Leaked Patch Confirms AMD Zen Will Have 32 Cores per Socket
A leaked Linux patch on LKML.org, first spotted by The New Citavia Blog, suggests that AMD Zen based processors will feature up to 32 physical cores. The patch also hints at the similarity of parts of the “Zen” and “Zeppelin” codenames.
The Zeppelin codename was first mentioned back in August last year, and parts of the patch identify it as a “family 17h model 00h” CPU:
AMD Zeppelin (Family 17h, Model 00h) introduces an instructionsretired performance counter which indicated byCPUID.8000_0008H:EBX. And dedicated Instructions Retired register(MSR 0xC000_000E9) increments on once for every instruction retired.
There’s also an older patch from November last year, on the same mailing list, which reveals a little more info about the upcoming Zen architecture. However, the most important part is the code which describes how the last level cache (LLC) ID is being calculated for Zen based MPUs:
+ core_complex_id = (apicid & ((1 << c->x86_coreid_bits) - 1)) >> 3;
+ per_cpu(cpu_llc_id, cpu) = (socket_id << 3) | core_complex_id;
The above code mentions "core complex," which accoding to Dresdenboy refers to "compute unit" and has already been used in some AMD patents. Dresdenboy explains the above code as follows:
The expression marked in red means a shift right by 3, which equals a division by 8. So with two logical cores per physical core due to SMT, a core complex should contain four Zen cores and a shared LLC.
The next line shows the socket ID being shifted left by 3, leaving 3 bits for the core complex ID, which suggests a maximum number of eight core complexes per socket, or 32 physical cores. This number should first be seen as a placeholder, but we've already seen rumours mentioning that many cores.
Built on a 14 nanometer architecture, Zen architecture features a unified AM4 socket with its GPU-equipped APU counterparts, as well as includes support for DDR4 memory. If a previous report is to be believed, it will bring greater than 40% IPC increase over their current generation.
The first Zen based chips are scheduled to arrive at the end of this year, with APUs arriving sometime in 2017.