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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eldakka - Wednesday, August 24, 2016 - link
The first page link, AMD Server CPUs and Motherboard Analysis, is wrong, it actually links to the ARM v8-A article.atlantico - Friday, August 26, 2016 - link
Yes, it's also wrong here: http://www.anandtech.com/show/10585/unpacking-amds...Sigh.
TristanSDX - Wednesday, August 24, 2016 - link
Zen do not support transactional memory, big disadvantage comparing to IntelSenti - Wednesday, August 24, 2016 - link
And how much does it matter? TSX is great thing no doubt there. But the adoption? What can you name of real software what uses and get significant benefit of it?I blame Intel stupid marketing for cutting TSX from too many versions and killing the adoption.
coder111 - Wednesday, August 24, 2016 - link
As far as I know, Azul JVMs do support transactional memory. So if you have a Java app, you can use it.Other than that, yes, I haven't seen TSX used much...
68k - Wednesday, August 24, 2016 - link
Isn't the version of glibc in recent Linux-distributions using the lock elision feature of TSX?https://lwn.net/Articles/534758/
https://01.org/blogs/tlcounts/2014/lock-elision-gl...
If so, then essentially every single Linux program does make use of TSX when present.
looncraz - Wednesday, August 24, 2016 - link
One of the most important features of TSX are checkpoints. Zen supports checkpoints in its execution pipeline. Otherwise, I've not seen anything that said Zen did or did not support TSX, not that the tech is widely used at this time.From there, you just need tagging and a few other features to add support. It's something that could be included in Zen+ if Zen does not have it.
silverblue - Wednesday, August 24, 2016 - link
It looks like Zen was developed to accelerate the vast majority of software, and rely on core count for everything else. It might explain the lack of focus on AVX.If cache stats were any indication of performance, it would appear that Zen was destined to compete with Broadwell, but not quite match the Lake CPUs; Zen+ would perhaps close the gap albeit a bit late. Bulldozer was hamstrung by half-speed writes and horrific L3 latency - would it be remiss to assume that they've at least fixed those two issues?
I'm not sure anybody can truly predict performance however, even with a Blender demonstration, and certainly not to work out prospective Cinebench or SuperPi performance. You could have a monster of an architecture, but if the software isn't optimised for it, it's not going to be representative of its true performance.
wumpus - Wednesday, August 24, 2016 - link
I'd still want the TSX instructions before even thinking about the server market. I guess they surrendered that before the overall architecture was finished. Although considering how badly it has worked for Intel (essentially turned off after errata was noted in the first generation), maybe it wasn't worth risk.Alexvrb - Sunday, August 28, 2016 - link
Yeah they need to take their time. A faulty implementation would do more harm than good at this point.