User:Becky Pippen

About

I love helping new residents get addicted... er, I mean acclimated to Second Life. Here are some notes I've picked up along the way that might be of help.

SL Glossary

Second Life's most complete glossary

What is Mono?

John and Mary

John and Mary are LSL scripters. John prefers the old-fashioned way of doing things, while Mary enjoys all the latest new technology, like Mono. They both write LSL scripts. John runs his scripts under the old way, and Mary runs hers using Mono.

When John writes an LSL script and clicks the Save button, the client viewer compiles the script into proprietary bytecode and uploads it to the servers. The server runs the script by using a proprietary interpreter to interpret the bytecode. It runs slow.

Mary writes the identical script -- no change in the LSL language syntax. When she presses "Save", the client uploads her script text to the servers where it gets compiled into standardized CIL assembly language. (CIL is a bytecode that originally came from Microsoft's .NET technology.) Because the Linden servers are Linux machines, there's no .NET framework available to run the CIL code. So the servers use the open-source Mono framework to execute the CIL. Mono includes an open-source JIT ("Just in Time") compiler that converts the CIL bytecode into machine code as it's needed, and that compiled machine code is what makes the script execute faster than the old way.

How it all Flows

Old way:

LSL  ---> LSL compiler ---> proprietary bytecode ---> LSL bytecode interpreter

New way:

LSL                        ---> LSL compiler   | 
C# (future)                ---> C# compiler    |  ---> CIL bytecode ---> Mono CIL execution engine
Other languages (future)   ---> other compiler |

Do Scripts Running in Mono Get More Memory?

Yes, it's true that each Mono script gets 64K of memory to use. Non-Mono scripts have to fit inside 16K of memory. (That's compiled code plus stack plus heap.) Unfortunately, Mono-compiled code is a bit of a memory hog and can take up to 4 times as much memory as the old runtime environment. So, scripts using Mono end up with a little more memory to play with depending on the code-to-data ratio, but not four times as much. The advantage to the servers is that small Mono scripts only consume as much server memory as they actually use, while the older LSL virtual machine had to allocate a chunk of 16K of memory to every script regardless of how little memory it actually needed.

Evolving Terminology

LSL - Used to refer to the whole system -- the language, the front-end compiler, and the back-end interpreter. Now it's confusing because it's used in one of two ways and its meaning depends on the context: (1) LSL is the language, the source code, which has not changed with Mono. E.g., "Take this LSL script and compile it with Mono and it will run faster." Or (2) LSL refers to the old compiler front-end and interpreter virtual machine back-end. E.g., "When I compile this script with LSL it runs very slowly." Both meanings are used in the sentence, "This LSL script was compiled with LSL, but that LSL script was compiled with Mono."

The term LSO refers to the bytecode generated by the traditional LSL compiler and executed by the old LSL runtime, and is a term that can be used to distinguish the old LSL tool set from the Mono tool set.

Can We Use Other Languages?

Mono just runs CIL bytecode. It doesn't know or care what the original language was. At this time (mid-2008) only LSL source code can be compiled into CIL and run on Mono. But maybe someday we will be able to compile C# and possibly other languages into CIL which will run under Mono. A single object could contain scripts that originally were written in different languages.

The LSL language -- the source syntax -- will probably be supported for a very long time.

However, the servers will support both back-ends for a very long time -- the one that executes the old proprietary LSL bytecode, and the new Mono virtual machine that executes standard CIL. All the millions of existing LSL scripts compiled the old way will continue to run indefinitely. New LSL scripts may be compiled using the old LSL compiler or the new Mono compiler.

What To Expect After Mono Hits the Main Grid

• Old LSL scripts already compiled: will continue to run indefinitely. You don't have to do a thing.

• Any LSL script if you have the source code: can be recompiled under the old LSL compiler or the new Mono compiler, at your choice, for a long time to come.

• C# and other languages: probably added at some future time.

LSL Timings and Rates

Here are some LSL function times and event rates comparing scripts compiled and run using the old LSL toolset and using Mono.

The basic test framework for these tests is to set i to a large value, and take the difference in llGetTime() before and after the loop shown below (the loop is partially unrolled to make the loop control structure overhead negligible), and take several measurements in multiple Class 5 sims that are running consistently with a total frame time under 10 ms, where each test runs for 20 - 90 minutes, and measured several times until the numbers converge with a variance of 10% or less:

while (--i >= 0) {
    f(); f(); f(); f(); f(); f(); f(); f();
    f(); f(); f(); f(); f(); f(); f(); f();
}

In a Mono sim, the result of the first run after script reset is ignored to eliminate the overhead of the VM initialization, JIT compilation, etc.

Note that how fast you can call a function doesn't necessarily correspond to how much a function will lag a sim because of all the throttling and scheduling going on inside the run-time engine.

Functions, operators, & control structures

call to empty f() { }	LSL	Mono	Units
	0.33	0.0026	ms/call

for loop overhead for (i = 0; i == 0; ++i) { }	LSL	Mono	Units
	0.07	0.001	ms/loop

This test measures the overhead of a for loop that iterates an empty block just once, including its setup. The code generated for this should be something like one integer assignment, one increment, and one test-and-branch. Or as one developer said, "if it's not, it should be."

state change	LSL	Mono	Units
	0.14	22.2	ms/transition

State changes in Mono are tied to the sim frame rate of 45 fps (per Vektor Linden). The test code for state changes is an outer loop around 16 state changes like this:

. . .
state stateN   { state_entry() { state stateN+1; } }
state stateN+1 { state_entry() { state stateN+2; } }
. . . etc.

v = <PI,PI,PI> + <PI,PI,PI> * PI;	LSL	Mono	Units
	0.43	0.025	ms/statement

call to llCSV2List(llList2CSV(x)); where x is a list of 10 integers	LSL	Mono	Units
	3.2	4 - 7	ms/call

So far, the Mono results vary too much for any meaningful result.

call to llGetInventoryName(INVENTORY_TEXTURE, 0);	LSL	Mono	Units
	0.52	0.21	ms/call

call to llGetPos();	LSL	Mono	Units
	0.44	0.045	ms/call

call to llList2Vector(x, 0); where x = [ ZERO_VECTOR ]	LSL	Mono	Units
	0.55	0.014	ms/call

call to llMD5String(s,0); where s is a 64-char string	LSL	Mono	Units
	1.4	1.4	ms/call

call to llMessageLinked(LINK_THIS, 0, "", NULL_KEY); One consumer script with empty link_message() { }	LSL	Mono	Units
	1.0	0.90	ms/call

The time it takes to call llMessageLinked() is dependent on the number of consumers of the message, up to about 20-30 consumer scripts. At that point, the time to send a link message becomes constant at one call per 11 or 22.2 ms (yes, the results are bimodal, depending on unknown factors), which is suspiciously similar to 1X or 2X the sim's basic frame rate. See the next test:

call to llMessageLinked(LINK_THIS, 0, "", NULL_KEY); 32 - 64 consumer scripts with empty link_message() { }	LSL	Mono	Units
	11 or 22.2	11 or 22.2	ms/call

call to llParseString2List(s,a,b) where s is 2000 random digits, a=["0","1"]; b=["2","3"];	LSL	Mono	Units
	45	24	ms/call

call to llSay(-2, "0123456789");	LSL	Mono	Units
	0.66	0.3 - 0.8	ms/call

The Mono results vary too much for meaningful results.

call to llSetLinkAlpha();	LSL	Mono	Units
	1.0	0.52	ms/call

call to llSetText(s,c,a) where string s is 20 chars	LSL	Mono	Units
	0.79	0.38	ms/call

call to llSubStringIndex(s, "X"); where s is 10000 digits not containing "X"	LSL	Mono	Units
	16	22	ms/call

Event handlers

Maximum rate of dataserver() events after llGetNotecardLine()	LSL	Mono	Units
	6.4	6.4	events/sec

Maximum rate of listen() events 1 listener per script, any number of scripts per prim, any number of prims in linkset	LSL	Mono	Units
	14.7	14.7	events/sec

Maximum rate of listen() events 2 listeners per script, any number of scripts per prim, any number of prims in linkset	LSL	Mono	Units
	11	11	events/sec

Maximum rate of listen() events 3 listeners per script, any number of scripts per prim, any number of prims in linkset	LSL	Mono	Units
	8.8	8.9	events/sec

Maximum rate of sensor() events after llSensorRepeat()	LSL	Mono	Units
	12	22	events/sec

Maximum rate of timer() events Maximum rate of touch() events	LSL	Mono	Units
	22	22	events/sec

About half the sim's basic frame rate of 45 fps.

Links

• The Community of the Nevia Archipelago Website
• JIRA Issue Tracker
• Architecture Working Group
• Mono wiki