The leaked presentation by AMD Senior Manager Martin Hilgeman shows that EPYC 3 « Milan » will, as promised and expected, reuse the current platform (SP3), and the system architecture and packaging looks to be the same, with the same 9-die chiplet design and the same maximum core and post-count (no SMT-4, contrary to rumour). The biggest change revealed so far is the enlargement of the compute complex from 4 cores to 8 cores, all sharing a larger L3 cache (« 32+ MB », likely to double to 64 MB, I think).
Hilgeman’s slides did also show that EPYC 4 « Genoa » is in the definition phase (or was at the time of the presentation in September, at least), and will come with a new platform (SP5), with new memory support (likely DDR5).
What else do you think we will see with Zen 4? PCI-Express 5 support? Increased core-count? 4-way SMT? New packaging (interposer, 2.5D, 3D)? Integrated memory on package (HBM)?
Vote in the poll and share your thoughts! 
