alt Hacker NewsJava already had M:N threads in the early days, aka green threads, because the JVM and Java specifications did not assert what kind of threading was to be provided.
Thus most JVM implementations had a mix of red (1:1:) and green (M:N) threads, eventually only red threads were kept in the surviving implementations.
With Project Loom now both models are officially supported and part of the specification.
Yeah I've heard of the Java 1-2 greenthreads, but supposedly those were M:1, ie you could only run them on one core. At least this SCO release note linked from Wikipedia says that: https://www.sco.com/developers/java/j2sdk122-001/ReleaseNote... And the pros are more about compatibility than performance