Sunny Cove Microarchitecture: A Peek At the Back End

One of the best parts when a new microarchitecture is announced is learning how it all works and how it has changed. Intel has been on close variants of the Skylake core since Skylake first launched in 2015, which has carried through Kaby Lake, Coffee Lake, and Coffee Lake refresh – and so getting an update to his old core design is more than welcome. While I applaud Intel for getting something new to put in front of our faces, the level of information was unfortunately less than complete. For reasons of ‘we’ll discuss more nearer the launch’, Intel was only set on discussing the back end of the microarchitecture design in any detail. In core design parlance, this means everything beyond the dispatch and order queues.

Intel framed its microarchitecture updates into two different sections: general purpose performance increases and special purpose performance increases. The company described the general purpose updates essentially as raw IPC (instructions per clock) throughput or an increase in frequency. An increase in either of these two leads to a performance increase no matter the code, at least as long as it is compute bound. Frequency is often a function of the implementation and process, whereas IPC increases can come from cores being wider (more executing instructions per clock), deeper (more parallelism per clock), and smarter (better data delivery through the front end).

The other type of performance increase is the special purpose kind, which means that certain workloads used in specific scenarios can be improved through additional acceleration methods, such as dedicated IP or specialized instructions.

Sunny Cove is set to have both.

On the back end of the microarchitecture design, Intel has done several things including increasing cache sizes, making the execution of the core wider, increasing L1 store bandwidth, and other things.

Starting with the cache improvements, the Sunny Cove core back end has a 48 KB L1 Data cache, upgraded from 32 KB. Normally cache misses decrease by a factor of a square root of the proportional size when the cache is increased, so in this case we should expect a 22% reduction in L1-Data cache misses. We are told that the L2 cache size has also been increased in both the Core and Xeon variants of Sunny Cove (256 KB and 1 MB respectively), although to exactly what size will be revealed at a later date.

While not back-end, the micro-op (uOp) cache has increased from a 2048-entry design, although exactly to what Intel is not saying at this point. The second level TLB size has also increased, which will help with machine address translation of recent addresses – normally this is increased when more lookups need to be kept and stored, which means that Intel identified a number of use cases where recent addresses had been evicted shortly before being needed.

From the diagram we can see some other changes, such as the increase in execution ports from eight to ten, allowing more instructions to come out of the schedulers at one time. Port 4 and Port 9 is linked to a cycling data store, doubling that bandwidth, but also the AGU store capabilities have doubled, for which the increased L1-D size will help. Previously on Skylake there was a potential bottleneck when all three AGUs could attempt a store, however only enough bandwidth was present for one per clock cycle. Load performance is unaffected.  There is also an allocation increase, moving from four to five. This means that the dispatch out of the reorder buffer can now hit five instructions per cycle, rather than four – however given that the ops/cycle out of the Skylake decoders and uOp cache was four-to-five (depending on certain factors), it will be interesting to see how the front end has changed as a result.

There are also substantial changes in the execution ports between Skylake and Sunny Cove, which is best represented in a diagram and a table:

Back-End Execution Resources
Skylake Port Sunny Cove
Integer
  JMP Shift ALU 0 ALU LEA Shift JMP
    LEA ALU 1 ALU LEA Mul iDIV
  Mul LEA ALU 5 ALU LEA MulHi  
  JMP Shift ALU 6 ALU LEA Shift JMP
Vector / FP
fpDIV Shift ALU FMA 0 FMA ALU Shift fpDIV
  Shift ALU FMA 1 FMA ALU Shift Shuffle
  Shuffle ALU FMA 5 FMA ALU Shuffle  

The parts highlighted in bold are different in the Sunny Cove core.

We see that Intel has equipped the Integer section of the core with more LEA units to help with memory addressing calculations, possibly to help improve performance loss with security mitigations that require frequent memory calculations or to help with high-performance array code with constant offsets. Port 1 gets the Mul unit from Skylake Port 5, probably for rebalancing purposes, but there is also an integer divider unit here. This addition is a bit of a tweak, as we know that Cannon Lake has a 64-bit IDIV unit in its design as well. In that case it brings 64-bit integer division down from 97 clocks (a mix of instructions) down to 18 clocks, and this is likely something similar. Also on the INT side is that Port 5’s multiply unit has become a ‘MulHi’ unit which in other implementations leave most-significant nibble of the result in a register for further use. I'm not sure exactly what its position is here in the Sunny Cove core though.

On the FP side, Intel has increased the shuffle resources, which we are told was because they had customers requesting this functionality to help remove bottlenecks in their code. Intel didn’t state the capabilities for the FMAs in the FP part of the core, however we do know that the core has an AVX-512 unit, so at least one of these FMAs should interface with it. Cannon Lake only has one 512-bit FMA, and that most likely carries over here, and the Xeon Scalable version might get two. I would like to get proved wrong though.

Other updates to the core as listed from Intel include improvements to the branch predictor and a reduced ‘effective load latency’ (due to the TLB and L1-D), although it was pointed out that these improvements won’t help everyone, and might require new algorithms in order to use specific parts of the core.

We’re waiting to see what changes Intel has made on the front-end, which is where a lot of low-hanging fruit often lies for performance.

On top of the microarchitecture differences, there are new instructions for the Sunny Cove core to help speed up specialist compute tasks. With the AVX-512 unit in play, the new core will support IFMA instructions for big arithmetic compute, and which are also useful in cryptography. Along the same lines, Vector-AES (supporting more AES concurrently) and Vector Carryless Multiply are also supported. SHA and SHA-NI instructrions are also added, along with Galois Field instructions, which are fundamental building blocks in some elements of cryptography as well.

For Sunny Cove, there is also built-in support for larger memory capacities. The paging table for main memory is now a five layer design, up from four layers, supporting a Linear Address space up to 57 bits and a physical address space up to 52 bits. This means, according to Intel, that the server processors could theoretically support 4 TB of memory per socket. We will see Sunny Cove hit the market in the server space with Ice Lake-SP in 2020 according to Intel’s previous Xeon roadmaps.   For security, Sunny Cove has access to Multi-Key Total Memory Encryption, and User Mode Instruction Prevention.

So where does this leave us in terms of Sunny Cove predicted performance compared to Skylake at a clock for clock level? Unfortunately we don’t have enough information. There is only enough to whet the appetite. Intel wants you (and me) to come back for more in 2019.

The CPU Core Roadmaps The Next Generation Gen11 Graphics: Playable Games and Adaptive Sync!
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  • nathanddrews - Wednesday, December 12, 2018 - link

    I know the meme about gaming on Intel graphics, but if they implement Adaptive Sync *combined* with some sort of low framerate compensation, it would make gaming on Intel IGP much less hilarious. Can Intel license FreeSync without using AMD GPU inside? I know FreeSync worked on KLG, but that had an AMD GPU. Reply
  • RarG123 - Wednesday, December 12, 2018 - link

    Like many of AMD's things, FS's an open standard and royalty free. Anyone can use it. Reply
  • Ryan Smith - Wednesday, December 12, 2018 - link

    More specifically, Freesync 1 just AMD's implementation of DisplayPort Adaptive Sync. Intel has to build their own implementation in their display controller and driver stack, but past that all the signaling aspects to the monitor are standardized. Reply
  • Topweasel - Wednesday, December 12, 2018 - link

    Ryan, I thought that was reversed, that AMD worked on adding Adaptive Sync into the specs and worked on making sure it's implementation matched what they were doing with Freesync. Reply
  • kpb321 - Wednesday, December 12, 2018 - link

    IIRC it's a bit of both. Adaptive Sync was present in the eDP standard for things like laptop monitors or tablets as a power saving feature. AMD brought this to the desktop side of things to use for variable framerates in games and helped the standard bring it over too. Reply
  • edzieba - Wednesday, December 12, 2018 - link

    'Adaptive Sync' is effectively the eDP Panel Self Refresh ported over to the full DP spec. Reply
  • drunkenmaster - Wednesday, December 12, 2018 - link

    Freesync utilises adaptive sync. Adaptive Sync is the technology on the screen side, a screen must support adaptive sync to be used by Freesync. Freesync is just the AMD side of it. If a adaptive sync capable screen is detected you can turn on freesync in drivers. Adaptive sync was a standard written up and proposed by AMD and given to I forget who it is now, Displayport group direct or to Vesa. They accepted it and implemented it pretty quickly but as with all things standards take a long time for get integrated into the next cycle or two of products.

    Anyone can use Adaptive sync panels, no one but AMD can use freesync as it's something specific to their hardware and drivers. intel will produce their own specific driver/implementation and just connected to adaptive sync panels in the same way.
    Reply
  • porcupineLTD - Wednesday, December 12, 2018 - link

    So Intel is going straight to chiplets on interposer, it will be interesting to see if AMD adopts this with Zen 3 or waits until Zen 4. Anyway its nice to see competition doing its job. Reply
  • Alexvrb - Wednesday, December 12, 2018 - link

    We don't know yet exactly how much logic Intel is moving to the interposer. It looks awesome for mobile form factors! I think they will face some challenges to bring it to high-TDP desktop solutions, though. Reply
  • ajc9988 - Wednesday, December 12, 2018 - link

    http://www.eecg.toronto.edu/~enright/micro14-inter... http://www.eecg.toronto.edu/~enright/Kannan_MICRO4... https://youtu.be/G3kGSbWFig4 https://seal.ece.ucsb.edu/sites/seal.ece.ucsb.edu/... https://www.youtube.com/watch?v=d3RVwLa3EmM&t=... Reply

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