AMD Zen Microarchitecture: Dual Schedulers, Micro-Op Cache and Memory Hierarchy Revealed
by Ian Cutress on August 18, 2016 9:00 AM ESTSimultaneous Multi-Threading
On Zen, each core will be able to support two threads in what is called ‘simulatenous multi-threading’. Intel has supported their version of SMT for a number of years, and other CPU manufacturers like IBM support up to 8 threads per core on their POWER8 platform designs. Building a core to be able to use multiple threads can be tough, as it requires a lot of resources to make sure that the threads do not block each other by consuming all the cache and buffers in play. But AMD will equip Zen with SMT which means we will see 8C/16T parts hitting the market.
Unlike Bulldozer, where having a shared FP unit between two threads was an issue for floating point performance, Zen’s design is more akin to Intel’s in that each thread will appear as an independent core and there is not that resource limitation that BD had. With sufficient resources, SMT will allow the core instructions per clock to improve, however it will be interesting to see what workloads will benefit and which ones will not.
Timeframe and Availability
At the presentation, it was given that Zen will be available in volume in 2017. As the AM4 platform will share a socket with Bristol Ridge, users are likely to see Bristol Ridge systems from AMD’s main OEM partners, like Dell and others, enter the market before separate Zen CPUs will hit the market for DIY builders. It’s a matter of principle that almost no consumer focused semiconductor company releases a product for the sale season, and Q1 features such events as CES, which gives a pretty clear indication of when we can expect to get our hands on one.
It’s worth noting that AMD said that as we get closer to launch, further details will come as well as deeper information about the design. It was also mentioned that the marketing strategy is also currently being determined, such that Zen may not actually be the retail product name for the line of processors (we already have Summit Ridge as the platform codename, but that could change for retail as well).
Wrap Up
AMD has gone much further into their core design than I expected this week. When we were told we had a briefing, and there were 200-odd press and analysts in the room, I was expecting to hear some high level puff about the brand and a reiteration of their commitment to the high end. To actually get some slides detailing parts of the microarchitecture, even at a basic cache level, was quite surprising and it somewhat means that AMD might have stolen the show with the news this week.
We’ve got another couple of pieces detailing some of the AMD internal/live benchmark numbers during the presentation, as well as the dual socket server platform, the 32-core Naples server CPU, and what we saw at the event in terms of motherboard design.
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DigitalFreak - Thursday, August 18, 2016 - link
Microsoft was already in the process of creating a 64bit version of Windows based on AMD's 64bit implementation (hence the reason you see AMD64 everywhere in 64bit Windows). Microsoft basically told Intel they were not going to support two competing implementations of "x64", so Intel caved and adopted the AMD64 implementation.tygrus - Thursday, September 8, 2016 - link
They license the ISA's ie. use of instructions and the expected output. The whole silicon designs are not cross-licensed. There probably have some IP of the silicon cross-licensed but the major point was they could handle the same instructions and be mostly compatible. AMD could only fully copy 486 and earlier designs. You can copy and implement the same ISA without having the same silicon. Intel had started a design for x86-64 but the front-end decoding and instructions were changed to be cmpatible. With micro/macro ops and microcoding there can be a lot of abstraction between ISA and execution. Intel made at least 1 mistake with their early AMD64 implementation that had to have work arounds and a later fix.frenchy_2001 - Thursday, August 18, 2016 - link
Opposite.Intel was vehement at the time that 64 bits needed to be a clean break from x86 and were pushing for their Itanium processors, implementing IA64 (completely incompatible with x86).
The market followed AMD, especially sice they had the better architecture at the time (Athlon64, with 64 bits and in processor memory controllers, faster interconnect, better server scaling...).
Intel then licensed AMD64 and and rebranded it EMT64 or x86-64.
wifiwolf - Friday, August 19, 2016 - link
wow. finally someone who remembers that time correctly. Intel pushed for Itanium for too much time, even after they adopted amd's 64bit implementation. They eventually had to drop it as it never got enough market.Samus - Sunday, August 21, 2016 - link
Microsoft did make an IA64 edition on NT and 2000 but without x86 compatibility there were no apps. The genius behind AMD's 64 bit implementation is it is simply a memory extension of x86 with 64 bit integer registers, maintaining complete 32-bit compatibility with no real impact on 32 bit performance, while costing very little die space for the extensions.Microsoft and software developers saw this and basically told Intel their Itanium dreams were not going to come true.
anubis44 - Monday, August 22, 2016 - link
And the genius behind that 'genius' was none other than Jim Keller, the man who also just designed the upcoming Zen processor family.Visual - Tuesday, August 23, 2016 - link
No, the IA64 architecture of Itanium does not try to keep any backwards-compatibility with x86, so any mention of it even being considered as an alternative to AMD64 is absurd. At that time the world was just not ready for a compatibility-breaking switch.Kevin G - Tuesday, August 23, 2016 - link
The ISA didn't directly try to keep backwards compatibility but Intel did put some x86 functionality into the first few generations of Itanium. This was later removed in chips post 2006.https://en.wikipedia.org/wiki/IA-32_Execution_Laye...
Gigaplex - Thursday, August 18, 2016 - link
Which is a legal term to describe "copying with permission".pikunsia - Friday, August 19, 2016 - link
AMD cannot copy ``TM'' Intel technologies as this is a crime with criminal consequences. All is managed through licenses and royalties.