Durango Memory System Example


This whole system example demonstrates what the memory bandwidth might look like when the whole system is working under a typical load (this numbers are only predictions not measured numbers)

This example assumes what’s expected to be a typical CPU load and a maximum GPU load:

  • Three display planes are enabled at 1080p resolution.
  • Display write-back is writing a 1080p image at 60 FPS.
  • Move engines are idle.
  • Read bandwidth of the command buffer and index buffer is 4 GB/s.
  • Regular GPU rendering consumes the rest of the available bandwidth.

durango memory2 Durango Memory System Example | VGLeaks 2.0

This diagram shows our prediction of the typical bandwidth for the north bridge clients and the typical available bandwidth for the GPU clients (which are shown in blue).

Let’s start by describing the CPU. Although each CPU module can request up to 20.8 GB/s of bandwidth for read and for write, the typical bandwidth you should expect for the CPU is 4 GB/s per CPU module per direction—about 16 GB/s altogether.

You can expect typical bandwidth to be around 3 GB/s per direction for the: audio, HDD, Camera, and USBs.

The Kinect Sensor is the main consumer of the bandwidth. For example, peak bandwidth to and from the HDD is only about 50 MB/s, so the HDD cannot be seen as a major bandwidth consumer.

Because the GPU is usually pushed to the maximum, you can expect typical coherent bandwidth to be about 25 GB/s. However, this amount depends on how many resources are made snoopable.

Currently, we are not able tell exactly how much of that access will be hitting the CPU’s caches and how much of the access much will go to DRAM. So as we said above, this figure is highly speculative at the moment.

The estimated 25 GB/s of bandwidth for coherent memory access does not account for the non-coherent memory access of the GPU.

The coherent bandwidth that can flow through the north bridge is a limited at 30 GB/s. Under typical conditions, this limit shouldn’t cause you problems. But during a high load on the coherent memory traffic, the north bridge might become saturated. Once the north bridge becomes saturated, you may notice increased latencies for memory access.

CPU memory access that is Write Combined does not fall under this limitation nor does GPU memory access that is non-coherent.

Finally let’s compute how much bandwidth is left for the non-coherent GPU access to consume. Let’s assume that:

  • The sum of bandwidth from the north bridge to DRAM is 25 GB/s.
  • Some portion of the GPU coherent bandwidth misses the L2 caches.
  • Non-coherent CPU bandwidth is 3 GB/s.

This leaves 42 GB/s of DRAM bandwidth available to the GPU clients.

  • stian

    Can you make a comparison with the memory of the PS4? I would like to see the difference.

  • Daniel Lawson

    I think maybe you’ve got some things wrong… >_>

  • Damn it! can’t someone just tell me which one is better? The PS4 or the leaked unofficial Xbox specs?

    • Nicholas Gatewood

      PS4, no doubt. It’s much more powerful than the next Xbox, but it might cost $50 more or something. Definitely worth it if you want the best experience next generation.

      • Thanks man, definitely getting the PS4 then

        • Kreten

          Ps3 is a lot more powerful than 360 but it still got short end of the stick

  • Joseph

    Guys, here are the PS4 and Xbox 720 comparisons I know so far based on what I know.

    – Both are 8-core
    – Xbox is x64, PS4 is x86
    – Xbox has 8GB of DRAM/ESRAM, PS4 has 8GB of DDR5 RAM (DDR5 is faster)
    – Xbox goes at 170 GB/s (combined from DRAM/ESRAM), PS4 goes at 176 GB/s

    – Xbox is 1.23 TFLOPS, PS4 is 1.84 TFLOPS
    – Xbox has 12 or 14 compute units, PS4 has 18 compute units
    – Xbox has 12 shader cores, PS4 has about 14(estimated)

    So overall, PS4 is winning, but Xbox could prevail in software and graphics again. And besides, all these specs are rumors of the Xbox. We all thought PS4 would be 4GB, but it turned out 8GB, so Xbox might change as well.

    • John Nunez

      Awesome man. Only thing that worries me is Sony deciding to go with shared GDDR5 memory, 8 whole gigs of it. Call me cheap, but that added maybe easily 100 bucks to the system.

      • plcn

        funny how this turned out

        • John Nunez

          My price guess was based on Sony not selling for a loss, which sadly, they are again. What they priced has nothing to do what the actual COST of the machine i.e there taking losses per unit.

          • plcn

            yeah, i would’ve made the same guess as you. who knows if they are actually selling for a loss, but seems pretty likely. but as the consumer, i can’t say i am sad about that. why push back the acceleration in adoption by 2 years like last time? the last thing the ecosystem needs is for developers/publishers to think there isn’t a very large install base out there. the premium profit they might get in the early days with a higher price probably isn’t enough to justify the loss in licensing revenue they probably get when the games come out and sell even half well, let alone the loss in customer goodwill/trust (which microsoft is now dealing with this time)…

          • John Nunez

            Oh yeah Microsoft will hit there limit maybe year one. Which is a shame because honestly some of their policies can be fixed, now I say that not knowing a thing about engineering of any sort so I wouldn’t TRULY know. However you are right, normally consoles are sold at loss to get into the consumers hands and build a bigger install base. My only concern with the price is with Sony as a business, it seems way to agressive of a push just to bring in more sales. Its been reported that Sony admitted to taking losses on PS4 with current price tag, and that worries me.