: 1.226400 millis
  -100000:    1.254300 millis
  -100:       1.296100 millis
  -1:         1.292400 millis
  0:          5.226000 millis
  1:          1.242300 millis
  100:        1.249100 millis
  100000:     1.219700 millis
  100000000:  1.228700 millis

Amount of text (llSay() and Channel 1 used for all):

   1 Character:    1.242300 millis
   10 Characters:  1.309700 millis
   100 Characters: 1.965600 millis