Buyer beware: AMD Zen2 and Zen 3 APUs are still on PCIe 3.0

Hardware CPU
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Hey, just wanted to discuss a quick thing that came to my mind that might be a big snafu for some…

O.k. so I was looking at the 5700G and noticed something really interesting with the 5700G. It does not run PCIe 4.0, only 3.0. Now, this is really wierd since the CPU part of this chip is a Zen 3, and AMD went to PCIe 4.0 on Zen 2 already.

So what gives here? Why would AMD do this? Then it hit me… They are still on PCIe 3.0 because Vega is still on 3.0. Soooooo, rather than include a bunch of extra conversion junk, AMD simply runs the whole system at 3.0.

I suspect AMD will move over to PCIe 4.0 once NAVI2 APUs come out, perhaps on the AM5 platform. Now, this interesting tidbit come with two issues;

  1. If you want to run PCIe 4.0 on a Ryzen 5x00 APU, you need an x570 board. Even then, most boards limit you to 8x 4.0 chipset lanes at most.

  2. Some newer cards, like the 6700 XT, only utilizes 8x PCIe lanes. For PCIe 4.0, this is virtually no slowdown, the bandwidth is enough to keep the card saturated. But on a PCIe 3.0 system, you might be bandwidth starved since the card runs @ 8xPCIe 3.0.

So, if you’re a sneaky chump like me and think “Hey, I’ma gonna buy a 5700G and game on that until prices finally come down” - Be aware you might lose up to 25%-30% performance due to bandwidth starvation (this all depends on if the card is saturated at 8x PCIe 3.0). This could be a dealbreaker for some though.

Has anyone done some benchmarks on this?

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I recall this was a known thing. I don’t remember if Wendell or ETA Prime or someone else entirely did the testing on the 5700G with a Radeon 6600XT vs a 5800x to check the PCI-E 3.0 vs 4.0 bottleneck on 8x lanes. In some games it was marginal, some have seen significant gains, so it depends on the game.

I want to buy a 5600G, but I don’t intend to put a GPU on it (or so is the initial plan).

Never mind, I found the video, it was Hardware Canucks:

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Yes, buuuuut… we will first have a Zen 3+ / refresh, which will still be AM4. AM5 or whatever it will be called when it comes out will feature Zen 4 cores, PCI-E gen 5 and DDR5 RAM. But if the Zen 4 APUs will come with Navi, we will see the same thing as with the current Vega. The PCI-E 5 will run at gen 4 speeds, unless AMD already saw that coming and are testing a Navi refresh or their next-gen GPU architecture on PCI-E gen 5. But that doesn’t change the fact that probably the first gen Navi APUs will probably still be on PCI-E gen 4.

IMO, it shouldn’t be that big of a deal, but it’s something to consider.

And I kinda want to do a DDR5 APU build with that sweet and fast RAM, but it’s probably at least 1 or 2 years away, so… big oof.

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Hmmm, true. Of course, there was a rumor that all Zen 4 or Zen 5 CPUs would be APUs, which would completely contradict this. Time will tell.

[edit]Ok, so after seeing the Canucks video, for gaming it doesn’t matter much, at least for this generation of cards. That is good news. So the only performance you leave on the table with an 8x 3.0 is like 10% or so, worst case - mostly harmless though.[/edit]

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Interesting. Does this mean that the Tuxedo Pulse 15, which has a PCIe 4.0 NVMe as an option, is only able to use it at PCIe 3.0 speeds? I’m especially curious because I recently ordered one with a PCIe 4.0 drive, and I may still be able to change the configuration before the machine ships.

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According to the website, that computer has:

So, yes, only PCIe 3.0 speeds. Some computers will have chipsets that support 4.0 (like the X570 boards), but the APUs only support 3.0.

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Thanks, well spotted. I remembered why I got the 4.0 drive instead of the 3.0 drive though: the 4.0 PRO drive is actually cheaper in this case than the 3.0 PRO drive, so no cause for alarm after all :slight_smile:

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