AMD Zen Microarchiture Part 2: Extracting Instruction-Level Parallelism
by Ian Cutress on August 23, 2016 8:45 PM EST- Posted in
- CPUs
- AMD
- x86
- Zen
- Microarchitecture
Some Final Thoughts and Comparisons
With the Hot Chips presentation we’ve been given more information on the Zen core microarchitecture than we expected to have at this point in the Zen design/launch cycle. AMD has already stated that general availability for Zen will be in Q1, and Zen might not be the final product launch name/brand when it comes to market. However, there are still plenty of gaps in our knowledge for the hardware, and AMD has promised to reveal this information as we get closer to launch.
We discussed in our earlier piece on the Zen performance metrics given mid-week that it can be hard to interpret any anecdotal benchmark data at this point when there is so much we don’t know (or can’t confirm). With the data in this talk at Hot Chips, we can fill out a lot of information for a direct comparison chart to AMD’s last product and Intel’s current offerings.
| CPU uArch Comparison | ||||
| AMD | Intel | |||
| Zen 8C/16T 2017 |
Bulldozer 4M / 8T 2010 |
Skylake 4C / 8T 2015 |
Broadwell 8C / 16T 2014 |
|
| L1-I Size | 64KB/core | 64KB/module | 32KB/core | 32KB/core |
| L1-I Assoc | 4-way | 2-way | 8-way | 8-way |
| L1-D Size | 32KB/core | 16KB/thread | 32KB/core | 32KB/core |
| L1-D Assoc | 8-way | 4-way | 8-way | 8-way |
| L2 Size | 512KB/core | 1MB/thread | 256KB/core | 256KB/core |
| L2 Assoc | 8-way | 16-way | 4-way | 8-way |
| L3 Size | 2MB/core | 1MB/thread | >2MB/cire | 1.5-3MB/core |
| L3 Assoc | 16-way | 64-way | 16-way | 16/20-way |
| L3 Type | Victim | Victim | Write-back | Write-back |
| L0 ITLB Entry | 8 | - | - | - |
| L0 ITLB Assoc | ? | - | - | - |
| L1 ITLB Entry | 64 | 72 | 128 | 128 |
| L1 ITLB Assoc | ? | Full | 8-way | 4-way |
| L2 ITLB Entry | 512 | 512 | 1536 | 1536 |
| L2 ITLB Assoc | ? | 4-way | 12-way | 4-way |
| L1 DTLB Entry | 64 | 32 | 64 | 64 |
| L1 DTLB Assoc | ? | Full | 4-way | 4-way |
| L2 DTLB Entry | 1536 | 1024 | - | - |
| L2 DTLB Assoc | ? | 8-way | - | - |
| Decode | 4 uops/cycle | 4 Mops/cycle | 5 uops/cycle | 4 uops/cycle |
| uOp Cache Size | ? | - | 1536 | 1536 |
| uOp Cache Assoc | ? | - | 8-way | 8-way |
| uOp Queue Size | ? | - | 128 | 64 |
| Dispatch / cycle | 6 uops/cycle | 4 Mops/cycle | 6 uops/cycle | 4 uops/cycle |
| INT Registers | 168 | 160 | 180 | 168 |
| FP Registers | 160 | 96 | 168 | 168 |
| Retire Queue | 192 | 128 | 224 | 192 |
| Retire Rate | 8/cycle | 4/cycle | 8/cycle | 4/cycle |
| Load Queue | 72 | 40 | 72 | 72 |
| Store Queue | 44 | 24 | 56 | 42 |
| ALU | 4 | 2 | 4 | 4 |
| AGU | 2 | 2 | 2+2 | 2+2 |
| FMAC | 2x128-bit | 2x128-bit 2x MMX 128-bit |
2x256-bit | 2x256-bit |
Bulldozer uses AMD-coined macro-ops, or Mops, which are internal fixed length instructions and can account for 3 smaller ops. These AMD Mops are different to Intel's 'macro-ops', which are variable length and different to Intel's 'micro-ops', which are simpler and fixed-length.
Excavator has a number of improvements over Bulldozer, such as a larger L1-D cache and a 768-entry L1 BTB size, however we were never given a full run-down of the core in a similar fashion and no high-end desktop version of Excavator will be made.
This isn’t an exhaustive list of all features (thanks to CPU World, Real World Tech and WikiChip for filling in some blanks) by any means, and doesn’t paint the whole story. For example, on the power side of the equation, AMD is stating that it has the ability to clock gate parts of the core and CCX that are not required to save power, and the L3 runs on its own clock domain shared across the cores. Or the latency to run certain operations, which is critical for workflow if a MUL operation takes 3, 4 or 5 cycles to complete. We have been told that the FPU load is two cycles quicker, which is something. The latency in the caches is also going to feature heavily in performance, and all we are told at this point is that L2 and L3 are lower latency than previous designs.
A number of these features we’ve already seen on Intel x86 CPUs, such as move elimination to reduce power, or the micro-op cache. The micro-op cache is a piece of the puzzle we want to know more about, especially the rate at which we get cache hits for a given workload. Also, the use of new instructions will adjust a number of workloads that rely on them. Some users will lament the lack of true single-instruction AVX-2 support, however I suspect AMD would argue that the die area cost might be excessive at this time. That’s not to say AMD won’t support it in the future – we were told quite clearly that there were a number of features originally listed internally for Zen which didn’t make it, either due to time constraints or a lack of transistors.
We are told that AMD has a clear internal roadmap for CPU microarchitecture design over the next few generations. As long as we don’t stay for so long on 14nm similar to what we did at 28/32nm, with IO updates over the coming years, a competitive clock-for-clock product (even to Broadwell) with good efficiency will be a welcome return.
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Krysto - Wednesday, August 24, 2016 - link
I think PCs in general run better on four cores than on two, even if most apps themselves can't take advantage of them, although I think in the next 5 years most new games will take advantage of 8 threads. But otherwise, it's just good for multitasking.tarqsharq - Wednesday, August 24, 2016 - link
I had an argument with one fellow on the internet regarding i7 being plenty for whatever I was doing in terms of core count. But streaming a show on one monitor while playing Overwatch was hitting 70%+ CPU usage, with all logical cores being 60-70% utilized consistently, with spikes up to 90%+.That was on my i7-4770K to be specific, running 1080P on a 144hz monitor for Overwatch, and Crunchyroll for 1080P anime stream on the second monitor.
So some games combined with slight multitasking is already taxing the 4C/8T environment.
galta - Wednesday, August 24, 2016 - link
And how much multitasking are we really using? If I had to guess, I would say not much, on average.You might have some folks here and there using it, but regular users need something between two and four cores, just as you said.
You have the OS, the software you're using, be it a game or not, plus everything that's running behind the scenes, including Windows ineficiencies, and that's it. But for some weird guy that spends his day on 7zip, more than 4 cores brings no extra power.
This is the reason why, no matter how excited we might get with 10 cores (I would love one, even if for bragging rights only), our i5s are enough for what we do.
Maybe in 5 years from now games will be multithreaded, but I'm not holding my breath: something similar was said 5 years ago, and here we are.
At the end of the day, we still need improvement in per core performance.
looncraz - Wednesday, August 24, 2016 - link
Browsers are becoming better and better at using more cores... and we're all running tens of processes in the background, some of which fire interrupts on a CPU. More cores allows for more going on at the same time without interruptions. You can actually feel this moving to an eight-core FX-8350 from a quad core i5... those eight cores provide a somewhat smoother multi-tasking environment, despite each core being slower and the overall performance being lower.Humans are simply sensitive to changes in timing - more cores and more threads reduces the variability in timing, which improves perceived performance.
galta - Thursday, August 25, 2016 - link
Hum....I don't know many people who share your opinion about FX-8350 vs i5.
Anyway, we have been multitasking for a while, a least to some extent: OS, Word, anti-virus, browser. The question is: for this light multitasking, are we better off with several cores with poor performance/core, or with less cores but with great performance/core.
Reviews and actual people generally prefer the later.
As of browsers, great news that they are improving, but download/upload speed is by far the most important factor in users experience.
Alexvrb - Sunday, August 28, 2016 - link
Download speed is fine for web browsing if you've got something faster than DSL. How much data exactly do you think you're consuming while browsing the web? Outside of streaming videos you won't use up a ton of bandwidth.Cooe - Thursday, May 6, 2021 - link
I know this is ANCIENT, but how the hell did you not realize that multi-core optimization was so bad only because nobody could afford greater than >4 core CPU's pre-Zen??? Modern games run freaking TERRIBLE now on 4c/4t i5's.Notmyusualid - Wednesday, August 24, 2016 - link
No, nope, nej, and nein.I see (FEEL) tangible improvements in my computing ever since dropped 2 cores for 4.
And it looks like others below agree....
galta - Thursday, August 25, 2016 - link
I believe you do, for the sweet spot is now around 4 cores, as I said before.The question is: do you believe that your experience will improve significantly if you mo to 6 or 8 cores?
Probably not, unless you spend your day zipping files or rendering images.
Alexvrb - Sunday, August 28, 2016 - link
They said the same thing about quad cores, and dual cores before that. AMD has to get on top of the curve, not behind it. They'll offer quad cores for more mainstream systems, and 8 for performance rigs. More for servers, and potentially less for low-power and/or low-cost.