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    Why AMD’s Zen Based Ryzen CPUs are So Good: Digging into the Bulldozer Design

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    AMD’s Ryzen 3000 CPUs have been in the market for more than two years, offering as much as twice the performance compared to Intel CPUs at unbeatable prices. This has resulted in the red CPUs taking the lead over big blue in many markets such as Japan, Korea, Germany, Netherlands, etc. The newly announced 7nm Ryzen 3000 and Epyc Rome processors are also giving Intel a tough time in both the consumer and server space. However, if you look at AMD’s performance over the past decade, it has been dismal, to say the least. Ever since the Core architecture landed, they have essentially been playing catch up.

    How did AMD go from “manufacturer of cheap, budget CPUs” to being a formidable competitor in less than four years? One word: Zen. This new architecture was manifold better than the Bulldozer design, especially if you consider the single-threaded performance and the IPC. But what makes it so good? Let’s find out.

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    For starters, let’s clear up a few things. Zen is good but it wouldn’t have been such a step up for AMD if the older Bulldozer design wasn’t so inferior with major major design flaws. After the third-gen K10 architecture, team red due to lack of funds decided to invest in a narrow, low-IPC, high-clock design. Due to this, they ended up losing their competitive edge, resulting in the abysmal FX processors.

    You needed high core counts and higher operating clocks to make this work which isn’t something AMD was able to pull off and the rest is history. To give a clearer picture of how disadvantaged the Bulldozer chips were compared to their predecessors, here’s an example: To offer performance in line with the older Phenom II processors, Bulldozer needed to have a 50% higher operating speed (on an average). This, of course, didn’t happen, and instead resulted in power-hungry CPUs that ran hot and unstable.

    https://h3g7w2e9.stackpathcdn.com/wp-content/uploads/2019/04/c1ob9l1ustu21.png

    Have a look at the above IPC chart. Instead of going up, the single-core performance actually fell with Bulldozer and it took three upgrade cycles to bring it back on par with K10. The company started to move in the right direction with Steamroller and Excavator but hit a roadblock soon after. The design limitations of the Bulldozer architecture prevented AMD’s engineers from making further improvements without overhauling the layout.

    This finally resulted in the brand new Zen architecture that ditched all the bottlenecks of Bulldozer and its successors, and here we are, with the 3rd Gen Ryzen lineup leveraging the Zen 2 architecture based on the 7nm node.

    Bulldozer Core vs Zen Core

    Let’s put the two architectures side-by-side and see how Zen varies from Bulldozer. Notice how the Excavator (last Bulldozer) design essentially makes one core into two by throwing in an extra Integer Scheduler and decoder. This may sound like a good idea on paper but it didn’t quite pan out. Yes, it did make the design simpler and cheaper, but the IPC went down the drain. Due to this shared logic design, the resources available to each core were severely compromised.

    An integer cluster basically counted as a “core” in Bulldozer and shared the same L2 cache and FP scheduler with the adjacent core. This prevented the decoders from performing efficiently and instead of increasing the level of instruction-level parallelism, it effectively crippled the decoders by keeping one of them idle most of the time. By stuffing two decoders side by side, there was a bottleneck leaving the lone FP scheduler to juggle between the workload of the two so-called cores, leading to especially poor floatingpoint performance.

    Have a look at Zen. Each core has two 256-bit FMACs under the FP scheduler, that’s four times as much as Bulldozer. The pipeline is also wider, with dedicated L2 cache per core, boosting singlecore performance by a notable margin.

    This is actually a form of Hyperthreading. Where Intel’s CPUs merge two threads into a single, large ALU cluster, AMD had two separate partitions. This is called Clustered Multi-Threading (CMT) while Intel leverages Simultaneous Multi-Threading (SMT), something which the Zen and Zen 2 designs also have. So there you have it. What do you think of AMD’s remarkable recovery in the past 4-5 years

    Further Reading:

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    Areej
    I love computer hardware and RPGs, and those two things are what drove me to start TechQuila. Other than that most of my time goes into reading psychology, writing (and reading) dark poetry and playing games.

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