How can I implement coroutines for a parallel task - kotlin

Question

So, I have this piece of code:
for (z in 0 until texture.extent.z) {
println(z)
for (y in 0 until texture.extent.y)
for (x in 0 until texture.extent.x) {
val v = Vec3(x, y, z) / texture.extent
var n = when {
FRACTAL -> FractalNoise().noise(v * noiseScale)
else -> 20f * glm.perlin(v)
}
n -= glm.floor(n)
data[x + y * texture.extent.x + z * texture.extent.x * texture.extent.y] = glm.floor(n * 255).b
}
}
That takes over 4m on the jvm. The original sample in cpp uses OpenMp to accelerate the calculation.
I've heard about coroutines and I hope I could take advantage of them in this case.
I tried first to wrap the whole fors into a runBlocking because I do want that all the coroutines have finished before I move on.
runBlocking {
for (z in 0 until texture.extent.z) {
println(z)
for (y in 0 until texture.extent.y)
for (x in 0 until texture.extent.x) {
launch {
val v = Vec3(x, y, z) / texture.extent
var n = when {
FRACTAL -> FractalNoise().noise(v * noiseScale)
else -> 20f * glm.perlin(v)
}
n -= glm.floor(n)
data[x + y * texture.extent.x + z * texture.extent.x * texture.extent.y] = glm.floor(n * 255).b
}
}
}
}
But this is throwing different thread errors plus a final jvm crash
[thread 27624 also had an error][thread 23784 also had an error]# A fatal error has been detected by the Java Runtime Environment:
#
[thread 27624 also had an error][thread 23784 also had an error]# A fatal error has been detected by the Java Runtime Environment:
#
# [thread 14004 also had an error]EXCEPTION_ACCESS_VIOLATION
[thread 32652 also had an error] (0xc0000005)[thread 32616 also had an error]
at pc=0x0000000002d2fd50
, pid=23452[thread 21264 also had an error], tid=0x0000000000007b68
#
# JRE version: Java(TM) SE Runtime Environment (8.0_144-b01) (build 1.8.0_144-b01)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (25.144-b01 mixed mode windows-amd64 compressed oops)
# Problematic frame:
# J 1431 C2 java.util.concurrent.ForkJoinPool$WorkQueue.runTask(Ljava/util/concurrent/ForkJoinTask;)V (86 bytes) # 0x0000000002d2fd50 [0x0000000002d2f100+0xc50]
#
# Failed to write core dump. Minidumps are not enabled by default on client versions of Windows
#
# An error report file with more information is saved as:
# C:\Users\gBarbieri\IdeaProjects\Vulkan\hs_err_pid23452.log
#
# If you would like to submit a bug report, please visit:
# http://bugreport.java.com/bugreport/crash.jsp
#
Process finished with exit code 1
I tried also to collect all the jobs into an arrayList and join() them at the end, but without success..
May coroutine be used for a parallel task like this one?
If yes, what am I doing wrong?

Answer 1

Instead of coroutines you should consider the parallel computation engine built into the JDK: java.util.stream. What you have here is an embarrassingly parallelizable task, a perfect use case for it.
I'd use something along these lines:
IntStream.range(0, extent.x)
.boxed()
.parallel()
.flatMap { x ->
IntStream.range(0, extent.y).boxed().flatMap { y ->
IntStream.range(0, extent.z).mapToObj { z ->
Vec(x, y, z)
}
}
}
.forEach { vec ->
data[vecToArrayIndex(vec)] = computeValue(vec)
}