For the launch of AMD’s Ryzen Threadripper processors, one of the features being advertised was Game Mode. This was a special profile under the updated Ryzen Master software that was designed to give the Threadripper CPU more performance in gaming, at the expense of peak performance in hard CPU tasks. AMD’s goal, as described to us, was to enable the user to have a choice: a CPU that can be fit for both CPU tasks and for gaming at the flick of a switch (and a reboot) by disabling half of the chip.

Initially, we interpreted this via one of AMD’s slides as half of the threads (simultaneous multi-threading off), as per the exact wording. However, in other places AMD had stated that it actually disables half the cores: AMD returned to us and said it was actually disabling one of the two active dies in the Threadripper processor. We swallowed our pride and set about retesting the effect of Game Mode.

A Rose By Any Other Name

It’s not very often we have to retract some of our content at AnandTech – research is paramount. However in this instance a couple of things led to confusion. First was assumption related: in the original piece, we had assumed that AMD was making Game mode available through both the BIOS and through the Ryzen Master software. Second was communication: AMD had described Game Mode (and specifically, the Legacy Compatibility Mode switch it uses) at the pre-briefing at SIGGRAPH as having half the threads, but offered in diagrams that it was half the cores.

Based on the wording, we had interpreted that this was the equivalent of SMT being disabled, and adjusted the BIOS as such. After our review went live, AMD published and also reached out to us to inform of the error: where we had tested the part of Game Mode that deals with legacy core counts, we had disabled SMT rather than disabling a die and made the 16C/32T into to a 16C/16T system rather than an 8C/16T system. We were informed that the settings that deal with this feature are more complex than simply SMT being disabled, and as such was being offered primarily through Ryzen Master.

From AMD's Gaming Blog. Emphasis ours.

So for this review, we’re going to set the record straight, and test Threadripper in its Game Mode 8C/16T version. The previous review will be updated appropriately.

So What Is Game Mode?

For Ryzen Threadripper, AMD has defined two modes of operation depending on the use case. The first is Creator Mode, which is enabled by default. This enables full cores, full threads, and gives the maximum available bandwidth across the two active Threadripper silicon dies in the package, at the expense of some potential peak latency. In our original review, we measured the performance of Creator Mode in our benchmarks as the default setting, but also looked into the memory latency.

Each die can communicate to all four memory channels, but is only directly connected to two memory channels. Depending on where the data in DRAM is located, a core may have to search in near memory (the two channels closest) or far memory (the two channels over). This is commonly referred to a non-uniform memory architecture (NUMA). In a unified memory system (UMA), such as Creator mode, the system sees no difference between near memory and far memory, citing a single latency value for both which is typically the average between the near latency and the far latency. At DDR4-2400, we recorded this as 108 nanoseconds.

Game Mode does two things over Creator Mode. First, it changes the memory from UMA to NUMA, so the system can determine between near and far memory. At DDR4-2400, that 108ns ‘average’ latency becomes 79ns for near memory and 136ns for far memory (as per our testing). The system will ensure to use up all available near memory first, before moving to the higher latency far memory.

Second, Game Mode disables the cores in one of the silicon dies. This isn’t a full shutdown of the 8-core Zeppelin die, just the cores. The PCIe lanes, the DRAM channels and the various IO are still active, but the cores themselves are power gated such that the system does not use them or migrate threads to them. In essence, the 16C/32T processor becomes 8C/16T, but with quad-channel memory and 60 PCIe lanes still: the 1950X becomes an uber 1800X, and the 1920X becomes an uber 1600X. The act of disabling dies is called ‘Legacy Compatibility Mode’, which ensures that all active cores have access to near memory at the expensive of immediate bandwidth but enables games that cannot handle more than 20 cores (some legacy titles) to run smoothly.

The core count on the left is the absolute core count, not the core count in Game Mode. Which is confusing.​

Some users might see paying $999 for a processor then disabling almost half of it as a major frustration (insert something about Intel integrated graphics). AMD’s argument is that the CPU is still good for gaming, and can offer a better gaming experience when given the choice. However if we consider the mantra surrounding these big processors around gaming adaptability: the ability to stream, transcode and game at the same time. It’s expected that in this mega-tasking (Intel’s term) scenario, having a beefy CPU helps even though there will be some game losses. Moving down to only 8 cores is likely to make this worse, and the only situation Game Mode assists is for a user who purely wants a gaming machine but quad-channel memory and all the PCIe lanes. There’s also a frequency argument – in a dual die configuration, active threads can be positioned at thermally beneficial points of the design to ensure the maximum frequency. Again, AMD reiterates that it offers choice, and users who want to stick with all or half the cores are free to do so, as this change in settings would have been available in BIOS even if AMD did not give a quick button to it.

As always, the proof is in the pudding. If there’s a significant advantage to gaming, then Game Mode will be a plus point in AMD’s cap.

With regards how the memory and memory latency operates, Game Mode still incorporates NUMA, ensuring near memory is used first. The memory latency results are still the same as we tested before:

For the 1950X in the two modes, the results are essentially equal until we hit 8MB, which is the L3 cache limit per CCX. After this, the core bounces out to main memory, where the Game mode sits around 79ns when it probes near memory while the Creator mode is at 108 ns average. By comparison the Ryzen 5 1600X seems to have a lower latency at 8MB (20ns vs 41 ns), and then sits between the Creator and Game modes at 87 ns. It would appear that the bigger downside of Creator mode in this way is the fact that main memory accesses are much slower than normal Ryzen or in Game mode.

If we crank up the DRAM frequency to DDR4-3200 for the Threadripper 1950X, the numbers change a fair bit:

Click for larger image

Up until the 8MB boundary where L3 hits main memory, everything is pretty much equal. At 8MB however, the latency at DDR4-2400 is 41ns compared to 18ns at DDR4-3200. Then out into full main memory sees a pattern: Creator mode at DDR4-3200 is close to Game Mode at DDR4-2400 (87ns vs 79ns), but taking Game mode to DDR4-3200 drops the latency down to 65ns.

Testing, Testing, One Two One Two

In our last review, we put the CPU in NUMA mode and disabled SMT. Both of the active dies were still active, although each thread had full CPU resources, and each set of CPUs would communicate to the nearest memory, however there would be potential die-to-die communication and more potential for far-memory access.

In this new testing, we use Ryzen Master to Game Mode, which enables NUMA and disables one of the silicon dies giving 8 cores and 16 threads. 

Related Reading

Test Bed and Setup
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  • peevee - Friday, August 18, 2017 - link

    Of course. Work CPUs must be tested at work. Kiddies are fine with i3s.
  • Ian Cutress - Sunday, August 20, 2017 - link hacking news hacking tutorials hacking ebooks
  • IGTrading - Thursday, August 17, 2017 - link

    It would be nice and very useful to post some power consumption results at the platform level, if we're doing "extra" additional testing.

    It is very important since we're paying for the motherboard just as much as we pay for a Ryzen 5 or even Ryzen 7 processor.

    And it will correctly compare the TCO of the X399 platform with the TCO of X299.
  • jordanclock - Thursday, August 17, 2017 - link

    So it looks like AMD should have gone with just disabling SMT for Game Mode. There are way more benefits and it is easier to understand the implications. I haven't seen similar comparisons for Intel in a while, perhaps that can be exploration for Skylake-X as well?
  • HStewart - Thursday, August 17, 2017 - link

    I would think disable SMT would be better, but the reason maybe in designed of link between the two 8 Core dies on chip.
  • GruenSein - Thursday, August 17, 2017 - link

    I'd really love to see a frame time probability distribution (Frame time on x-axis, rate of occurrence on y-axis). Especially in cases with very unlikely frames below a 60Hz rate, the difference between TR and TR-GM/1800X seem most apparent. Without the distribution, we will never know if we are seeing the same distribution but slightly shifted towards lower frame rates as the slopes of the distribution might be steep. However, those frames with frame times above a 60Hz rate might be real stutters down to a 30Hz rate but they might just as well be frames at a 59,7Hz rate. I realize why this threshold was selected but every threshold is quite arbitrary.
  • MrSpadge - Thursday, August 17, 2017 - link

    Does AMD comment on the update? What's their reason for choosing 8C/16T over 16C/16T?

    > One could postulate that Windows could do something similar with the equivalent of hyperthreads.

    They're actually already doing that. Loading 50% of all threads on an SMT machine will result in ~50% average load on every logical core, i.e. all physical cores are only working on 1 thread at a time.

    I know mathematically other schedulings are possible, leading to the same result - but by now I think it's common knowledge that the default Win scheduler works like that. Hence most lightly threaded software is indifferent to SMT. Except games.
  • NetMage - Sunday, August 20, 2017 - link

    Then why did SMT mode show differences from Creator mode in the original review?
  • Dribble - Thursday, August 17, 2017 - link

    No one is ever going to run game mode - why buy a really expensive chip and then disable half of it, especially as you have to reboot to do it? It's only use is to make threadripper look slightly better in reviews. Imo it would be more honest as a reviewer to just run it in creator mode all the time.
  • jordanclock - Thursday, August 17, 2017 - link

    The point is compatibility, as mentioned in the article multiple times. AMD is offering this as an option for applications (mainly games) that do not run correctly, if at all, on >16 core CPUs.

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