So, how to use MT in AviSynth+? Most of it has been posted earlier actually, but let me summarize it.
By default, your script will run in single-threaded mode, just like with SEt's build. Also, just like in SEt's build, you'll have to make sure that filters use the correct MT mode, or else they might wreak havoc. There are three MT modes (1,2,3), and they are the same modes as in (yeah you guessed correctly) SEt's build. Which means you can use the same modes that you have used with AviSynth-MT.
There are some things though that are different and/or new in AviSynth+. The first difference is *how* you set the MT mode. In AviSynth-MT, you had to use SetMTMode(X), which caused all filters following that line to use mode X (until the next call to SetMTMode()). This meant if you needed to use multiple MT modes, you had to insert all those calls in the middle of your script, littered over many places.
Setting MT modes
AviSynth+ does it differently. In AviSynth+, you specify the MT-mode for only specific filters, and those filters will then automatically use their own mode, even if there were other MT-modes inbetween. This means you can specify all the MT modes at the beginning without polluting your script. You can even make a SetMTMode.avsi if you wish and let it autoload for all of your scripts, or import() it from their top. This is much cleaner, and it allows you to maintain all your MT-modes centrally at a single place. To make this distinction clear from AviSynth+, SetMTMode() is called SetFilterMTMode() in AviSynth+.
Enabling MT
The other difference is how you actually enable multithreading. Calling SetFilterMTMode() is not enough, it sets the MT mode, but the MT mode only has an effect if MT is enabled at all. Note this means you can safely include/import/autoload your SetFilterMTMode() calls in even single-threaded scripts, and they will not be messed up. Uhm, onto the point: You enable MT by placing a single call to Prefetch(X) at the *end* of your script, where X is the number of threads to use.
Example
Code:
# This line causes all filters that don't have an MT mode explicitly use mode 2 by default.
# Mode 2 is a relatively safe choice until you don't know most of your calls to be either mode 1 or 3.
# Compared with mode 1, mode 2 trades memory for MT-safety, but only a select few filters will work with mode 1.
SetFilterMTMode("DEFAULT_MT_MODE", 2)
# FFVideoSource(), like most source filters, needs MT mode 3
SetFilterMTMode("FFVideoSource", 3)
# Now comes your script as usual
FFVideoSource(...)
trim(...)
MCTemporalDenoise(...)
...
# Enable MT!
Prefetch(4)
Closing notes (don't skip!)- Remember that MT is only stable as long as you have specified a correct MT mode for all filters.
- Instead of the numbers 1-2-3, you can also use symbolic names for MT modes: MT_NICE_FILTER (1), MT_MULTI_INSTANCE (2), MT_SERIALIZED (3)
- Mode 3 is evil. It is necessary for some filters, and it is usually no problem for source filters, but it can literally completely negate all advantages of MT, if such a filter is placed near the end of your script. Let us know if you meet a non-source mode 3 filter, we might be able to do something about it, but in general, avoid such calls if you want performance. (And of course, insert what you have found into here.)
- The new caches will save you a lot of memory in single-threaded scripts, but due to the way they work, they will also use more memory than before with MT enabled. The memory usage will scale much closer with the number of threads you have. Just something to keep in mind.
- MT-enabled AviSynth+ triggers a latent bug in AvsPmod. Until a new version of AvsPmod is officially released, use this build. A thousand thanks to vdcrim for the fix.
- Using too many threads can easily hurt performance a lot, because there are other bottlenecks too in your PC than just the CPU. For example, if you have a quad-core machine with 8 logical cores, less than 8 threads will often work much better than 8 or more.