Recursive async method looses its context

Question

I am writing a raytracer in F# and I am trying to use multithreading in the montecarlo sampling step.

However, when I run my code with the async variant, the program never returns and runs indefinitely.

My Code Currently:

let rec rayTrace previousTraceDepth ((ray : Ray) , (accEmitted : Color) , (accScatter :Color)) =
if previousTraceDepth > maxTraceDepth 
then  
    accEmitted + backgroundColor*accScatter
else
    let newTraceDepth = previousTraceDepth + 1us
    let (realSolution,t,surface) = findClosestIntersection ray surfaces
    let surfaceGeometry : Hitable = surface.Geometry
    if surfaceGeometry.IntersectionAcceptable realSolution t 1.0f (PointForRay ray t)
    then
        let emittedShading = surface.Emitted
        let e = accEmitted + accScatter*emittedShading 
        let mcSamples = surface.SampleCount

        //Synchronous
        // let mutable totalShading = e/surface.MCNormalization
        // for _ in 1..mcSamples do
        //     let (doesRayContribute,outRay,cosOfIncidence) = surface.Scatter ray t ((int)newTraceDepth)
        //     let shading = surface.BRDF*cosOfIncidence / (surface.PDF*surface.MCNormalization)
        //     let s = accScatter*shading
        //     totalShading <- totalShading + (rayTrace newTraceDepth (outRay , e , s))
        // totalShading

        let eMCAdjusted = e / surface.MCNormalization
        let shadingSamplesAsync = 
           [|
               for _ in 1..mcSamples -> async {
                       let (doesRayContribute,outRay,cosOfIncidence) = surface.Scatter ray t ((int)newTraceDepth)
                       let shading = surface.BRDF*cosOfIncidence / (surface.PDF*surface.MCNormalization)
                       let s = accScatter*shading
                       return rayTrace newTraceDepth (outRay,e,s)
                   }|]

        if Array.isEmpty shadingSamplesAsync then 
            e
        else
            let shadingSamplesSync = shadingSamplesAsync |> Async.Parallel |> Async.RunSynchronously
            Array.sumBy (fun x -> eMCAdjusted + x) shadingSamplesSync

    else 
       accEmitted + backgroundColor*accScatter

How can I make this method async recursive?

Answer 1

I found the cause of the problem:

I was actually calling Async.RunSynchronously from the entry point method i.e rayTraceBase. Apparently nested Async.RunSynchronously is not a good thing. After I removed the second Async.RunSynchronously the code worked.
But it is painfully slow since async contexts are created all over the place and the GC runs non stop.