Performance - An Update

The Chipworks PS4 teardown last week told us a lot about what’s happened between the Xbox One and PlayStation 4 in terms of hardware. It turns out that Microsoft’s silicon budget was actually a little more than Sony’s, at least for the main APU. The Xbox One APU is a 363mm^2 die, compared to 348mm^2 for the PS4’s APU. Both use a similar 8-core Jaguar CPU (2 x quad-core islands), but they feature different implementations of AMD’s Graphics Core Next GPUs. Microsoft elected to implement 12 compute units, two geometry engines and 16 ROPs, while Sony went for 18 CUs, two geometry engines and 32 ROPs. How did Sony manage to fit in more compute and ROP partitions into a smaller die area? By not including any eSRAM on-die.

While both APUs implement a 256-bit wide memory interface, Sony chose to use GDDR5 memory running at a 5.5GHz data rate. Microsoft stuck to more conventionally available DDR3 memory running at less than half the speed (2133MHz data rate). In order to make up for the bandwidth deficit, Microsoft included 32MB of eSRAM on its APU in order to alleviate some of the GPU bandwidth needs. The eSRAM is accessible in 8MB chunks, with a total of 204GB/s of bandwidth offered (102GB/s in each direction) to the memory. The eSRAM is designed for GPU access only, CPU access requires a copy to main memory.

Unlike Intel’s Crystalwell, the eSRAM isn’t a cache - instead it’s mapped to a specific address range in memory. And unlike the embedded DRAM in the Xbox 360, the eSRAM in the One can hold more than just a render target or Z-buffer. Virtually any type of GPU accessible surface/buffer type can now be stored in eSRAM (e.g. z-buffer, G-buffer, stencil buffers, shadow buffer, etc…). Developers could also choose to store things like important textures in this eSRAM as well, there’s nothing that states it needs to be one of these buffers just anything the developer finds important. It’s also possible for a single surface to be split between main memory and eSRAM.

Obviously sticking important buffers and other frequently used data here can definitely reduce demands on the memory interface, which should help Microsoft get by with only having ~68GB/s of system memory bandwidth. Microsoft has claimed publicly that actual bandwidth to the eSRAM is somewhere in the 140 - 150GB/s range, which is likely equal to the effective memory bandwidth (after overhead/efficiency losses) to the PS4’s GDDR5 memory interface. The difference being that you only get that bandwidth to your most frequently used data on the Xbox One. It’s still not clear to me what effective memory bandwidth looks like on the Xbox One, I suspect it’s still a bit lower than on the PS4, but after talking with Ryan Smith (AT’s Senior GPU Editor) I’m now wondering if memory bandwidth isn’t really the issue here.

Microsoft Xbox One vs. Sony PlayStation 4 Spec comparison
  Xbox 360 Xbox One PlayStation 4
CPU Cores/Threads 3/6 8/8 8/8
CPU Frequency 3.2GHz 1.75GHz 1.6GHz
CPU µArch IBM PowerPC AMD Jaguar AMD Jaguar
Shared L2 Cache 1MB 2 x 2MB 2 x 2MB
GPU Cores   768 1152
GCN Geometry Engines   2 2
GCN ROPs   16 32
GPU Frequency   853MHz 800MHz
Peak Shader Throughput 0.24 TFLOPS 1.31 TFLOPS 1.84 TFLOPS
Embedded Memory 10MB eDRAM 32MB eSRAM -
Embedded Memory Bandwidth 32GB/s 102GB/s bi-directional (204GB/s total) -
System Memory 512MB 1400MHz GDDR3 8GB 2133MHz DDR3 8GB 5500MHz GDDR5
System Memory Bus 128-bits 256-bits 256-bits
System Memory Bandwidth 22.4 GB/s 68.3 GB/s 176.0 GB/s
Manufacturing Process   28nm 28nm

In order to accommodate the eSRAM on die Microsoft not only had to move to a 12 CU GPU configuration, but it’s also only down to 16 ROPs (half of that of the PS4). The ROPs (render outputs/raster operations pipes) are responsible for final pixel output, and at the resolutions these consoles are targeting having 16 ROPs definitely puts the Xbox One as the odd man out in comparison to PC GPUs. Typically AMD’s GPU targeting 1080p come with 32 ROPs, which is where the PS4 is, but the Xbox One ships with half that. The difference in raw shader performance (12 CUs vs 18 CUs) can definitely creep up in games that run more complex lighting routines and other long shader programs on each pixel, but all of the more recent reports of resolution differences between Xbox One and PS4 games at launch are likely the result of being ROP bound on the One. This is probably why Microsoft claimed it saw a bigger increase in realized performance from increasing the GPU clock from 800MHz to 853MHz vs. adding two extra CUs. The ROPs operate at GPU clock, so an increase in GPU clock in a ROP bound scenario would increase performance more than adding more compute hardware.

The PS4's APU - Courtesy Chipworks

Microsoft’s admission that the Xbox One dev kits have 14 CUs does make me wonder what the Xbox One die looks like. Chipworks found that the PS4’s APU actually features 20 CUs, despite only exposing 18 to game developers. I suspect those last two are there for defect mitigation/to increase effective yields in the case of bad CUs, I wonder if the same isn’t true for the Xbox One.

At the end of the day Microsoft appears to have ended up with its GPU configuration not for silicon cost reasons, but for platform power/cost and component availability reasons. Sourcing DDR3 is much easier than sourcing high density GDDR5. Sony managed to obviously launch with a ton of GDDR5 just fine, but I can definitely understand why Microsoft would be hesitant to go down that route in the planning stages of Xbox One. To put some numbers in perspective, Sony has shipped 1 million PS4s thus far. That's 16 million GDDR5 chips, or 7.6 Petabytes of RAM. Had both Sony and Microsot tried to do this, I do wonder if GDDR5 supply would've become a problem. That's a ton of RAM in a very short period of time. The only other major consumer of GDDR5 are video cards, and the number of cards sold in the last couple of months that would ever use that RAM is a narrow list. 

Microsoft will obviously have an easier time scaling its platform down over the years (eSRAM should shrink nicely at smaller geometry processes), but that’s not a concern to the end user unless Microsoft chooses to aggressively pass along cost savings.

Introduction, Hardware, Controller & OS Image Quality - Xbox 360 vs. Xbox One
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  • 3DoubleD - Thursday, November 21, 2013 - link

    You sit closer than 7 ft (4 ft optimal) to your 60" TV? This must be in a bedroom, office, or a tiny apartment. I live in what I consider a small apartment and I still sit 10 ft away. Perhaps you just put your couch in the center of the room so that it is really close to your TV? Either way, this is not most people's setup. Average seating distances are far greater than 7 ft. UHD TVs will need to be ~100+" for any benefit to be apparent to regular consumers.

    You must also live in Europe or Asia to get an internet rate like that. I pay $45/mo for 45/4 Mbit with a 300GB cap - although it's unlimited between 2am - 8am, which I take full advantage of.
  • nathanddrews - Thursday, November 21, 2013 - link

    We've got three rows of seating in our home theater. 115" 1080p projection with seating at approximately 7', 11', and 15'. I choose my seating positions based completely upon my audio configuration which is calibrated to the room's acoustic strengths, not upon one-size-fits-all visual acuity seating calculators. We generally sit in the front row when we don't have guests. It's immersive without being nauseating. Pixels are visible in the back row with good eyesight, so I'm anxiously awaiting a 4K upgrade, but probably not until laser projection becomes affordable.

    We've got Comcast Business Class 50/10 for $99/mo. No cap and 50/10 are the guaranteed minimum speeds, unlike the residential service which has a cap (sort of) and sells you a max speed instead of a minimum. Comcast BC also has a $59 plan with no cap that is 12/3, but we wanted more speed. Still can't get gigabit fiber... :-(
  • 3DoubleD - Friday, November 22, 2013 - link

    Sweet setup! You definitely have the screen real estate and seating arrangement to take advantage of 4k. I'd like a similar setup when I move on from apartment style living to a house. Awesome Internet setup too. I could get unlimited as well, and did for a while, but I realized I could pay half as much and get away without hitting my cap by downloading during "happy hours", but that takes some planning.

    I've been anxiously waiting for laser projection systems as well... Will they ever come or is it vaporware? Hopefully that is what my next TV purchase will be.
  • douglord - Thursday, November 21, 2013 - link

    more BS from the anti 4k crowd. I'm sitting 8 feet from my TV right now. In fact it's difficult to sit further away in your standard apartment living room. For a 60 inch TV 4k resolution is recommended for anything 8 feet or closer. For a 85 inch TV its 11 feet. For a 100 inch screen its 13 feet.
  • 3DoubleD - Friday, November 22, 2013 - link

    I'm hardly the anti 4k crowd. I think 4k is great, I just think it is only great when properly implemented. This means that 4k TVs should start at 60", since only very few people sit close enough to begin to see the difference. At 8ft,that is the optimal for 1080p for 60". If you really want to take advantage of 4k you'd sit at 4ft for a 60" set.
  • A5 - Wednesday, November 20, 2013 - link

    PS3 didn't launch with DLNA support, either. I'm guessing it will get patched in at some point.

    As for the rest of it, I'm guessing they made a guess that 4K won't really catch on during the lifespan of these systems, which seems like a fairly safe bet to me.
  • Hubb1e - Wednesday, November 20, 2013 - link

    And with only 16 ROPs Microsoft has trouble even pushing 1080p gaming. It seems that they targeted 720p gaming which is fine with me since most of the time TVs aren't big enough for this to matter. Microsoft did target 4K video though and they designed the video decode blocks specifically to handle this load. It will likely be high resolution but low bitrate video which in most cases is not an improvement over 1080p with high bitrate.
  • piroroadkill - Wednesday, November 20, 2013 - link

    2005? Well the consoles then being well specc'd? I disagree, they were mostly pretty great, but I recall very distinctly thinking 512MiB RAM was pretty poor.
  • airmantharp - Wednesday, November 20, 2013 - link

    It was horrific, and the effects of that decision still haunt us today.
  • bill5 - Wednesday, November 20, 2013 - link

    of course it matters. here xbox one has an edge with an awesome 204 gb/s of esram bandwidth p;us 68 gb/s of ddr bw for a total of 272 gb/s.

    and yes, you can add them together. so dont even start that noob talk.

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