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.

All three CPUs exhibit the same behaviour - one core seems to be given high priority, while the rest are not.

Frequency Ramping

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.

From an idle frequency of 800 MHz, It takes ~16 ms for Intel to boost to the top frequency for both the i9 and the i5. The i7 was most of the way there, but took an addition 10 ms or so. 

Power Consumption: Caution on Core i9 CPU Tests: Office and Science
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  • ozzuneoj86 - Thursday, April 1, 2021 - link

    "Rocket Lake also gets you PCIe 4.0, however users might feel that is a small add-in when AMD has PCIe 4.0, lower power, and better general performance for the same price."

    If a time traveling tech journalist would have told us back in the Bulldozer days that Anandtech would be writing this sentence in 2021 in a nonchalant way (because AMD having better CPUs is the new normal), we wouldn't have believed him.
    Reply
  • Hrel - Friday, April 2, 2021 - link

    Just in case anyone able to actually affect change reads these comments, I'm not even interested in these because the computer I built in 2014 has a 14nm processor too... albeit with DDR 3 RAM but come on, DDR4 isn't even much of a real world difference outside ultra specific niche scenarios.

    Intel, this is ridiculous, you're going to have been on the SAME NODE for a DECADE HERE!!!!

    Crying out loud 10nm has been around for longer than Intels 14nm, this is nuts!
    Reply
  • James5mith - Saturday, April 3, 2021 - link

    " More and more NAS and routers are coming with one or more 2.5 GbE ports as standard"

    No, they most definitely are not. lol
    Reply
  • Linustechtips12#6900xt - Monday, April 5, 2021 - link

    gotta say, love the arguments on page 9 lol Reply
  • peevee - Monday, April 5, 2021 - link

    "the latest microcode from Intel should help increase performance and cache latency"

    Do we really want the increase in cache latency? ;) :)
    Reply
  • statisticsexplained - Monday, April 12, 2021 - link

    I really like this stuff. you are providing nice content in a appropriate way. Reply
  • statisticsexplained - Monday, April 12, 2021 - link

    We also provide service online. can you please come and have a look on our service also Reply

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