When running an infinite loop or other computationally expensive function, you will often want to release processing time back to the region to avoid causing undue ‘lag’. For instance – in a sim with 4,000 colour changing scripts, each will be trying to pummel the CPU simultaneously to get their updates onccuring.
What ends up happening is each will run for a fairly long period of time until broken up by a processing expensive “thread switch” at which point the next gets a long run before going to the next and the next and the next, or alternatively – none will finish and only one will be running.
The following diagram explains this in action – script A runs until there is an expensive thread switch, at which point script B will commence operation. You can manually force a thread switch through the use of the “llSleep(0)” statement in LSL, however the cost of switching threads is still present.
Fig 1. Normal Threaded Scripting
Microthreading allows you to say “You can pause and give time to another script” – in a way that is very lightweight and easy for the runtime VM. You manually insert breakpoints into the script which define points between the ‘tasklets’, at these points the VM can switch between runtime contexts without any penalty. As a result you can have thousands of them without problem; when running time is divided between all the running threads evenly – broken up at the tasklet borders.
The following diagram shows the same as above, in a microthreading environment.
Fig 2. Microthreaded Runtime
C# supports a limited form of microthreading via something called the “yield” keyword – and a lot of people have recognised you can use it to build a state machine — the basis of microthreading. Where we apply this to MRM is with some additional keywords.
The basic C# microthreading can be implemented in MRM without any additional work – however there is a significant manual cost in typing the construction of a microthreaded function. I have added two new keywords to MRM scripts today, which map to the C# yield statements.
The first is “microthreaded” – this keyword is inserted between the public/private/protected/internal keywords at the beggining of the function, and the variable type.For example, “private microthreaded void MyFunction(…) {”. Microthreaded functions can only work on functions with a no return type (void) – and internally it works by replacing “microthreaded void” with “IEnumerable”.
The second important keyword is “relax” – this keyword is defined as a breakpoint – an area where the VM can switch into another microthread without a penalty. There is a very small cost in placing these statements into your code, so consider placing them liberally. Place them inside of any loops or other routines which have a total processing time in excess of 2 milliseconds.
The relax keyword can only be placed inside of functions marked microthreaded – and you cannot call microthreaded functions directly (as in “Function(params)”). You must place microthreaded functions into the host scheduler marked as ‘Host.Microthreads.Run(myfunc(params));’
Another important note is that microthreaded functions will run asynchronously to your code, so do not expect “Microthreads.Run(…)” to block, consider it more a ‘Microthreads.ScheduleToRun(…)’. If you take these penalties into consideration however, these Microthreads can provide an excellent way to optimize your scripting and write coroutines relatively easily and quickly.
By default, the MRM VM will run 1,000 microthread tasklets per frame (or optimally 45,000 per second), this can be tweaked from inside of MRMModule.cs, however will eventually move into the OpenSim.ini [MRM] section.
The following is an example script that uses MRM microthreads.
Enjoy.
