The script execution model/Performance considerations
This section presents some performance-related issues that originate from the way AviSynth scripts are executed; it also provides advice on how to optimise your scripts and AviSynth configuration so that your scripts are parsed and/or encoded faster.
 Plugin auto-loading
An important thing to note is that auto-loading, although a convenient method to have all your favorite filters on hand, does incur a speed penalty. The penalty is twofold:
- The loading, registering and unloading of plugins takes some time. The parsing of .avsi scripts also takes some time. This time, although small, is paid in every AviSynth script invocation.
- The registering of many functions and globals increases the size of internal AviSynth data structures, which in turn increases the seek time to locate a filter / variable during the parsing phase as well as during runtime script parsing.
For small scripts and / or small number of auto-loading plugins the ease of use outweighs the above speed penalty (since there is also a speed penalty in writing a lot of LoadPlugin calls in every script that needs them). However if you regularly write large and complex scripts and have a large number of plugins / include scripts in your AviSynth plugin folder, you should consider a more granular approach to increase overall script parsing / encoding performance.
For example, you could group LoadPlugin calls for related plugins in separate .avsi scripts and have a central .avsi script with a config function that loads different .avsi scripts depending on its arguments. Then place in the plugin folder only the central .avsi script and the bare-essential plugins that you use almost every time.
 Frame caching and the effect on splitting filter graph's paths
In order to improve performance AviSynth places, transparently to script writers, a specialised Cache filter just after each filter. The purpose of the cache is to avoid the computationally expensive generation of a video frame that has recently been created; if the frame is in the cache then it is returned immediately, avoiding a possibly long chain of filter calls.
The presence of the cache gives a speed and memory advantage to filter graphs that split processing paths as late as possible. In our filter graph example above, if instead of:
ov = AviSource("clip2.avi") ov1 = Lanczos4Resize(ov, 280, 210) ov2 = ov1.Invert()
we had used the following code:
ov = AviSource("clip2.avi") ov2 = ov ov1 = Lanczos4Resize(ov, 280, 210) ov2 = ov2.Lanczos4Resize(280, 210).Invert()
then the respective part of the filter graph would have been:
... | AviSource(clip2) <--+-- Lanczos4Resize <--+-- Overlay <--+ | | +-- Lanczos4Resize <-- Invert <------+-- Overlay (filter graph's root)
In the latter case we would have one more filter (and cache) in the filter chain and -more importantly- we would have to generate two resized frames for each call by the host application to get a frame instead of one.
Therefore, always try to split processing paths as late as possible; it will make your scripts faster.
 What not to include in runtime scripts
Although, as said above, runtime scripts are parsed as regular scripts and thus every statement allowed to a regular script is allowed in a runtime script, some statements are not advisable from a performance point of view.
The principal reason is that runtime script parsing occurs in every frame requested. Therefore, as a rule of thumb, computationally expensive actions should in general be placed outside the runtime environment (in the main script) in order to be executed only once. This practice trades some start-up overhead with savings during frame serving, which in general dominates the overall clip rendering / encoding time; thus it is justified as an optimisation. This is of course to be taken with a grain of salt because there are circumstance where the application needs force the (balanced) use of such statements.
Having said all that, let's see our not-to-do-in-runtime-scripts list (and some interesting counter-examples):
- The following actions should most of the time be avoided:
- Importing a script.
- Loading a plugin.
- Defining a user function.
- Issuing them on every frame will slow down (maybe significantly) encoding speed and (subject to implementation details) eat valuable memory. Moreover, this overhead will be borne without returning significant gains. It is in general much better to place them in the main script.
- See however an example of acceptable use: Subtitles from a changing text-file.
- Calling a lot of filters / functions inside the runtime script will slow down your encoding speed. Those filters will be created and destroyed on every frame; thus you pay initialisation/cleanup costs at every frame.
- If you can, put outside the runtime environment those filter calls not essential for the runtime processing; break the runtime script into more scripts if you have to. For example, instead of doing this:
AviSource("myclip.avi") total_frames = Framecount() ScriptClip(""" Levels(0, 0.9, 255, 5, 250) total_frames % current_frame < 2 ? FlipHorizontal : last Tweak(hue=18) Subtitle("frame: " + String(current_frame), y=320) """)
AviSource("myclip.avi") total_frames = Framecount() Levels(0, 0.9, 255, 5, 250) ConditionalFilter(FlipHorizontal, last, "total_frames % current_frame", "<", "2") Tweak(hue=18) ScriptClip("""Subtitle("frame: " + String(current_frame), y=320)""")
- Arrays, due to their recursive, script-based implementation can be expensive to parse, especially if they host a large number of elements. Using them without paying attention to minimize operations will slow down your encoding speed.
- See however an example of acceptable use: Per frame filtering, exporting specific frame(s) (note that FrameFilter is a wrapper around ScriptClip).
Back to the script execution model.