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Comment by treprinum | Hacker News Reader

7940HS in my 4090 laptop is cool, though I am worried about the future starting with Phoenix 2 that will feature few good cores (Zen 5) and many crappy cores (Zen 5c). Not sure why AMD had to ape a failing Intel arch here that was introduced only to improve Intel's multicore benchmarks. Now with AMD we will get all the "heterogenous goodies" when programs keep running into weird states due to scheduler not being 100% in its core allocations. Even if 5c's only difference is in the smaller cache and slower AVX-512, this could bite in unpredictable ways. Moreover, if AMD really needed to go with this hybrid design, why not ape Apple M1 or M2 with only 2 or 4 crappy cores?

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> Phoenix 2 that will feature few good cores (Zen 5) and many crappy cores (Zen 5c).

Phoenix 2 is already starting to ship, with Zen 4 and Zen 4c cores in a 2+4 arrangement. I don't think there's anything wrong with this approach: the main downside of Zen 4c cores is they cannot clock as high, but the kind of systems the SoC is going into are thermally limited to the point that using more than two CPU cores is already guaranteed to drop you down to the clock speed range that Zen 4c runs at. Ignoring the impact on chip floorplan and just looking at the implications for software, it's more like a more pronounced version of Intel's Turbo Boost Max 3.0 (preferred cores) rather than the heterogeneous P vs E core situation. All the cores have the same feature support and same performance per clock and the same sizes for their private caches.

The smaller cache will make it a heterogeneous situation for some workloads and likely games as well (seeing how 5800x3D/7800x3D are demolishing the competition thanks to a larger cache).

There is no smaller cache. L1 and L2 are the same capacity (but smaller area) for Zen 4 and Zen 4c: 32kB and 1MB respectively. The L3 cache is 16MB for Phoenix and Phoenix 2, so it's actually more L3 per core for the 2+4-core Phoenix 2 than the 8-core Phoenix. It's only in the server Zen 4c processors (Bergamo) that they've cut L3 per CCX in half.

Performance per clock has already been verified as the same: https://pic2.zhimg.com/v2-866cc994355fb54831c7398c97b20479_b...

The only heterogeneity experienced by software will be due to the lower maximum clocks on the Zen 4c cores, which is a far simpler problem to deal with than Intel's P vs E cores or AMD's 7950X3D with 3D V-Cache on only one of the two CPU chiplets.

You can do more power optimizations with efficiency cores than the performance cores. There are power tradeoffs to get max perf. You can significantly reduce vcore if your max frequency is much lower. Cache is also power hungry & takes up space.

5c should be the same as 5 but with a smaller cache (unlike Core vs Atom distinction in Intel CPUs). I'd rather have a CPU full of 5c than a mix of 5 and 5c.

Early indications are that Zen 4c actually requires significantly higher voltage for the same clock speed as plain Zen 4 cores (though still a lower voltage than Zen 4 at speeds far beyond the reach of Zen 4c). All the savings AMD gets by targeting ~3.5GHz peak rather than 5+ GHz has been put into shrinking the core area: https://zhuanlan.zhihu.com/p/653961282

interesting, I wonder why. I guess it's saving via current reductions despite higher vcore.

It seems 4c requires .1v more through most of the clock speed range. It's purely a cost and density play.

> It seems 4c requires .1v more through most of the clock speed range.

It doesn't look anything like a constant offset to me: https://pic2.zhimg.com/v2-3246ea92d494fffe1ea26f30bc9d9d79_b...

That seems like the same voltage at about 1 Ghz less clock, which makes sense.

AMD has already released a product featuring Zen4c cores designed for servers. In contrast to Intel, AMD's small cores are not crappy; they are real Zen4 cores, with a reduction in size achieved by sacrificing some of the L3 cache and clock frequencies.

As for the future of Zen 5, we are uncertain about its development, but I am hopeful that AMD will follow a similar approach to Zen4c.

By the way, Intel employs different cores derived from their Atom line of low-power processors, specifically the Gracemont microarchitecture in their current products. Unfortunately, these cores are indeed crappy, particularly in terms of floating-point performance.

I am fine with a CPU full of 4c/5c cores, I however don't want a mix of 4/4c or 5/5c.

> "heterogenous goodies"

Like battery life

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