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Scalability of JVM processes is limited by GC rate.

For contemporary CPUs and GCs that provide the best throughput the allocation limit is ~11Gb/sec.

After reaching the allocation limit the JVM process doesn't scale.

Having 7.5Gb/sec in 1 thread the limit will be reached at 2-4 threads and adding more threads only decrease the total troughput.

Steps to reproduce that by running JSON parsing benchmarks on your JDK:

1. Install latest version of sbt and/or ensure that it already installed properly:

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andriy@notebook:~$ /opt/scala-2.13.0-RC1/bin/scala
Welcome to Scala 2.13.0-RC1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_212).
Type in expressions for evaluation. Or try :help.

scala> def timed[A](f: => A): A = { val t = System.currentTimeMillis; val r = f; println(s"Elapsed time (ms): ${System.currentTimeMillis - t}"); r }
timed: [A](f: => A)A

scala> timed(BigDecimal("1e-1000000000") + 1)
Elapsed time (ms): 1
res0: scala.math.BigDecimal = 1.000000000000000000000000000000000

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