Core-to-Core, Cache Latency, Ramp

For some of our standard tests, we look at how the CPU performs in a series of synthetic workloads to example any microarchitectural changes or differences. This includes our core-to-core latency test, a cache latency sweep across the memory space, and a ramp test to see how quick a system runs from idle to load.

Core-to-Core

Inside the chip are eight cores connected through a bi-directional ring, each direction capable of transmitting 32 bytes per cycle. In this test we test how long it takes to probe an L3 cache line from a different core on the chip and return the result.

For two threads on the same core, we’re seeing a 7 nanosecond difference, whereas for two separate cores we’re seeing a latency from 15.5 nanoseconds up to 21.2 nanoseconds, which is a wide gap. Finding out exactly how much each jump takes is a bit tricky, as the overall time is reliant on the frequency of the core, of the cache, and of the fabric over the time of the test. It also doesn’t tell us if there is anything else on the ring aside from the cores, as there is also going to be some form of external connectivity to other elements of the SoC.

However, compared to the Zen3 numbers we saw on the Ryzen 9 5980HS, they are practically the same.

Cache Latency Ramp

This test showcases the access latency at all the points in the cache hierarchy for a single core. We start at 2 KiB, and probe the latency all the way through to 256 MB, which for most CPUs sits inside the DRAM.

Part of this test helps us understand the range of latencies for accessing a given level of cache, but also the transition between the cache levels gives insight into how different parts of the cache microarchitecture work, such as TLBs. As CPU microarchitects look at interesting and novel ways to design caches upon caches inside caches, this basic test proves to be very valuable.

The data here again mirrors exactly what we saw with the previous generation on Zen3.

Frequency Ramp

Both AMD and Intel over the past few years have introduced features to their processors that speed up the time from when a CPU moves from idle into a high-powered state. The effect of this means that users can get peak performance quicker, but the biggest knock-on effect for this is with battery life in mobile devices, especially if a system can turbo up quick and turbo down quick, ensuring that it stays in the lowest and most efficient power state for as long as possible.

Intel’s technology is called SpeedShift, although SpeedShift was not enabled until Skylake.

One of the issues though with this technology is that sometimes the adjustments in frequency can be so fast, software cannot detect them. If the frequency is changing on the order of microseconds, but your software is only probing frequency in milliseconds (or seconds), then quick changes will be missed. Not only that, as an observer probing the frequency, you could be affecting the actual turbo performance. When the CPU is changing frequency, it essentially has to pause all compute while it aligns the frequency rate of the whole core.

We wrote an extensive review analysis piece on this, called ‘Reaching for Turbo: Aligning Perception with AMD’s Frequency Metrics’, due to an issue where users were not observing the peak turbo speeds for AMD’s processors.

We got around the issue by making the frequency probing the workload causing the turbo. The software is able to detect frequency adjustments on a microsecond scale, so we can see how well a system can get to those boost frequencies. Our Frequency Ramp tool has already been in use in a number of reviews.

A ramp time of within one millisecond is as expected for modern AMD platforms, although we didn’t see the high 4.9 GHz that AMD has listed this processor as being able to obtain. We saw it hit that frequency in a number of tests, but not this one. AMD’s previous generation took a couple of milliseconds to hit around the 4.0 GHz mark, but then another 16 milliseconds to go full speed. We didn’t see it in this test, perhaps due to some of the new measurements AMD is doing on core workload and power. We will have to try this on a different AMD Ryzen 6000 Mobile system to see if we get the same result.

AMD's Ryzen 9 6900HS Rembrandt Benchmarked Power Consumption
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  • mode_13h - Sunday, March 13, 2022 - link

    > M1 is cute, nothing more.

    Cool story bro. I guess the original M1 rather skimps on RAM. Maybe that's what's limiting your "first-hand experience".
    Reply
  • Dug - Friday, March 11, 2022 - link

    You don't know much about the architecture, do you? Reply
  • Marlin1975 - Tuesday, March 1, 2022 - link

    From what I have seen Intel will hold the 45watt+ performance crown but the AMD 6000 series will do better at lower wattage. Reply
  • Fulljack - Tuesday, March 1, 2022 - link

    not to mention the battery life. of course, if you're doing power consuming task like render, 4k video playback, or gaming, keep the device at wall power. but if you simply browsing, watching vids, or typing and doesn't need to use wall power continuously, the battery life are much much better than Alder Lake or Cezanne. Reply
  • yankeeDDL - Tuesday, March 1, 2022 - link

    I have the same impression, but I did not see any solid data confirming this. Reply
  • trivik12 - Tuesday, March 1, 2022 - link

    Rembrandt is a solid upgrade considering it was not a new cpu core. Still its good time to be a consumer. I hope anandtech reviews a 1280p laptops as well. That supposedly does very well at lower wattages as well. Reply
  • skavi - Tuesday, March 1, 2022 - link

    The comparison I most want to see is the XPS 13 Plus with a 1280P up against the ThinkPad Z13 with a 6860Z. The ThinkPad looks like it may be the first AMD laptop in the same league as the XPS. Reply
  • skavi - Tuesday, March 1, 2022 - link

    https://www.notebookcheck.net/Dell-XPS-13-Plus-932...

    https://www.notebookcheck.net/Lenovo-ThinkPad-Z13-...

    Links for anyone interested.
    Reply
  • DannyH246 - Tuesday, March 1, 2022 - link

    "We haven't touched battery life or graphics" - the two areas where AMD completely destroys Intel. And arguably the 2 areas that most consumers will be bothered about. What a joke of an article. Reply
  • GoldenBullet - Tuesday, March 1, 2022 - link

    Agreed, the igpu is amazing in ryzen 6000. Reply

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