Avoid opening a new thread for TruffleCompiler

[nsoubelet]

Hello team,
I recently noticed that when evaluating a JavaScript source, Truffle enqueues the operation and opens a new thread called TruffleCompilerThread, something I presume happens here.

Since on my app's side the evaluation is done already in a separate Thread, I would like to know whether it is possible to tell Truffle to avoid opening a new one but use the current instead.
First look suggests it is not possible since it must be always enqueued, but probably I'm missing something.

Thanks!

[chumer]

Hi!

Yes, you can perform the compilation on the main thread by setting the engine.BackgroundCompilation option to false. However, we do not recommend doing this, as it can cause latency spikes in the evaluation of code (up to seconds). I have yet to encounter a use case where I would recommend turning off background compilation.

The compilation is expected to be performed only once per polyglot Source, so if you are running in a long-running server process, I would recommend keeping background compilation on, but at the same time making sure code caching across contexts is enabled, so scripts only need to be compiled once per server process. See here for instructions:
https://www.graalvm.org/latest/reference-manual/embed-languages/#code-caching-across-multiple-contexts

If your scripts are short-lived and very simple, it can make sense to turn off compilation entirely by setting the engine.Compilation option to false. With this option set to false Truffle will never create a compiler thread.

If the compiler uses too much CPU resources, you can, for example, reduce the number of compiler threads by setting engine.CompilerThreads to 1. By default, it selects a number depending on the number of available CPU cores.

More info on setting options: https://www.graalvm.org/latest/reference-manual/polyglot-programming/#polyglot-options
A full list of available options can be found here: https://github.com/oracle/graal/blob/master/truffle/docs/Options.md

Hope this helps.

[nsoubelet]

Hello @chumer,
Thank you for the detailed answer. That helps for sure.

Unfortunately, my scripts are short-lived but can be complex (using resources-needed+lines-of-code as a way of measuring complexity), a rare combination. So, yes, disabling engine.BackgroundCompilation can be potentially problematic.
I will opt then for caching Sources as the number is deterministic and, if I find something interesting, will share some benchmark with you.

[nsoubelet]

Hello @chumer, bothering you with this subject again.

I finally implemented a caching mechanism for Source but I still see several TruffleCompilerThread being created.
After some investigation, and due to the fact that I implemented a custom FileSystem, I noticed that when the Source is evaluated, all the imports are compiled "again".

Hence my question: is it possible to create a Source with the full tree, that is, including its dependencies, so we avoid getting them from the FileSystem and, more important, recompiling them each time?
Note: Source.Builder has cache flag enabled.

Thanks!