Balancing The System With Other Hardware Features

The biggest technological advantage consoles have over PCs is that consoles are a fully-integrated fixed platform specified by a single manufacturer. In theory, the manufacturer can ensure that the system is properly balanced for the use case, something PC OEMs are notoriously bad at. Consoles generally don't have the problem of wasting a large chunk of the budget on a single high-end component that the rest of the system cannot keep up with, and consoles can more easily incorporate custom hardware when suitable off-the-shelf components aren't available. (This is why the outgoing console generation didn't use desktop-class CPU cores, but dedicated a huge amount of the silicon budget to the GPUs.)

By now, PC gaming has thoroughly demonstrated that increasing SSD speed has little or no impact on gaming performance. NVMe SSDs are several times faster than SATA SSDs on paper, but for almost all PC games that extra performance goes largely unused. In part, this is due to bottlenecks elsewhere in the system that are revealed when storage performance is fast enough to no longer be a serious limitation. The upcoming consoles will include a number of hardware features designed to make it easier for games to take advantage of fast storage, and to alleviate bottlenecks that would be troublesome on a standard PC platform. This is where the console storage tech gets actually interesting, since the SSDs themselves are relatively unremarkable.

Compression: Amplifying SSD Performance

The most important specialized hardware feature the consoles will include to complement storage performance is dedicated data decompression hardware. Game assets must be stored on disk in a compressed form to keep storage requirements somewhat reasonable. Games usually rely on multiple compression methods—some lossy compression methods specialized for certain types of data (eg. audio and images), and some lossless general-purpose algorithm, but almost everything goes through at least one compression method that is fairly computationally complex. GPU architectures have long included hardware to handle decoding video streams and support simple but fast lossy texture compression methods like S3TC and its successors, but that leaves a lot of data to be decompressed by the CPU. Desktop CPUs don't have dedicated decompression engines or instructions, though many instructions in the various SIMD extensions are intended to help with tasks like this. Even so, decompressing a stream of data at several GB/s is not trivial, and special-purpose hardware can do it more efficiently while freeing up CPU time for other tasks. The decompression offload hardware in the upcoming consoles is implemented on the main SoC so that it can unpack data after it traverses the PCIe link from the SSD and resides in the main RAM pool shared by the GPU and CPU cores.

Decompression offload hardware like this isn't found on typical desktop PC platforms, but it's hardly a novel idea. Previous consoles have included decompression hardware, though nothing that would be able to keep pace with NVMe SSDs. Server platforms often include compression accelerators, usually paired with cryptography accelerators: Intel has done such accelerators both as discrete peripherals and integrated into some server chipsets, and IBM's POWER9 and later CPUs have similar accelerator units. These server accelerators more comparable to what the new consoles need, with throughput of several GB/s.

Microsoft and Sony each have tuned their decompression units to fit the performance expected from their chosen SSD designs. They've chosen different proprietary compression algorithms to target: Sony is using RAD's Kraken, a general-purpose algorithm which was originally designed to be used on the current consoles with relatively weak CPUs but vastly lower throughput requirements. Microsoft focused specifically on texture compression, reasoning that textures account for the largest volume of data that games need to read and decompress. They developed a new texture compression algorithm and dubbed it BCPack in a slight departure from their existing DirectX naming conventions for texture compression methods already supported by GPUs.

Compression Offload Hardware
  Microsoft
Xbox Series X
Sony
Playstation 5
Algorithm BCPack Kraken (and ZLib?)
Maximum Output Rate 6 GB/s 22 GB/s
Typical Output Rate 4.8 GB/s 8–9 GB/s
Equivalent Zen 2 CPU Cores 5 9

Sony states that their Kraken-based decompression hardware can unpack the 5.5GB/s stream from the SSD into a typical 8-9 GB/s of uncompressed data, but that can theoretically reach up to 22 GB/s if the data was redundant enough to be highly compressible. Microsoft states their BCPack decompressor can output a typical 4.8 GB/s from the 2.4 GB/s input, but potentially up to 6 GB/s. So Microsoft is claiming slightly higher typical compression ratios, but still a slower output stream due to the much slower SSD, and Microsoft's hardware decompression is apparently only for texture data.

The CPU time saved by these decompression units sounds astounding: the equivalent of about 9 Zen 2 CPU cores for the PS5, and about 5 for the Xbox Series X. Keep in mind these are peak numbers that assume the SSD bandwidth is being fully utilized—real games won't be able to keep these SSDs 100% busy, so they wouldn't need quite so much CPU power for decompression.

The storage acceleration features on the console SoCs aren't limited to just compression offload, and Sony in particular has described quite a few features, but this is where the information released so far is really vague, unsatisfying and open to interpretation. Most of this functionality seems to be intended to reduce overhead, handling some of the more mundane aspects of moving data around without having to get the CPU involved as often, and making sure the hardware decompression process is invisible to the game software.

DMA Engines

Direct Memory Access (DMA) refers to the ability for a peripheral device to read and write to the CPU's RAM without the CPU being involved. All modern high-speed peripherals use DMA for most of their communication with the CPU, but that's not the only use for DMA. A DMA Engine is a peripheral device that exists solely to move data around; it usually doesn't do anything to that data. The CPU can instruct the DMA engine to perform a copy from one region of RAM to another, and the DMA engine does the rote work of copying potentially gigabytes of data without the CPU having to do a mov (or SIMD equivalent) instruction for every piece, and without polluting CPU caches. DMA engines can also often do more than just offload simple copy operations: they commonly support scatter/gather operations to rearrange data somewhat in the process of moving it around. NVMe already has features like scatter/gather lists that can remove the need for a separate DMA engine to provide that feature, but the NVMe commands in these consoles are acting mostly on compressed data.

Even though DMA engines are a peripheral device, you usually won't find them as a standalone PCIe card. It makes the most sense for them to be as close to the memory controller as possible, which means on the chipset or on the CPU die itself.The PS5 SoC includes a DMA engine to handle copying around data coming out of the compression unit. As with the compression engines, this isn't a novel invention so much as a feature missing from standard desktop PCs, which means it's something custom that Sony has to add to what would otherwise be a fairly straightforward AMD APU configuration.

IO Coprocessor

The IO complex in the PS5's SoC also includes a dual-core processor with its own pool of SRAM. Sony has said almost nothing about the internals of this: Mark Cerny describes one core as dedicated to SSD IO, allowing games to "bypass traditional file IO", while the other core is described simply as helping with "memory mapping". For more detail, we have to turn to a patent Sony filed years ago, and hope it reflects what's actually in the PS5.

The IO coprocessor described in Sony's patent offloads portions of what would normally be the operating system's storage drivers. One of its most important duties is to translate between various address spaces. When the game requests a certain range of bytes from one of its files, the game is looking for the uncompressed data. The IO coprocessor figures out which chunks of compressed data are needed and sends NVMe read commands to the SSD. Once the SSD has returned the data, the IO coprocessor sets up the decompression unit to process that data, and the DMA engine to deliver it to the requested locations in the game's memory.

Since the IO coprocessor's two cores are each much less powerful than a Zen 2 CPU core, they cannot be in charge of all interaction with the SSD. The coprocessor handles the most common cases of reading data, and the system falls back to the OS running on the Zen 2 cores for the rest. The coprocessor's SRAM isn't used to buffer the vast amounts of game data flowing through the IO complex; instead this memory holds the various lookup tables used by the IO coprocessor. In this respect, it is similar to an SSD controller with a pool of RAM for its mapping tables, but the job of the IO coprocessor is completely different from what an SSD controller does. This is why it will be useful even with aftermarket third-party SSDs.

Cache Coherency

The last somewhat storage-related hardware feature Sony has disclosed is a set of cache coherency engines. The CPU and GPU on the PS5 SoC share the same 16 GB of RAM, which eliminates the step of copying assets from main RAM to VRAM after they're loaded from the SSD and decompressed. But to get the most benefit from the shared pool of memory, the hardware has to ensure cache coherency not just between the several CPU cores, but also with the GPU's various caches. That's all normal for an APU, but what's novel with the PS5 is that the IO complex also participates. When new graphics assets are loaded into memory through the IO complex and overwrite older assets, it sends cache invalidation signals to any relevant caches—to discard only the stale data, rather than flush the entire GPU caches.

What about the Xbox Series X?

There's a lot of information above about the Playstation 5's custom IO complex, and it's natural to wonder whether the Xbox Series X will have similar capabilities or if it's limited to just the decompression hardware. Microsoft has lumped the storage-related technologies in the new Xbox under the heading of "Xbox Velocity Architecture":

Microsoft defines this as having four components: the SSD itself, the compression engine, a new software API for accessing storage (more on this later), and a hardware feature called Sampler Feedback Streaming. That last one is only distantly related to storage; it's a GPU feature that makes partially resident textures more useful by allowing shader programs to keep a record of which portions of a texture are actually being used. This information can be used to decide what data to evict from RAM and what to load next—such as a higher-resolution version of the texture regions that are actually visible at the moment.

Since Microsoft doesn't mention anything like the other PS5 IO complex features, it's reasonable to assume the Xbox Series X doesn't have those capabilities and its IO is largely managed by the CPU cores. But I wouldn't be too surprised to find out the Series X has a comparable DMA engine, because that's kind of feature has historically shown up in many console architectures.

SSD Details: Xbox Series X and Playstation 5 What To Expect From Next-gen Games
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  • JStacts - Tuesday, June 30, 2020 - link

    "The problem is that upgrading is a necessity." Upgrading a computer is no more necessary that buying a new console (I would argue less). The difference is that the PC game can upgrade when he wants and is not forced into a cycle by a multi-national corporation. I built my computer a few years ago with an RX480. Could I upgrade? Sure. I could have upgraded last year, or the year before. I could upgrade next year.

    It is entirely my choice when or even if I upgrade. It is my choice how much to spend on the upgrade. It is my choice because of the incredible wealth of options offered by the PC market.

    If I had a PS4, I had to wait until Sony CHOSE to release the Pro version (which they had no obligation to do) or the PS5.

    Next year, I will very probably buy a couple monitors and upgrade my GPU, but I certainly don't feel compelled to do so. I enjoy my gaming experience.

    It is this difference of choice that makes my say that "I get to upgrade" instead of "I have to upgrade".

    You mentioned that we (PC gamers) spend thousands to keep up with the state of software. While there certainly are those people (and you will meet a disproportionate number of them in the AT comment section), I would hardly call that a fair generalization of the PC gaming community. Most of us keep our components for many years before upgrading. Most of us are perfectly happy buying low or midrage parts and the gaming experiences those offer. A significant portion of us use very low end hardware.

    If you're not already acquainted with the channel, I recommend "The LowSpec Gamer" on Youtube. His entire channel is dedicated to making AAA games run on extremely low-end hardware.
    Reply
  • Oxford Guy - Monday, June 15, 2020 - link

    "What makes a console a good piece of hardware is that I do not have to even care what is inside the box that runs the games since the games that I buy for said console will just work as intended. Every generation has extensive discussion about hardware and the usual comparing and penis measuring between brands. It all ends up being meaningless when you pick up a controller and play a game on it which is far and above the hassle of PC gaming in terms of equipment apathy. In short, IDGAF what drive is inside the thing. Insert disc (or download digital copy) and game on!"

    Thank you MS or Sony employee for this attempt to justify the "console" scam.
    Reply
  • zmatt - Tuesday, June 16, 2020 - link

    Adding internal storage and internet connectivity destroyed what I saw as the two big advantages with consoles. It used to be that you didn't have to install anything and you didn't have to patch anything. Developers were forced to released games as complete and working products and playing them was truly as simple as sticking a disc in and playing.

    Sure load times and graphics suffered because of it but consoles shouldn't try to compete with PCs on performance anyways. Its a fool's errand. Back then you could truly say consoles just worked. Having to wait for games to install, having to manage updates and dealing with buggy releases were largely problems that PC users had to put up with.

    Now consoles have all of those problems too. You can't seem to power on a console today without first waiting for it to update before letting you do anything. And unlike on a PC where you have control of such things the console will force you to update both it and the games before letting you play them. The only way around it is to disconnect networking entirely which renders half of its functionality broken.

    If I can't just slap in a disc and play then why bother? If I have to expend effort then I might as well get the full benefit of a PC. Not this halfway point consoles have reached.
    Reply
  • PeachNCream - Tuesday, June 16, 2020 - link

    I think its actually a good thing that consoles have access to internet-based patching and multiplayer capabilities as well. There is a lot more code in a modern game than was the case with cartridge-based systems and early even the first couple of generations of disc-based consoles. Deploying an update is a reality of modern software we just have to cope with and work around these days regardless of the platform.

    In addition, if you keep an eye on the ROM hacking and modding community, you will see there are still hobbyists out there finding and fixing bugs that are present in 8- and 16-bit era cartridge based games that have a total memory footprint of a few hundred kilobytes to a single digit number of megabytes due to technology and cost limits of the time period. (Lots of Final Fantasy 3/6 fixes for the SNES for example).
    Reply
  • Gastec - Sunday, January 3, 2021 - link

    Then why are you here, reading this article and bothering to comment? Go...play video games on consoles with that amazing input device you call "controller". It just works! Reply
  • DrogoAnthonyson - Thursday, November 11, 2021 - link

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    https://coimobile.com/minecraft/
    Reply
  • ToTTenTranz - Friday, June 12, 2020 - link

    Small correction at the end of page 1:

    Cerny said the 3rd party SSDs will need to be faster than the PS5's rated 5.5GB/s, because consumer SSDs only have 2 priority lanes, whereas the PS5's has 6 lanes.
    So the extra speed is needed to compensate for a less effective system of I/O interrupts. A 3rd party 5.5GB/s SSD is supposedly not capable enough to put in the PS5 as extra storage.
    Reply
  • close - Friday, June 12, 2020 - link

    I wonder if there will be a market for after market SSDs that do match (or get sufficiently close to) the performance of the built in one. Also if MS plans on introducing a higher performance builtin SSD in future model refreshes. Reply
  • ToTTenTranz - Friday, June 12, 2020 - link

    Even if Microsoft does introduce a higher performance SSD in later models, the baseline is set in stone with this 2.4GB/s + BCPack & Zlib architecture. Reply
  • brucethemoose - Friday, June 12, 2020 - link

    They could easily do another mid gen update with a faster SSD. Reply

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