The AMD Zen and Ryzen 7 Review: A Deep Dive on 1800X, 1700X and 1700
by Ian Cutress on March 2, 2017 9:00 AM ESTSimultaneous MultiThreading (SMT)
Zen will be AMD’s first foray into a true simultaneous multithreading structure, and certain parts of the core will act differently depending on their implementation. There are many ways to manage threads, particularly to avoid stalls where one thread is blocking another that ends in the system hanging or crashing. The drivers that communicate with the OS also have to make sure they can distinguish between threads running on new cores or when a core is already occupied – to achieve maximum throughput then four threads should be across two cores, but for efficiency where speed isn’t a factor, perhaps power gating/clock gating half the cores in a CCX is a good idea.
There are a number of ways that AMD will deal with thread management. The basic way is time slicing, and giving each thread an equal share of the pie. This is not always the best policy, especially when you have one performance dominant thread, or one thread that creates a lot of stalls, or a thread where latency is vital. In some methodologies the importance of a thread can be tagged or determined, and this is what we get here, though for some of the structures in the core it has to revert to a basic model.
With each thread, AMD performs internal analysis on the data stream for each to see which thread has algorithmic priority. This means that certain threads will require more resources, or that a branch miss needs to be prioritized to avoid long stall delays. The elements in blue (Branch Prediction, INT/FP Rename) operate on this methodology.
A thread can also be tagged with higher priority. This is important for latency sensitive operations, such as a touch-screen input or immediate user input elements required. The Translation Lookaside Buffers work in this way, to prioritize looking for recent virtual memory address translations. The Load Queue is similarly enabled this way, as typically low latency workloads require data as soon as possible, so the load queue is perfect for this.
Certain parts of the core are statically partitioned, giving each thread an equal timing. This is implemented mostly for anything that is typically processed in-order, such as anything coming out of the micro-op queue, the retire queue and the store queue. However, when running in SMT mode but only with a single thread, the statically partitioned parts of the core can end up as a bottleneck, as they are idle half the time.
The rest of the core is done via competitive scheduling, meaning that if a thread demands more resources it will try to get there first if there is space to do so each cycle.
New Instructions
AMD has a couple of tricks up its sleeve for Zen. Along with including the standard ISA, there are a few new custom instructions that are AMD only.
Some of the new commands are linked with ones that Intel already uses, such as RDSEED for random number generation, or SHA1/SHA256 for cryptography (even with the recent breakthrough in security). The two new instructions are CLZERO and PTE Coalescing.
The first, CLZERO, is aimed to clear a cache line and is more aimed at the data center and HPC crowds. This allows a thread to clear a poisoned cache line atomically (in one cycle) in preparation for zero data structures. It also allows a level of repeatability when the cache line is filled with expected data. CLZERO support will be determined by a CPUID bit.
PTE (Page Table Entry) Coalescing is the ability to combine small 4K page tables into 32K page tables, and is a software transparent implementation. This is useful for reducing the number of entries in the TLBs and the queues, but requires certain criteria of the data to be used within the branch predictor to be met.
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lakerssuperman - Thursday, March 2, 2017 - link
People like me. I was previously running a 2600k overclocked. Nice chip. Still runs great, but I was looking for an upgrade about a year ago as one of the things I do a lot of is Handbrake conversion for my HTPC. Going to even the newest Intel 4 core got me maybe 20% improvement on one of my major workloads for insane amounts of money and going to the high end to get 8-10 cores was just not justifiable.I ended up buying a used Xeon/X79 motherboard combo for around $300 off ebay. 8 cores/16 threads and it works great for Handbrake. I lost some clock speed in the move so single thread performance took a bit of a hit, but was more than made up for in multi-thread performance. I can still game on this CPU just fine and I don't play the newest stuff right away anyway just because of time constraints.
The X79 platform is fine for what I'm doing with it. Would I like the new stuff? Sure. And if I was in the position I was last year looking for an upgrade I don't see how I wouldn't get an 1800x. It gives me the right balance of features for what I do with my computer.
If I was just gaming, I'd look at Intel currently because their 4 core i5 is the sweet spot for this. But I'm not just gaming so this chip is infinitely more attractive to someone like me. With the price and features I can't see how it isn't a winner and when the 4 and 6 core parts come out at likely higher frequencies, I think they are going to be the real winners for gaming.
rarson - Thursday, March 2, 2017 - link
Ryzen is clearly well-suited to anyone who values high performance in a multitude of usage scenarios over one single usage scenario, especially if one cares about how much money they need to spend to achieve those results.injurer - Friday, March 3, 2017 - link
1800X is definitely designed for enthusiast, and AMD fans, but when you go to 1700X this is a price killer targeting the mainstream. 1700 is on the same boat but at even lower price. All the 3 are 8 core chips and are quite close to the 6900K but at 2-4 times lower price.At the end I really believe AMD are still having to show us the real potential of their architecture. Those chips are just the start. Remember Ryzen design is a new from its core, so they definitely have room to ecpand and enhance it.
bill.rookard - Thursday, March 2, 2017 - link
Well, thing to remember is that for those looking for a new build, they now have a legitimate choice. I still do see in the future that things will only go more multithreaded, and even though the i7-7700k is still a great chip, having more physical cores and resources to throw at it will only help.To that end, again, anyone planning a NEW build from the ground up will be able to seriously consider a Ryzen system.
Worst case, think about it. In the deep dive they had mention of 'competitive resource sharing' with SMT enabled. If you were to disable SMT on Ryzen - it would give you 8 PHYSICAL cores versus the 4 physical/4 logical cores of the 7700k. Without those resources being partially used across 16 threads - all resources would be allocated to the physical cores instead, potentially allowing more processing power per physical core.
There's still quite a bit to be checked out and dug through.
lilmoe - Thursday, March 2, 2017 - link
This. I want 2 things dug deeper in follow ups:1) Single/multi threaded performance with SMT disabled VS SMT enabled.
2) Game comparisons with more sensible GPUs (which actually ship and sell in volume, IE: the ones people actually buy), like the GTX 1060 and/or RX 480.
BurntMyBacon - Friday, March 3, 2017 - link
@lilmoeI agree with 1). Intel had HT for several generations before it was universally better to leave it enabled (still needs to be disabled some times, but these are more the edge cases now).
Not so sure I'm onboard with 2). Pairing a $200 GPU with a $500 processor for gaming purposes seems a little backwards. I'd like to see that (GTX1060 / RX480) gaming comparison on a higher clocked R5 or R3 processor when they are released.
Meteor2 - Friday, March 3, 2017 - link
I'd rather see tests paired with a 1080 Ti. At RX480/1060 level, it's well known the bottleneck is GPU performance not CPU. A 1080 Ti should be fast enough to show up the CPU.lilmoe - Friday, March 3, 2017 - link
@BurntMyBacon @Meteor2Lots of people, like me, are more into CPU power. I'm OK with a mid-range GPU. Gaming is not my top priority, and when I do, It's never above 1080p.
It'd be interesting to see if there are differences. I wouldn't dismiss it, saying the GPU would be the bottleneck so fast.
bigboxes - Sunday, March 5, 2017 - link
I'm with you on that. Gaming is way down in my priority list. I do it occasionally just because I love to see what my hardware can do. I currently have a ultrawide 1080p monitor. When I move to 4K then hopefully midrange GPU will cover that. My CPU is a 4790K. It's great for most tasks. I've been wanting to go to 6/8 core for some time, but the cost for the platform was too high. I think in a couple of years I will seriously think about Ryzen when building a new workstation.rarson - Thursday, March 2, 2017 - link
I am interested in seeing potential improvement due to BIOS updates. Additionally, I'm interested in seeing potential improvement due to better multi-threaded software. My hunch is that AMD is either on-par or better than Intel, or maybe damn near that prediction, so I think the 4-core parts will compare well to the current Skylake SKUs. I also expect them to overclock better than the 8-core chips. I guess we'll just have to wait for them to release.8 physical cores is definitely better than 4 cores with SMT/HTT/whatever you want to call it.