Could you post some quotes?
This is Durango Memory System Overview:
Some Quotes :
8 GB of DRAM.
32 MB of ESRAM.
DRAM
The maximum combined read and write bandwidth to DRAM is 68 GB/s (gigabytes per second). In other words, the sum of read and write bandwidth to DRAM cannot exceed 68 GB/s. You can realistically expect that about 80 – 85% of that bandwidth will be achievable (54.4 GB/s – 57.8 GB/s).
DRAM bandwidth is shared between the following components:
CPU
GPU
Display scan out
Move engines
Audio system
ESRAM
The maximum combined ESRAM read and write bandwidth is 102 GB/s. Having high bandwidth and lower latency makes ESRAM a really valuable memory resource for the GPU.
ESRAM bandwidth is shared between the following components:
GPU
Move engines
Video encode/decode engine. System coherency
There are two types of coherency in the Durango memory system:
Fully hardware coherent
I/O coherent
The two CPU modules are fully coherent. The term fully coherent means that the CPUs do not need to explicitly flush in order for the latest copy of modified data to be available (except when using Write Combined access).
The rest of the Durango infrastructure (the GPU and I/O devices such as, Audio and the Kinect Sensor) is I/O coherent. The term I/O coherent means that those clients can access data in the CPU caches, but that their own caches cannot be probed.
When the CPU produces data, other system clients can choose to consume that data without any extra synchronization work from the CPU.
The total coherent bandwidth through the north bridge is limited to about 30 GB/s.
The CPU requests do not probe any other non-CPU clients, even if the clients have caches. (For example, the GPU has its own cache hierarchy, but the GPU is not probed by the CPU requests.) Therefore, I/O coherent clients must explicitly flush modified data for any latest-modified copy to become visible to the CPUs and to the other I/O coherent clients.
The GPU can perform both coherent and non-coherent memory access. Coherent read-bandwidth of the GPU is limited to 30 GB/s when there is a cache miss, and it’s limited to 10 – 15 GB/s when there is a hit. A GPU memory page attribute determines the coherency of memory access.
The CPU
The Durango console has two CPU modules, and each module has its own 2 MB L2 cache. Each module has four cores, and each of the four cores in each module also has its own 32 KB L1 cache.
When a local L2 miss occurs, the Durango console probes the adjacent L2 cache via the north bridge. Since there is no fast path between the two L2 caches, to avoid cache thrashing, it’s important that you maximize the sharing of data between cores in a module, and that you minimize the sharing between the two CPU modules.
Typical latencies for local and remote cache hits are shown in this table.
Remote L2 hit approximately 100 cycles
Remote L1 hit approximately 120 cycles
Local L1 Hit 3 cycles for 64-bit values
5 cycles for 128-bit values
Local L2 Hit approximately 30 cycles
Each of the two CPU modules connects to the north bridge by a bus that can carry up to 20.8 GB/s in each direction.
Keep in mind that if the CPU uses Write Combined memory writes, then a memory synchronization instruction (SFENCE) must follow to ensure that the writes are visible to the other client devices.
The GPU
The GPU can read at 170 GB/s and write at 102 GB/s through multiple combinations of its clients. Examples of GPU clients are the Color/Depth Blocks and the GPU L2 cache.
The GPU has a direct non-coherent connection to the DRAM memory controller and to ESRAM. The GPU also has a coherent read/write path to the CPU’s L2 caches and to DRAM.
For each read and write request from the GPU, the request uses one path depending on whether the accessed resource is located in “coherent” or “non-coherent” memory.
Some GPU functions share a lower-bandwidth (25.6 GB/s), bidirectional read/write path. Those GPU functions include:
Command buffer and vertex index fetch
Move engines
Video encoding/decoding engines
Front buffer scan out
As the GPU is I/O coherent, data in the GPU caches must be flushed before that data is visible to other components of the system.
Although ESRAM has 102.4 GB/s of bandwidth available, in a transfer case, the DRAM bandwidth limits the speed of the transfer.
Move engines
The Durango console has 25.6 GB/s of read and 25.6 GB/s of write bandwidth shared between:
Four move engines
Display scan out and write-back
Video encoding and decoding
The display scan out consumes a maximum of 3.9 GB/s of read bandwidth (multiply 3 display planes × 4 bytes per pixel × HDMI limit of 300 megapixels per second), and display write-back consumes a maximum of 1.1 GB/s of write bandwidth (multiply 30 bits per pixel × 300 megapixels per second).
