Here's my lab experience with UnixWare from Novell:

Making the kernel preemptable and the UFS (SVR4's FFS) reentrant sped up UnixWare 2.0 SMP over Unixware 1.0 UP ~150%. That's measured, in a lab.

UnixWare uses a method called DOW or Delayed Ordered Writes (the ReiserFS is in violation of this patent, if SCO wanted to push it); it is inferior to Soft Updates. It introduces a synchronization point, rather than an update, when a modification would require an update occur and a dependency entered into the dependency tree.

In addition, UnixWare did (does) not have a unified VM and buffer cache. The per vnode working set quota code I had proposed to resolve the "move mouse, wiggle cursor" problem with page stealing by a single hungry process was not adopted, and a fixed scheduling class was adopted instead (giving X the cycles to page itself back in -- stealing the pages back -- in the process), so the results were poorer than they would have otherwise been.

Combined, these mean that this lab test was biased against the speed improvement. On a reasonable non-benchmark workload where there were no stalling points being introduced by all processes whacking the same directory, and where no single process was allowed to force another out of core via page steal DOS attacks, and where the buffer cache and VM system didn't result in extra copies and "useless" reserve memory for one system or another, I'd expect this to increase to 250% or more (I was able to get ~190% overall by introducing my working set quota changes on a per process, rather than a per vnode, basis for experimental purposes).

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