TECH TALK The Folly of Counting Cores
I’VE BEEN REVIEWING the Core i9-9980XE (pg. 76), and there’s a lot of supplementary material I didn’t have space to discuss. So, if you’ll indulge me a moment, let’s talk a bit more about CPUs and architectures.
What’s the best processor on the planet? You might think there’s a simple answer, but it depends on what you plan to do. Just about any modern CPU is “fast enough” for general applications, and upgrading other components might be more worth your time. But let’s just focus on CPUs for now.
The past two years, AMD and Intel have pushed the boundaries on core counts to potentially obscene levels. AMD’s updated second-gen Threadripper processors now offer up to 32 cores/64 threads in a single package, and at first you might be tempted to take the plunge. But when you start digging into the benchmarks, things get a lot more complex. Some heavily threaded workloads have excellent performance on Threadripper 2990WX, others do better on a Threadripper 2950X, and some even perform best on a Ryzen 7 2700X. What’s going on?
It goes back to AMD’s Zen architecture and how it chose to scale from quad-core up to 64-core CPU packages. Socket TR4 may look like Epyc’s socket SP3, but a bunch of the pins are unused. Epyc CPUs have octal-channel memory, but Threadripper is limited to quad-channel. The second-generation WX parts have four CPU packages—basically four Ryzen 7 processors—but two of the CPUs have to talk to system RAM via the memory controllers on the other chips. At the same time, AMD altered the die-to-die Infinity Fabric, and cut the bandwidth in half (from 50GB/s to 25GB/s).
Combined, these changes cause much higher latencies for some workloads, and the result can be disastrous. In fact, it can be so bad that AMD even provides a way to disable the extra cores, effectively turning the 2990WX into a 2950X. But what happens if you leave all the cores enabled?
In testing, applications including video encoding, 7-zip compression, web browsing, photo editing, and more tend to underperform thanks to the 2990WX’s design. Often, the 2950X beats the 2990WX, despite having half as many cores. Most games also perform better on the 2950X, and better still on a Ryzen 7 2700X— and Intel’s CPUs take top honors. In other words, more cores aren’t always better, and in some cases, they can be substantially worse.
Other applications that make good use of AVX instructions, such as video editing and scientific computing, also favor CPU architectures with better AVX support. Despite their core deficit, Intel’s Skylake-X CPUs do very well in Handbrake x265 and y-cruncher, thanks to their use of AVX512. Other applications favor higher clock speeds and lower latencies. Most games, for example, provided you’re using a fast enough graphics card (for example, a GTX 1080 Ti or RTX 2080, or above), run best on a Core i9-9900K. Numerous professional apps such as AutoCAD, Solidworks, Maya, and more favor Intel’s Coffee Lake processors, in part because a professional GPU like a Quadro P6000 offloads a lot of the work.
It may not be a huge performance deficit, but at roughly six times the cost, you might mistakenly think a Core i9-9980XE would be clearly better for professional use than a Core i7-8700K. Sometimes it is, sometimes it isn’t—there are times when the i7-8700K is up to 15–20 percent faster. I suspect the future 28-core X-series CPU will follow a similar pattern. It will generally perform worse in games and certain professional applications, because it will of necessity have lower clock speeds, and its mesh network will further increase latencies for memory accesses.
Elsewhere, however, core and thread counts reign supreme. Cinebench and several other CPUbased 3D rendering applications all scale extremely well with additional cores. A Threadripper 2990WX is about 40–50 percent faster than a Core i9-9980XE in Cinebench, POV-ray, and Corona, for instance.
In short, more than ever before, it’s important to know your workloads as well as the CPU and platform architectures and features. Only then can you determine which CPU is best, and it may not be the most expensive part, or the one with the most cores. For a lot of people, including even many professionals, the mainstream AMD and Intel CPUs are the best overall options—core counts be damned.