Red Hat's Matt Wilson has posted an explanation on the Phoebe mailing list:

In the past, this was indeed the case. Red Hat Linux 9's incorporation of NPTL does mean that certain applications that function on older versions of Red Hat Linux (like 8.0) will not work without intervention on Red Hat Linux 9. For example, some Java JVMs do not work properly because they make certain assumptions about the thread model that are no longer true. Most of these applications can still be used by specifying that you wish the older thread libraries to be used through LD_ASSUME_KERNEL=2.4.1 and LD_ASSUME_KERNEL=2.2.5.

But there's something a bit more fundamental that I want people to be aware of. In the past we would never have tackled something as massive and invasive as a new threads implementation just after a ".0" release (in this case, 8.0). We were able to do this, and bring this great new technology to a mass audience, because we've changed the way we consider technology to incorporate in Red Hat Linux. In the past we would have felt it necessary to wait a while for a ".0" release because we had to support a series of releases for years.

With the introduction of the full family of Red Hat Enterprise Linux product we now have the flexibility to incorporate the best technology that both the Open Source communities and Red Hat have to offer when they're ready, instead of having to hold back.

One example of this sort of thing that caused a lot of negative feedback in the past was the delayed incorporation of Python 2.0 in the Red Hat Linux 7.x series. In the new model we would be able to get the new releases of major subsystems like Python in the distribution as soon as they have been proven stable.

I hope this sheds a little light on "why 9 and not 8.1".

Cheers,

Matt
msw@redhat.com
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Matt Wilson
Manager, Base Operating Systems
Red Hat, Inc.