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Carrizo vs. Kaveri: Which low-end AMD CPU is a better buy?

When AMD launched its Carrizo CPU refresh last year, it made it clear that the chip would focus almost entirely on notebooks rather than desktops. The company just one low-cost part for the desktop space — the Athlon X4 845. This chip doesn’t use Carrizo’s updated integrated GPU, but packs in four cores in two CPU modules with a base clock at 3.5GHz and a 3.8GHz Turbo Mode. Based on AMD’s disclosures regarding Carrizo, the new chip should be faster and more efficient than the Kaveri cores it ostensibly replaced — but the truth turns out to be a bit more complicated.

Over at Anandtech, they’ve taken AMD’s latest core and matched it against previous parts based on Kaveri, Richland, and Trinity. The result is a 26-page magnum opus that compares the various chips in a huge range of scenarios and tests, from gaming to general-purpose compute in Windows. Linux benchmarks are included, as are results on a number of Intel products. There’s a great deal of information packed into the article and I highly recommend it.

The big-picture takeaway on Carrizo versus Kaveri at the CPU level is this: There’s a solid group of tests where Carrizo shows real efficiency improvements over Kaveri. The graph below compares each APU to the previous generation and gives the improvement in percentage terms. A negative percentage means that the APU in question is slower than its predecessor, a positive percentage means the newer chip is faster.

This is just one of the overall graphs in the review. Anandtech’s entire non-gaming Windows test suite measured the Athlon X4 845 as being 7.3% faster than Kaveri when all of the APUs were locked to 3GHz and tested at that clock speed. That’s not bad for a generational improvement, especially considering that the X4 845 is a 65W part compared with a 95W X4 860K.

Unfortunately, Carrizo is dogged by two issues. First, its gains are situational. In some workloads, Carrizo is as much as 32% faster than Kaveri. In others, it’s 8-12% slower. Gaming takes a particular hit — Kaveri is approximately 6% faster than Carrizo when gaming, almost across the board.

Second, AMD was forced to pull clock speeds down when it shifted to Carrizo, just as it was when Kaveri debuted. The 65W Carrizo tops out at 3.8GHz with a 3.5GHz boost, while the X4 860K is a 3.7GHz / 4GHz CPU. Anandtech reports that the overclocking headroom with their particular sample is small, at roughly 10%. Users would need to push the chip’s clock at least that high to count on matching Kaveri’s performance in the worst-case scenarios, though an OC’d X4 845 could also be substantially faster than the X4 860K.

Workloads that fit comfortably within Carrizo’s larger L1 cache (128K L1-D, compared to 64K for Carrizo) or benefit from its increased cache associativity (8-way, up from 4-way) show the largest improvements. Other tests show Kaveri winning past its newer cousin, presumably thanks to a combination of higher clocks and a larger L2 cache. This core was originally designed for laptops and it shows — the smaller L2 cache and eight lanes of PCIe 3.0 may have been smart tradeoffs in the 15-25W space, but this is a 15W chip competing against desktop processors. Just pushing the TDP up to 65W doesn’t mean that Carrizo was actually designed to compete in these power envelopes (as we discussed last year, Carrizo is actually optimized to outstrip Kaveri at lower power envelopes, but may not compete well against it in the 65W+ space).

Those of you who have followed AMD’s designs over the past few years are likely aware that we saw a very similar pattern when Kaveri launched. Back in 2014, Kaveri proved it was an extremely potent replacement for Richland at the 45W TDP envelope but less persuasive at the 65W and 95W targets. Chips clocked above their sweet spot tend to require more voltage, which in turn generates more heat, which then requires more voltage… you get the picture.

On a more positive note, the competitive price ($70) and its quad-threaded design makes the X4 845 a very potent competitor against some of Intel’s dual-core CPUs like the 20th Anniversary Pentium it replaced last year. In the nearly four years since AMD’s first Piledriver-based APU launched, the company has managed to improve IPC (instructions per clock cycle, a measure of efficiency) by between 10% and 20% while simultaneously reducing power consumption. That’s a significant achievement, particularly for a company as cash-strapped as AMD, but it’s going to take Zen to really move the bar on the company’s overall performance-per-watt story.

BristolRidge-Features

When AMD announced Bristol Ridge earlier this year, we thought the chips and chipsets would be debuting already — but Computex has come and gone with no sign of the refresh. If Bristol Ridge doesn’t debut soon, it’s possible that AMD will hold the cycle for a CES announcement, presumably alongside Zen. Sunnyvale continues to insist that its next-generation CPU will sample late in Q4 for a Q1 2017 launch, but Zen’s first iteration is CPU-only. AMD will still need an APU to pair it with, which means 28nm Bristol Ridge APUs based on Carrizo will share space with 14nm Zen cores based on AMD’s new architecture. AMD hasn’t said when it’ll push Zen into APUs, but it’s safe to assume the company will make that transition as quickly as it can. Even if Zen-based APUs stick with DDR4 as their memory standard, the additional CPU performance and superior 14nm process make it a much more attractive part — assuming it hits its power and performance targets.

Bristol Ridge is unlikely to shake up the overall roadmap or Carrizo’s performance very much. While the chip will have some improvements and tweaks (and should support dual-channel memory in laptops) the typical gain for this kind of refresh is in the 3-5% range. Another 5% sure wouldn’t hurt the core’s CPU performance, but Zen’s 40% is what people are going to be watching for. If you’re looking to build an entry-level AMD gaming rig, Kaveri is probably the better option. If, on the other hand, you want a general-purpose system, Carrizo and the X4 845 may be the better core.

Now read: How L1 and L2 CPU caches work, and why they’re an essential part of modern chips

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