Estimate number of cores/threads your system can scale to aka cpu count

estimate_cpu_count_scala.sh

#!/bin/sh
exec scala -Dscala.concurrent.context.minThreads=16 -Dscala.concurrent.context.maxThreads=64 "$0" "$@"
!#

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._
import java.util.concurrent.Executors
import scala.concurrent._


def time[R](block: => R): Long = {
    val t0 = System.nanoTime()
    val result = block
    val t1 = System.nanoTime()
    t1 - t0
}

def slowFunction = {
  var i = 0L
  while(i < Long.MaxValue / 1024 / 1024 / 1024) {
    i += 1
    //math.sqrt(i)
  }
}

val maxCpuCount = 64
val errorMargin = 1.1
val warmupTimes = 10
//implicit val ec = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(maxCpuCount))
//implicit val ec = ExecutionContext.fromExecutorService(Executors.newSingleThreadExecutor)  

def estimate_cpu_count(concurrencyLevel: Int = 1, firstTimingNanos: Long = 0): Int = {
  val elapsed = time(Await.result(Future.sequence(List.fill(concurrencyLevel)(Future{slowFunction})),Duration.Inf))
  val expected = (if(firstTimingNanos == 0) elapsed else firstTimingNanos)
  val maxExpected = (expected * errorMargin).toLong
  println(s"threads: $concurrencyLevel, (elapsed)$elapsed >? ${maxExpected}(max expected for this level)," +
          s" elapsed/max:  ${elapsed*100/maxExpected}%")
  if(elapsed > maxExpected)
    concurrencyLevel
  else
    estimate_cpu_count(concurrencyLevel + 1, expected)
}

def warmup() = (1 to warmupTimes).foreach(_ => slowFunction)

println(s"Warmup started, running $warmupTimes times before the actual test...")
warmup()
println("Warmup finished.")

println("Estimating CPU count...")
val estimatedCores = estimate_cpu_count()
val runtimeCores = Runtime.getRuntime.availableProcessors
println(s"Estimated cores: $estimatedCores, runtime cores: $runtimeCores")

another machine (both i7 but different gen):

$ ./estimate_cpu_count_scala.sh 
Warmup started, running 10 times before the actual test...
Warmup finished.
Estimating CPU count...
threads: 1, (elapsed)2605980421 >? 2866578463(max expected for this level), elapsed/max:  90%
threads: 2, (elapsed)2549883180 >? 2866578463(max expected for this level), elapsed/max:  88%
threads: 3, (elapsed)2623480059 >? 2866578463(max expected for this level), elapsed/max:  91%
threads: 4, (elapsed)4214595473 >? 2866578463(max expected for this level), elapsed/max:  147%
Estimated cores: 4, runtime cores: 8

hyperthreading is enabled, 1 CPU, 4 cores, 8 threads, i7 various generations