Why isn't SMT Xeon scaling?

Intel has extended its SMT, "HyperThreaded" Xeon processors to large servers. But the processors don't give the dramatic performance benefits that they should, given the higher frequencies, better bandwidth and virtual processor technology.

The chips use ServerWorks GC-HE custom chipset and come in 1.4Ghz, 1.5Ghz and 1.6Ghz frequencies, with L3 cache up to 1MB.

As with the earlier SMT Xeon for workstations, the chips are built to a .18 micron process, use DDR memory and the P4's 400Mhz bus. There's a significant bandwidth improvement over the PIII, too.

But does this add up to a linear scaling? Not quite.

Full story at The Register USA