Improvements to the Cache Hierarchy

The biggest under-the-hood change for the Ryzen 2000-series processors is in the cache latency. AMD is claiming that they were able to knock one-cycle from L1 and L2 caches, several cycles from L3, and better DRAM performance. Because pure core IPC is intimately intertwined with the caches (the size, the latency, the bandwidth), these new numbers are leading AMD to claim that these new processors can offer a +3% IPC gain over the previous generation.

The numbers AMD gives are:

  • 13% Better L1 Latency (1.10ns vs 0.95ns)
  • 34% Better L2 Latency (4.6ns vs 3.0ns)
  • 16% Better L3 Latency (11.0ns vs 9.2ns)
  • 11% Better Memory Latency (74ns vs 66ns at DDR4-3200)
  • Increased DRAM Frequency Support (DDR4-2666 vs DDR4-2933)

It is interesting that in the official slide deck AMD quotes latency measured as time, although in private conversations in our briefing it was discussed in terms of clock cycles. Ultimately latency measured as time can take advantage of other internal enhancements; however a pure engineer prefers to discuss clock cycles.

Naturally we went ahead to test the two aspects of this equation: are the cache metrics actually lower, and do we get an IPC uplift?

Cache Me Ousside, How Bow Dah?

For our testing, we use a memory latency checker over the stride range of the cache hierarchy of a single core. For this test we used the following:

  • Ryzen 7 2700X (Zen+)
  • Ryzen 5 2400G (Zen APU)
  • Ryzen 7 1800X (Zen)
  • Intel Core i7-8700K (Coffee Lake)
  • Intel Core i7-7700K (Kaby Lake)

The most obvious comparison is between the AMD processors. Here we have the Ryzen 7 1800X from the initial launch, the Ryzen 5 2400G APU that pairs Zen cores with Vega graphics, and the new Ryzen 7 2700X processor.

This graph is logarithmic in both axes.

This graph shows that in every phase of the cache design, the newest Ryzen 7 2700X requires fewer core clocks. The biggest difference is on the L2 cache latency, but L3 has a sizeable gain as well. The reason that the L2 gain is so large, especially between the 1800X and 2700X, is an interesting story.

When AMD first launched the Ryzen 7 1800X, the L2 latency was tested and listed at 17 clocks. This was a little high – it turns out that the engineers had intended for the L2 latency to be 12 clocks initially, but run out of time to tune the firmware and layout before sending the design off to be manufactured, leaving 17 cycles as the best compromise based on what the design was capable of and did not cause issues. With Threadripper and the Ryzen APUs, AMD tweaked the design enough to hit an L2 latency of 12 cycles, which was not specifically promoted at the time despite the benefits it provides. Now with the Ryzen 2000-series, AMD has reduced it down further to 11 cycles. We were told that this was due to both the new manufacturing process but also additional tweaks made to ensure signal coherency. In our testing, we actually saw an average L2 latency of 10.4 cycles, down from 16.9 cycles in on the Ryzen 7 1800X.

The L3 difference is a little unexpected: AMD stated a 16% better latency: 11.0 ns to 9.2 ns. We saw a change from 10.7 ns to 8.1 ns, which was a drop from 39 cycles to 30 cycles.

Of course, we could not go without comparing AMD to Intel. This is where it got very interesting. Now the cache configurations between the Ryzen 7 2700X and Core i7-8700K are different:

CPU Cache uArch Comparison
  AMD
Zen (Ryzen 1000)
Zen+ (Ryzen 2000)
Intel
Kaby Lake (Core 7000)
Coffee Lake (Core 8000)
L1-I Size 64 KB/core 32 KB/core
L1-I Assoc 4-way 8-way
L1-D Size 32 KB/core 32 KB/core
L1-D Assoc 8-way 8-way
L2 Size 512 KB/core 256 KB/core
L2 Assoc 8-way 4-way
L3 Size 8 MB/CCX
(2 MB/core)
2 MB/core
L3 Assoc 16-way 16-way
L3 Type Victim Write-back

AMD has a larger L2 cache, however the AMD L3 cache is a non-inclusive victim cache, which means it cannot be pre-fetched into unlike the Intel L3 cache.

This was an unexpected result, but we can see clearly that AMD has a latency timing advantage across the L2 and L3 caches. There is a sizable difference in DRAM, however the core performance metrics are here in the lower caches.

We can expand this out to include the three AMD chips, as well as Intel’s Coffee Lake and Kaby Lake cores.

This is a graph using cycles rather than timing latency: Intel has a small L1 advantage, however the larger L2 caches in AMD’s Zen designs mean that Intel has to hit the higher latency L3 earlier. Intel makes quick work of DRAM cycle latency however.

Talking 12nm and Zen+ Translating to IPC: All This for 3%?
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  • spdragoo - Thursday, April 19, 2018 - link

    Per Tom's Hardware (https://www.tomshardware.com/reviews/amd-ryzen-7-2...

    "Our test rigs now include Meltdown And Spectre Variant 1 mitigations. Spectre Variant 2 requires both motherboard firmware/microcode and operating system patches. We have installed the operating system patches for Variant 2.

    Today's performance measurements do not include Intel's motherboard firmware mitigations for Spectre Variant 2 though, as we've been waiting for AMD patches to level the playing field. Last week, AMD announced that it’s making the mitigations available to motherboard vendors and OEMs, which the company says should take time to appear in the wild. We checked MSI's website for firmware updates applicable to our X370 platforms when AMD made its announcement, but no new BIOSes were available (and still aren't).

    Unfortunately, we were only made aware that Variant 2 mitigations are present in our X470 board's firmware just before launch, precluding us from re-testing the Intel platforms with patches applied. We're working on this now, and plan to post updated results in future reviews.

    The lack of Spectre Variant 2 patches in our Intel results likely give the Core CPUs a slight advantage over AMD's patched platforms. But the performance difference should be minimal with modern processors."

    For those that are TL:DR in their viewpoint: unlike Anandtech, TH did NOT include all of the Spectre/Meltdown patches, & even said that there might be differences in their test results.
    Reply
  • Chris113q - Thursday, April 19, 2018 - link

    Other reviewers also had their setups meltdown/spectre patched and it's been already confirmed that these patches don't greatly impact gaming performance at all.
    It's clear that Anandtech's results are wrong here. I have read 12 other reviews and most of their results differ from the ones you got. You'd have to be delusional to take just 1 review as the absolute truth.
    Reply
  • Ninjawithagun - Thursday, April 19, 2018 - link

    Incorrect. Those reviews were conducted back in January 2018 (look at the review dates). Microsoft issued new patches for Meltdown and Spectre earier this month (April 2018). I could find no other performance review showing performance gain/loss for Intel CPUs based upon the new patches other than the one posted now by AnandTech. Reply
  • Ninjawithagun - Thursday, April 19, 2018 - link

    The only way to know for sure is for each hardware reviewer to provide the exact version of Windows 10 they used for testing. This will prove whether or not they ran benchmarks with the most current Windows updates/patches. Reply
  • Intel999 - Thursday, April 19, 2018 - link

    It is plausible that many reviewers were lazy and carried over data from earlier reviews on Intel and 1000 series Ryzen CPUs.

    Thank you Anandtech for doing aa genuinely unbiased review that required a great deal of extra work compared to others.
    Reply
  • 5080 - Thursday, April 19, 2018 - link

    And don't forget BIOS patches as well. If you have a fully patched system the impact is even bigger than just updating with the Windows KB patches. Reply
  • sor - Thursday, April 19, 2018 - link

    Looking at Tom’s results, they have OC intels in first place. Other than that it’s damn close. Is there a chance you’re just browsing graphs to see who is in the top spot and not really comprehending the results?

    Aside from that, the test setups and even benchmarks used are different. You owe Ian an apology for not realizing you’re comparing OC results to his.
    Reply
  • Silma - Thursday, April 19, 2018 - link

    Yes. Ian is a top reviewer. At worst he made a mistake in this evaluations. It happens to the best of us.
    However, I have an issue with non OC test. It seems to me people will purchase overclockable processors and graphic cards to overclock them. At least game results should probably be based on OC benchmarks.
    Reply
  • pogostick - Thursday, April 19, 2018 - link

    @Silma No, it makes more sense to do it this way. Everyone who buys these processors are guaranteed to have a part that will run the manufacturer spec. OC is a random lottery. Reply
  • ACE76 - Thursday, April 19, 2018 - link

    Wrong... majority of even gamers DON'T overclock...that us relagated to a niche market of enthusiasts. Reply

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