AMD’s Carrizo: Higher efficiency, better performance, and a huge leap for battery life

AMD’s Carrizo: Higher efficiency, better performance, and a huge leap for battery life
AMD is unveiling its Carrizo APU at ISSCC this week.

AMD is unveiling its Carrizo APU at ISSCC this week. This new chip occupies an odd intersection between the old and new. On the one hand, it’s the last of the Bulldozer-derived architectures and we’ve known for months that the core would remain on the now-mature 28nm process. Despite these apparent limitations, AMD is quite bullish on this core’s potential.

New features and capabilities

As expected, Carrizo will be the first AMD APU to integrate the use of High Density Libraries (HDL)’s on the PC side. AMD made a number of changes to the CPU’s structure to support this — Excavator uses nine metal layers as compared to 15 for Steamroller. We also know that the L1 data cache is larger in this core, from 16KB to 32KB per core. Total IPC improvements for Excavator are estimated at ~5%. That’s on top of the 7-10% that AMD delivered with Steamroller— but unlike that chip, the only major changes disclosed so far is the increased L1 data cache.
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The graphs and diagrams to the left show the advantage of using HDL as opposed to HP (high performance) libraries for Carrizo’s design. The CPU is dramatically smaller than it would’ve been otherwise, with a 30-40% reduction for specific areas. AMD claimed that it could get the benefits of a full node shrink from moving to HDL and based on die and feature sizes, it appears to have succeeded. As for performance, the graph on the right shows how the power-optimized version of Excavator is able to hit higher normalized frequency compared to Kaveri. The implication of the chart is that Excavator / Carrizo is capable of ~10% higher frequencies at 15W TDP as compared to Kaveri at the same power point.

The power saving mechanisms and metrics in Carrizo will have the most benefit at lower TDPs. This duplicates the pattern we saw with Kaveri, where the 95W desktop chips were often a wash when compared to the previous Richland processors but 15-35W mobile Kaveri was significantly faster than Trinity/Richland chips. Carrizo will target 12W – 35W TDPs, but the chip will shine the brightest around the 15W point.

The CPU inside Carrizo changed more than the GPU, but the graphics core inside Carrizo is getting a point update, to Tonga. AMD rebuilt the GPU core — the orange dots show the percentage of total transistors in the core that were previously of a particular type, while the blue dots show the percentage of transistors in Carrizo of that same type at normalized Ion / Ioff ratios. The entire GPU has shifted down and to the left, meaning it draws significantly less power. AMD claims that they can drive 10% higher frequency on the GPU core at the same power level or cut power consumption by 20% at the same frequency.

One other major change actually addresses an issue in the lowest-power Kaveri cores that we weren’t previously aware of. Some of AMD’s low power mobile cores had eight GPU clusters for a total of 512 cores — but didn’t actually enable all eight Compute Units (CUs) at the same time.  All of AMD’s 20W chips were limited to six or fewer CUs to keep the core from overheating. Carrizo removes this limitation, which should substantially improve performance.

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