AMD Ryzen Threadripper Memory and PCIe Detailed: What an MCM Entails
AMD built its Ryzen Threadripper HEDT (high-end desktop) processor as a multi-chip module (MCM) of two 8-core "Summit Ridge" dies, each with its own dual-channel memory controller, and PCI-Express interface. This is unlike the competing Core "Skylake-X" from Intel, which is a monolithic 18-core die with a quad-channel DDR4 interface and 44-lane PCIe on one die. AMD has devised some innovative methods of overcoming the latency issues inherent to an MCM arrangement like the Ryzen Threadripper, by tapping into its nUMA technology innovation.
To the hardware, four 8 GB DDR4 memory modules populating the four memory channels of a Ryzen Threadripper chip is seen as 16 GB controlled by each of the two "Summit Ridge" dies. To the software, it is a seamless block of 32 GB. Blindly interleaving the four 8 GB memory modules for four times the bandwidth of a single module isn't as straightforward as it is on the Core X, and is fraught with latency issues. A thread being processed by a core on die-A, having half of its memory allocation on memory controlled by a different die, is hit with latency. AMD is overcoming this by treating memory on a Ryzen Threadripper machine like a 2-socket machine, in which each socket has its own memory.
To the hardware, four 8 GB DDR4 memory modules populating the four memory channels of a Ryzen Threadripper chip is seen as 16 GB controlled by each of the two "Summit Ridge" dies. To the software, it is a seamless block of 32 GB. Blindly interleaving the four 8 GB memory modules for four times the bandwidth of a single module isn't as straightforward as it is on the Core X, and is fraught with latency issues. A thread being processed by a core on die-A, having half of its memory allocation on memory controlled by a different die, is hit with latency. AMD is overcoming this by treating memory on a Ryzen Threadripper machine like a 2-socket machine, in which each socket has its own memory.