This would only make things more complicated since then the primitives would have to start parallel native threads working on the same object memory. The problem with native threads is that the current object memory is not designed to work with multiple independent mutator threads. There are GC algorithms which work with parallel threads, but AFAIK they all have quite some overhead relative to the single-thread situation.
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Dear Hans-Martin,
Thanks for your clear explanation. It is really instructive.
Regards, Alexandre