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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mikeZZZ - Friday, March 3, 2017 - link
Anadtech, can we please run closer to real life scenarios such as a gaming benchmark with a file compression benchmark running at the same time. Even gaming enthusiasts run more than one program at a time. For example, file decompression in the background while playing a game, or baseball game streaming in a small window while playing a game. You already have many individual benchmarks, so why not go the extra but significant benchmark of running two? We know this favors the higher core CPUs (maybe even Ryzen 7 1700 over all other lower core ones CPUs) but that is closer to real life and should be very meaningful to someone wanting to make an informed purchase.ValiumMm - Saturday, March 4, 2017 - link
Would also like to see thisUrQuan3 - Friday, March 3, 2017 - link
Just want to put out a quick comment about benchmarking with Handbrake. In dealing with Broadwell-E, and especially ThunderX, I've found that Handbrake often doesn't scale well past about 10 cores, and really doesn't scale well past 16 or so. What seems to happen is that the single-threaded parts of Handbrake tend to dominate the encode time. In extreme cases, ultra-fast and placebo will take almost the same amount of time as x264 is consuming input faster than the rest of Handbrake can generate it. On ThunderX, I've found I can complete four 1080p placebo encodes in the same amount of time that I can complete one. I would expect a similar result on a 48 core Intel, though I do not have access to one beyond 24 cores. Turbo boost would hide this effect a bit.I am not knocking using Handbrake for benchmarking. The Handbrake and ray-trace results are the two that I care about most. I just thought I'd give a heads up about this limitation. You can check CPU usage statistics to get an indication of when you are running up against this limit.
Oh, and I am very excited to see multiple ray-tracers in your runs. Please continue.
Meteor2 - Saturday, March 4, 2017 - link
Presumably though you can have several x264 jobs running simultaneously on that hardware? So while your time to encode a certain piece doesn't decrease, you have more total-throughput (e.g. encoding several different bitrates for adaptive streaming). Should give good efficiency too on a larger Broadwell-E or a ThunderX.UrQuan3 - Tuesday, March 7, 2017 - link
Exactly. It's the first time I've thought about installing a queue manager for a single computer.jade5419 - Saturday, March 4, 2017 - link
I agree with this. In my experience Handbrake has a core / thread limit.I have a Z600 system with dual Xeon 5570 @ 2.93GHz, 6 core / 12 threads (total 24 threads), 48GB of RAM and a Z620 system with dual Xeon E5-2690 @ 2.9GHz 8 core / 16 threads (total 32 threads), 64GB RAM.
The two systems transcode video at the same speed using Handbrake 1.0.3. Monitoring CPU usage shows all threads of the Z600 at 100% utilization whereas the CPU utilization on the Z620 is approximately 80%.
Notmyusualid - Sunday, March 5, 2017 - link
Ever tried running GTA5 on 28 cores?It doesn't work. You have to adjust the game 'launchers' core affinity to < 26 cores or it won't even load.
Given this discovery, I expect there are many more applications out there, that may crap-out as we see more and more cores come into the mainstream.
Just a thought.
mapesdhs - Sunday, March 5, 2017 - link
I'd love to know why this happens. I'm guessing something dumb within Windows.Outlander_04 - Friday, March 3, 2017 - link
There is more than enough good news to make me want to buy a 6 core Ryzen when they become available .Likely that will be the sweet spot for gamers
0ldman79 - Saturday, March 4, 2017 - link
I'm looking forward to seeing Ryzen updated in the bench.There aren't any apps or benchmarks that cross over between the FX series and the Ryzen series, so we can't do any side by side comparison.
Great review guys. Looking forward to the six core Ryzen. I think just like the FX series the six core will be the sweet spot.