CPU Tests: Microbenchmarks

Core-to-Core Latency

As the core count of modern CPUs is growing, we are reaching a time when the time to access each core from a different core is no longer a constant. Even before the advent of heterogeneous SoC designs, processors built on large rings or meshes can have different latencies to access the nearest core compared to the furthest core. This rings true especially in multi-socket server environments.

But modern CPUs, even desktop and consumer CPUs, can have variable access latency to get to another core. For example, in the first generation Threadripper CPUs, we had four chips on the package, each with 8 threads, and each with a different core-to-core latency depending on if it was on-die or off-die. This gets more complex with products like Lakefield, which has two different communication buses depending on which core is talking to which.

If you are a regular reader of AnandTech’s CPU reviews, you will recognize our Core-to-Core latency test. It’s a great way to show exactly how groups of cores are laid out on the silicon. This is a custom in-house test built by Andrei, and we know there are competing tests out there, but we feel ours is the most accurate to how quick an access between two cores can happen.

In terms of the core-to-core tests on the Tiger Lake-H 11980HK, it’s best to actually compare results 1:1 alongside the 4-core Tiger Lake design such as the i7-1185G7:

What’s very interesting in these results is that although the new 8-core design features double the cores, representing a larger ring-bus with more ring stops and cache slices, is that the core-to-core latencies are actually lower both in terms of best-case and worst-case results compared to the 4-core Tiger Lake chip.

This is generally a bit perplexing and confusing, generally the one thing to account for such a difference would be either faster CPU frequencies, or a faster clock of lower cycle latency of the L3 and the ring bus. Given that TGL-H comes 8 months after TGL-U, it is plausible that the newer chip has a more matured implementation and Intel would have been able to optimise access latencies.

Due to AMD’s recent shift to a 8-core core complex, Intel no longer has an advantage in core-to-core latencies this generation, and AMD’s more hierarchical cache structure and interconnect fabric is able to showcase better performance.

Cache & DRAM Latency

This is another in-house test built by Andrei, which 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 (before you start saying 64-core TR has 256 MB of L3, it’s only 16 MB per core, so at 20 MB you are in 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.

What’s of particular note for TGL-H is the fact that the new higher-end chip does not have support for LPDDR4, instead exclusively relying on DDR4-3200 as on this reference laptop configuration. This does favour the chip in terms of memory latency, which now falls in at a measured 101ns versus 108ns on the reference TGL-U platform we tested last year, but does come at a cost of memory bandwidth, which is now only reaching a theoretical peak of 51.2GB/s instead of 68.2GB/s – even with double the core count.

What’s in favour of the TGL-H system is the increased L3 cache from 12MB to 24MB – this is still 3MB per core slice as on TGL-U, so it does come with the newer L3 design which was introduced in TGL-U. Nevertheless, this fact, we do see some differences in the L3 behaviour; the TGL-H system has slightly higher access latencies at the same test depth than the TGL-U system, even accounting for the fact that the TGL-H CPUs are clocked slightly higher and have better L1 and L2 latencies. This is an interesting contradiction in context of the improved core-to-core latency results we just saw before, which means that for the latter Intel did make some changes to the fabric. Furthermore, we see flatter access latencies across the L3 depth, which isn’t quite how the TGL-U system behaved, meaning Intel definitely has made some changes as to how the L3 is accessed.

Power Consumption - Up to 65W or not? SPEC CPU - Single-Threaded Performance
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  • Andrei Frumusanu - Monday, May 17, 2021 - link

    As a note, we're just finishing up this review at the very last minute due to us getting our hands on the reference laptop only in the last 48h. I'll be completing the missing page texts in the next few hours as we're tidying up the article. Reply
  • EliteRetard - Monday, May 17, 2021 - link

    Did you have a description/specs of the test systems?
    If it was there I missed it, even after going back to look.
    Differences in RAM / storage etc. can affect some tests.

    I'm guessing the size based on the name of the Asus...
    Looks like you're comparing a 16" workstation vs a 13" thin/light?

    Would the AMD CPU perform better in a larger/cooler chassis?
    Reply
  • timecop1818 - Tuesday, May 18, 2021 - link

    > Would the AMD CPU perform better

    lol
    Reply
  • Spunjji - Tuesday, May 18, 2021 - link

    > timecop1818

    lol
    Reply
  • Qasar - Tuesday, May 18, 2021 - link

    gotta love timecrap181... Reply
  • at_clucks - Wednesday, May 19, 2021 - link

    Come on, the Intel CPU actually performs decently... for a slowish desktop CPU stuck in a laptop chassis. Still not that bad. Reply
  • Spunjji - Thursday, May 20, 2021 - link

    It performs very well, but timcarp1488 was completely misreading what had actually been said just to shitpost his usual anti-AMD nonsense. Reply
  • Spunjji - Tuesday, May 18, 2021 - link

    "Did you have a description/specs of the test systems?"
    A brief description of the Intel reference system is in this review, more detail of the AMD system is available in the review these test results came from.

    "Would the AMD CPU perform better in a larger/cooler chassis?"
    A 45W variant of the AMD CPU in a larger chassis would see higher sustained multi-core performance, but single-core is probably quite similar.
    Reply
  • Gondalf - Wednesday, May 19, 2021 - link

    Strange article Andrei.
    5980HS is rated 35-54 W or 45W+. How can you judge that 45W Intel is less efficient?? Have you data about TDP settings of Asus X13 ? Likely the AMD SKU run at the highest TDP for more performance on Asus device, for several minutes or continuously.
    Bet you neeed to be more informed in your articles, OEMs can go at the max TDP of a cpu since the Tskin of the laptop allow this.
    Bet Intel Tiger Lake H will be faster than in your article on the right chassis ?
    Bet direct power measures are better than generic comments ?
    Reply
  • Retycint - Thursday, May 20, 2021 - link

    Strange comment Gondalf.
    The graph of the 5980HS on page 2 shows that the Asus X13 runs at 42W for about 300s and then drops to 35W for the rest of the time.
    Bet you didn't read the article and just came down instantly to try and feel smug?
    Bet you need to be more informed when making hate posts?
    Reply

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