``And there are also some cases where we took a direction with the
underlying technology that turned out to be a mistake. An example is the
two-level thread scheduling model, where thread scheduling happens both
at user level and in the kernel. Although this approach had some
theoretical advantages in terms of thread creation and context switch
time, it turned out to be enormously complicated, particularly when
dealing with traditional Unix process semantics like signals. In Solaris
8, we made an "alternate" version of the threads library available that
relied solely on kernel-based scheduling; it turned out to be not only
much simpler and easier to maintain, but also faster in almost every
case. It particularly sped up Java code, which is obviously important to
us. In Solaris 9 (and later) we switched over to the single-level
library as the only one available.''

  --  http://www.osnews.com/story.php?news_id=3830