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Summary

This is a fully functional alternative to XEngine for LSL/OSSL scripts, featuring

• Preemptive Multitasking
• Improved script syntax
• Expressions evaluation order closer to original LSL specification
• Memory heap and stack use control per script
• Optional new language extensions

YEngine does direct translation from script language to .net IL code, this makes it compile faster than XEngine. XEngine translates script language to C#, and then uses .net compiler to generate IL, this is naturally slower. While XEngine could compile scripts in other languages like C#, YEngine only supports LSL. Due to security issues, those other languages could not be actually used, except on very special and restricted cases.

Once a script code is loaded on XEngine, the memory used by it can not be recovered whne the script is deleted, unless a .net feature called AppDomains are used
But AppDomains are very heavy, use a lot more memory and worse, have a huge negative impact on scripts performance
On calls crossing domains ( ie the script and main framework) all parameters and return values are serialized and deserialized by sender and receiver domains.

Yengine loads scripts code in a diferent way, most their memory is recovered on delete without the need for those AppDomains.

Preemptive Multitasking

YEngine executes script events using preemptive multitasking.
This means that, at certain control points or when told, a script may release execution, being placed on a queue waiting for its turn to execute again.
The released execution thread goes on processing other scripts.

When a new event started execution, it will be allowed to run for about 60ms (subject to change) until releasing execution. When it is is turn to execute again it will be allowed to run for another 30ms(subject to change). This will be repeated until all the event code is executed
Engine will give more priority to short fast events.

The script can also be placed in a sleep state until a sleep time lapses.
This, for example, solves one of XEngine worse problems: llSleep() and other internal script sleeps. On YEngine, this just releases execution and waits in a queue, until the sleep time expires. On XEngine an "expensive" execution thread was placed in Sleep mode, so not available to do anything.

Improved script syntax

YEngine follows more closely the LSL script syntax and execution order. statements like

if(oneKey)
   ...

should now work.

llSomething; // missing()
break; // but see new extensions

are now errors

Expressions evaluation order

Complex statements execution order is closer to LSL specification than XEngine execution order.

for example, with i = 1;

((i == 2) && (i++ == 1))

will evaluate to true as by LSL spec, while on XEngine it will evaluate to false, because it will do (i == 2) first

Even if not useful in this case, always use parentheses (...) to enforce the order you need (and do this in any language)!

New language extensions

If the second line of a script is (currently first line needs to be present, empty or with script engine selection)

yoptions;

The following Yengine specific language extensions are activated

• switch
• break
• continue
• try catch and finally
• constant
• short circuit AND and OR

Scripts using these features will not compile or run on XEngine.

Memory heap and stack use control

YEngine keeps control of the memory a script uses.
There are two types of memory:

• Stack holds function arguments and simple local variables.
  
• Heap holds global variables and complex variables like lists or strings, even if they are local to a method/event.
  

The maximum memory a script can use can be configured in the OpenSim.ini section [YEngine]

; maximum stack a script can use in KB
;ScriptStackSize = 2048
   
; maximum heap memory a script can use in KB
;ScriptHeapSize = 1024

You may need to increase these values

Activation

OpenSimulator default configuration selects XEngine. To change to YEngine you need to change OpenSim.ini:
[Startup] section:

DefaultScriptEngine= "YEngine"

[YEngine] section:

Enable = true

[XEngine] section:

Enable = false

Note: in theory, OpenSimulator could run several engines at the same time, but we should not do that with X and Y engines.

Configuration

Please see file OpenSimDefaults.ini, section [YEngine] for details.
As in all case, if you need to change something, copy respective lines to similar location on file OpenSim.ini, and change there

Origins

YEngine is a modified derivative of XMREngine.
XMREngine was developed by teams of DreamNation and Avination grids, based on early work by Meta7.
It is still in use by DreamNation.

http://wiki.dreamnation.net/index.php/XMREngine_Script_Engine

A lot of information about XMREngine no longer applies to YEngine. Some features may still work, but may be removed or changed.