[Manuel Calavera]

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Kitguru:

The only thing currently known about AMD’s Zen is that it will drop clustered multi-thread (CMT) design in favour of more traditional simultaneous multi-threaded (SMT) design. This may result in decrease of the amount of cores inside AMD’s processors, but will increase their efficiency.

But if the initial rumors are anything to go by then Zen will likely adopt an SMT style microarchitecture which indicates that it’s quite a large core since SMT (Simultaneous multithreading) is usually leveraged in large CPU cores to opportunistically take advantage of the various resources in the core and dedicate it to an additional execution thread for added throughput.
This adds to the area efficiency of the core design and reduces the effect of stalls and pipeline back pressure leading to improved resource utilization inside the core which in turn improves overall performance.

Read more: http://wccftech.com/breaking-amds-ge...#ixzz3X8Cx6DpV

SMT is not a new technology, but it is an approach AMD has been reluctant to use, as opposed to the Cluster Multithreading from Bulldozer (CMT for short).
In CMT, a unified front end sends instructions to two separate integer pipelines, so that the benefits of having two full integer pipelines would be gained while also saving die space and power draw.
SMT, or Hyper-Threading as Intel calls it, can schedule and execute two different instructions in the same clock style. CMT should have given AMD Bulldozer an advantage, but the chip's low efficiency killed that possibility.
Using SMT in Zen (though perhaps not like in Hyper-Threading) could be a saving grace if the underlying CPU architecture is OK with a die size increase. 15-20% performance boosts can be achieved, as Intel has revealed repeatedly.

Opt for Simultaneous multithreading as opposed to Cluster Multithreading: With Bulldozer, AMD opted for an arrangement called cluster multi-threading, or CMT. This is the strategy used by Bulldozer, in which a unified front end issues instructions to two separate integer pipelines. The idea behind the Bulldozer design was that AMD would gain the benefits of having two full integer pipelines but save die space and power consumption compared to building a conventional multi-core design.

Intel, in contrast, has long used simultaneous multithreading (SMT), which they call Hyper-Threading, in which two instructions from different threads can be executed in the same clock cycle. In theory, AMD’s design could have given it an advantage, since each core contains a full set of execution units as opposed to SMT, where those resources are shared, but in practice Bulldozer’s low efficiency crippled its scaling.
The rumor now is that AMD will include an SMT-style design with Zen. It’s entirely possible that the company will do this — Hyper-Threading is one example of SMT, but it’s not the only implementation — IBM, for example, uses SMT extensively in its POWER architectures. The reason I’m not willing to completely sign off on this rumor is that it’s a rumor that’s dogged AMD literally since Intel introduced Hyper-Threading 15 years ago.

The benefits of using SMT are always dependent on the underlying CPU architecture, but Intel has demonstrated that the technology is often good for a 15-20% performance increase in exchange for a minimal die penalty. If AMD can achieve similar results, the net effect will be quite positive.