Clean Sheet and New Focus

AMD has taken the wraps off of architectural details of Zen.

It is no secret that AMD has been struggling for some time. The company has had success through the years, but it seems that the last decade has been somewhat bleak in terms of competitive advantages. The company has certainly made an impact in throughout the decades with their 486 products, K6, the original Athlon, and the industry changing Athlon 64. Since that time we have had a couple of bright spots with the Phenom II being far more competitive than expected, and the introduction of very solid graphics performance in their APUs.

Sadly for AMD their investment in the “Bulldozer” architecture was misplaced for where the industry was heading. While we certainly see far more software support for multi-threaded CPUs, IPC is still extremely important for most workloads. The original Bulldozer was somewhat rushed to market and was not fully optimized, while the “Piledriver” based Vishera products fixed many of these issues we have not seen the non-APU products updated to the latest Steamroller and Excavator architectures. The non-APU desktop market has been served for the past four years with 32nm PD-SOI based parts that utilize a rebranded chipset base that has not changed since 2010.

Four years ago AMD decided to change course entirely with their desktop and server CPUs. Instead of evolving the “Bulldozer” style architecture featuring CMT (Core Multi-Threading) they were going to do a clean sheet design that focused on efficiency, IPC, and scalability. While Bulldozer certainly could scale the thread count fairly effectively, the overall performance targets and clockspeeds needed to compete with Intel were just not feasible considering the challenges of process technology. AMD brought back Jim Keller to lead this effort, an industry veteran with a huge amount of experience across multiple architectures. Zen was born.

Hot Chips 28

This year’s Hot Chips is the first deep dive that we have received about the features of the Zen architecture. Mike Clark is taking us through all of the changes and advances that we can expect with the upcoming Zen products.

Zen is a clean sheet design that borrows very little from previous architectures. This is not to say that concepts that worked well in previous architectures were not revisited and optimized, but the overall floorplan has changed dramatically from what we have seen in the past. AMD did not stand still with their Bulldozer products, and the latest Excavator core does improve upon the power consumption and performance of the original. This evolution was simply not enough considering market pressures and Intel’s steady improvement of their core architecture year upon year. Zen was designed to significantly improve IPC and AMD claims that this product has a whopping 40% increase in IPC (instructions per clock) from the latest Excavator core.

AMD also has focused on scaling the Zen architecture from low power envelopes up to server level TDPs. The company looks to have pushed down the top end power envelope of Zen from the 125+ watts of Bulldozer/Vishera into the more acceptable 95 to 100 watt range. This also has allowed them to scale Zen down to the 15 to 25 watt TDP levels without sacrificing performance or overall efficiency. Most architectures have sweet spots where they tend to perform best. Vishera for example could scale nicely from 95 to 220 watts, but the design did not translate well into sub-65 watt envelopes. Excavator based “Carrizo” products on the other hand could scale from 15 watts to 65 watts without real problems, but became terribly inefficient above 65 watts with increased clockspeeds. Zen looks to address these differences by being able to scale from sub-25 watt TDPs up to 95 or 100. In theory this should allow AMD to simplify their product stack by offering a common architecture across multiple platforms.

In overall power per cycle, Zen is very similar to what we see with current Carrizo parts. It will consume as much power at 3 GHz while providing that 40% boost in performance. This is a tremendous efficiency gain, but it is not entirely done by design alone. Carrizo is still being produced on GLOBALFOUNDRIES 28nm HKMG process while Zen will be introduced on the new 14nm LPP process that has been ramping up at GF. While design does play a significant part, AMD is getting a near two generation jump in process technology to apply to the Zen architecture.

The above list shows many of the improvements that Zen has received as compared to the previous Excavator based products. It truly is a clean sheet design that borrows very little from the previous generation other than feature support (SSE, AVX, FMA3, etc.). Of interest is the drop of support of FMA4. This was introduced some years ago by AMD and included FMA3 functionality. FMA4 was not easy to implement effectively and never saw wide support in software. Intel championed FMA3 and it is a more effective implementation and has wider software support.

The front end of the Zen CPU was again changed dramatically from previous generations. There are many new additions here that help keep the execution units fed. Engineers were able to improve the L1 and L2 cache systems to offer nearly double the bandwidth of the previous generation as well as lower latency and improve accessibility from other cores.