Ryzen 5, Core Allocation, and Power

In our original review of Ryzen 7, we showed that the underlying silicon design of the Ryzen package consists of a single eight-core Zeppelin die with Zen microarchitecture cores.

The silicon design consists of two core complexes (CCX) of four cores apiece. Each CCX comes with 512 KB of L2 cache per core, which is disabled when a core is disabled, and each CCX has 8MB of L3 cache which can remain enabled even when cores are disabled. This L3 cache is an exclusive victim cache, meaning that it only accepts evicted L2 cache entries, rather than loading data straight into it (which is how Intel builds their current L3 cache designs).

One of the suggestions regarding Ryzen 7’s performance was about thread migration and scheduling on the core design, especially as core-to-core latency varies depending on where the cores are located (and there’s a jump between CCXes). Despite the use of AMD’s new Infinity Fabric, which is ultimately a superset of HyperTransport, there is still a slightly longer delay jumping over that CCX boundary, although the default Windows scheduler knows how to manage that boundary as demonstrated by Allyn at PCPerspective.

So when dealing with a four-core or six-core CPU, and the base core design has eight-cores, how does AMD cut them up? It is possible for AMD to offer a 4+0, 3+1 or 2+2 design for its quad-core parts, or 4+2 and 3+3 variants for its hexacore parts, similar to the way that Intel cuts up its integrated graphics for GT1 variants.

There are some positives and negatives to each configuration, some of which we have managed to view through this review. The main downside from high level to a configuration split across CCXes, such as a 2+2 or 3+3, is that CCX boundary. Given that the Windows scheduler knows how to deal with this means this is less of an issue, but it is still present.

There are a couple of upsides. Firstly is related to binning – if the 2+2 chips didn’t exist, and AMD only supported 4+0 configurations, then if the binning of such processors was down to silicon defects, fewer silicon dies would be able to be used, as one CCX would have to be perfect. Depending on yield this may or may not be an issue to begin with, but having a 2+2 (and AMD states that all 2+2 configs will be performance equivalent) means more silicon available, driving down cost by having more viable CPUs per wafer out of the fabs.

Secondly, there’s the power argument. Logic inside a processor expends energy, and more energy when using a higher voltage/frequency. When placing lots of high-energy logic next to each other, the behavior becomes erratic and the logic has to reduce in voltage/frequency to remain stable. This is why AVX/AVX2 from Intel causes those cores to run at a lower frequency compared to the rest of the core. A similar thing can occur within a CCX: if all four cores of a CCX are loaded (and going by Windows Scheduler that is what happens in order), then the power available to each core has to be reduced to remain stable. Ideally, if there’s no cross communication between threads, you want the computation to be in opposite cores as threads increase. This is not a new concept – some core designs intentionally put in ‘dark silicon’ - silicon of no use apart from providing extra space/area between high power consuming logic. By placing the cores in a 2+2 and 3+3 design for Ryzen 5, this allows the cores to run at a higher power than if they were in 4+0 and 4+2 configurations.

Here’s some power numbers to show this. First, let’s start with a core diagram.

Where exactly the 0/1/2/3 cores are labelled is not really important, except 0-3 are in one CCX and 4-7 are in another CCX. As we load up the cores with two threads each, we can see the power allocation change between them. It is worth noting that the Ryzen cores have a realistic voltage/frequency limit near 4.0-4.1 GHz due to the manufacturing process – getting near or above this frequency requires a lot of voltage, which translates into power.

First up is the 1800X, which is a 4+4 configuration with a maximum TDP of 95W. One fully loaded core gets 22.6W, and represents the core at its maximum frequency with XFR also enabled. The same thing happens with two cores fully loaded, but at 20.6 W apiece. Moving onto three cores loaded is where XFR is disabled, and we see the drop to 3.7 GHz is saving power, as we only consume +1.33W compared to the two cores loaded situation. Three to four cores, still all on the same CCX, shows a decrease in power per core.

As we load up the first core of the second CCX, we see an interesting change. The core on CCX-2 has a bigger power allocation than any core in CCX-1. This can be interpreted in two ways: there is more dark silicon around, leading to a higher potential for this core on CCX-2, or that more power is required given the core is on its own. Technically it is still running at the same frequency as the cores on CCX1. Now as we populate the cores on CCX-2, they still consume more power per core until we hit the situation where all cores are loaded and the system is more or less equal.

Moving to the Ryzen 5 1600X, which is a 3+3 configuration, nets more of the same. During XFR with one or two cores loaded, the power consumption is high. As we move onto the second CCX, the cores on CCX-2 consumer more power per core than those already loaded on CCX-1.

It is worth noting here that the jump from two cores loaded to three cores loaded on the 3+3 gives a drop in the total power consumption of the cores. Checking my raw data numbers, and this also translates to a total package power drop as well, showing how much extra effort it is to run these cores near 4.0 GHz with XFR enabled.

On the Ryzen 5 1500X, using a 2+2 configuration, the situation is again duplicated. The hard comparison here is the 2+2 of the 1500X to the 4+0 on the 1800X, because the TDP of each of the processors is different.

It should be noted however the total package power consumption (cores plus IO plus memory controller and so on) is actually another 10W or so above these numbers per chip.  

Power: Cores Only (Full Load)

The cache configurations play an important role in the power consumption numbers as well. In a 3+3 or a 2+2 configuration, despite one or two cores per CCX being disabled, the L3 cache is still fully enabled in these processors. As a result, cutting 25% of the cores doesn’t cut 25% of the total core power, depending on how the L3 cache is being used.

Nonetheless, the Ryzen 5 1600X, despite being at the same rated TDP as the Ryzen 7 1800X, does not get close to matching the power consumption. This ropes back into the point at the top of the page – usually we see fewer cores giving a higher frequency to match the power consumption with parts that have more cores. Because the silicon design has such a high barrier to get over 4.0 GHz with voltage and power, AMD has decided that it is too big a jump to remain stable, but still given the 1600X the higher TDP rating anyway. This may be a nod to the fact that it will cause users to go out and buy bigger cooling solutions, providing sufficient headroom for Turbo modes and XFR, giving better performance.

Despite this, we see the 1800X and 1500X each tear their TDP rating for power consumption (92W vs 95W and 67W vs 65W respectively).

However, enough talking about the power consumption. Time for benchmarks!

MOAR CORES Test Bed Setup and Hardware
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  • Notmyusualid - Tuesday, April 18, 2017 - link

    @zenith

    Go ahead and release a DX12-only game.

    Let me know how you get on with sales...
    Reply
  • Arbie - Wednesday, April 12, 2017 - link

    When I do upgrade it will be with AMD. Even where (and if) Intel offers a little more performance per dollar, AMD has amazingly reduced the difference to the point where I can accept it in order to help fuel competition. If the market does not reward AMD for their valiant effort in Zen, the company may be forced to give up. It seems impossible for them to come from behind yet again in such a high-stakes arena. Then Intel will really slack off, and several years from now we'll ALL be worse off than if they were still duking it out.

    Everyone has to make their own decision, and I couldn't buy the Excavator etc fiascos, but the AMD product is now a real contender - and we need to keep them there.
    Reply
  • bodonnell - Wednesday, April 12, 2017 - link

    Agreed. I updated my main rig a couple years ago and Intel was really the only option at the time, but if I was in the market now I would definitely be looking at a Ryzen 5 as keeping AMD around is better for consumers. For the money where Ryzen 5 lags it doesn't lag by much (and honestly legacy software that is single threaded was made to work on much lower performance cores) and where it shines (multi-threaded performance) it often beats price comparable Intel processors by a healthy margin. Reply
  • npz - Wednesday, April 12, 2017 - link

    "*A note on Ashes. During our testing, the 2.2 update came through automatically, and broke our scripting methods due to a new splashscreen/popup. We worked to find a solution that worked one minute, and then stopped working 30 minutes later, and it was decided due to time limits that we'd look into the matter after the review."

    Will you be doing an updated review of Ryzen following the recent optimizations that Ashes on Singularity, a few other games have made, and the upcoming Bethesda optimizations (i.e. Quake Champions)? I suspect we'll see optimizations trickle in to other game engines.
    Reply
  • BrokenCrayons - Wednesday, April 12, 2017 - link

    Ryzen 5 is an interesting CPU, worth a careful look given the outcome of the benchmarks in this article. Modern workloads seem to be much more likely to use more than one thread and legacy workloads that are single threaded would perform perfectly well on just about any modern CPU so it really isn't a difficult choice to look into a Ryzen 5 if you fall into its price bracket. AMD's APU offerings in the future might offer a better value for some customers since the price of a Ryzen CPU doesn't currently include graphics. People happy with iGPU performance would either require a dedicated graphics card purchase or reuse one thy already have available to build a complete system around a Zen-based processor so those sorts might be better off waiting until the APU versions are released later this year or they might be compelled to purchase a competing Intel product with an iGPU. Reply
  • bodonnell - Wednesday, April 12, 2017 - link

    Can't wait to see what AMD does with the Zen core in the mainstream and mobile markets. A well balanced quad core design with a good Polaris based iGPU will be all most consumers need for their day to day use. Reply
  • ehfield7 - Wednesday, April 12, 2017 - link

    Is there a reason you don't put in the 7700k in these charts? I mean if you're going to put in 1700X and 1800X, you should put in 7700k too. Plus at just $80 more it's honestly a CPU being considered too people consider the 1600X. Reply
  • vladx - Wednesday, April 12, 2017 - link

    Reason is obvious, anandTech have an AMD bias. Reply
  • Outlander_04 - Thursday, April 13, 2017 - link

    Unlike yourself, and your well respected neutrality ? Reply
  • vladx - Thursday, April 13, 2017 - link

    Yes as someone with both a 7700k system and a 1700X system I can safely call myself unbiased as I hold no special loyalty towards any brand. Reply

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