Custom scheduler: Using loom for deterministic simulation testing

[Alan Bateman]

On 05/07/2025 21:25, Ryan Yeats wrote:
> :
>
>  1. Its not easy to replace the virtual thread scheduler
>  2. The aforementioned scheduler has no understanding of delays so any
>     Thread.sleep or Object.wait introduces continuations back into the
>     scheduled execution pool non-deterministically. This affects the
>     execution order so bugs can't always be reproduced.
>  3. IO also introduces continuations back to the scheduler execution
>     pool non-deterministically.
>
> The first issue can currently be solved by using reflection to make 
> the virtual thread constructor public 
> <https://github.com/ryeats/loom-dst/blob/main/src/main/java/org/example/SchedulableVirtualThreadFactory.java>. The 
> second issue can currently be solved by controlling system time by 
> replacing the byte code for all calls to System.nanoTime(), 
> System.onCurrentTimeMillis() and Instant.now() using an agent at 
> runtime 
> <https://github.com/cmu-pasta/fray/blob/main/instrumentation/base/src/main/kotlin/org/pastalab/fray/instrumentation/base/visitors/TimeInstrumenter.kt>. 
> The third likely has no general solution but I am interested in 
> hearing ideas.
>
There are experimental changes in the loom repo to allow the virtual 
thread scheduler be replaced or wrapped. There is also a prototype API 
to set a scheduler when creating a virtual Thread. There is a doc in the 
repo with more on this [1]. This project hasn't decided whether to 
expose anything so any reports from experiments would be useful to hear 
about.

-Alan

[1] 
https://github.com/openjdk/loom/blob/fibers/loom-docs/CustomSchedulers.md
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