Intel's Lion Cove Architecture Preview

https://chipsandcheese.com/2024/06/03/intels-lion-cove-architecture-preview/

by zdw • 2 years ago

175 105 2 years ago

andrewia4• 2 years ago

I'm very interested to see independent testing of cores without SMT/hyperthreading. Of course it's one less function for the hardware and thread scheduler to worry about. But hyperthreading was a useful way to share resources between multiple threads that had light-to-intermediate workloads. Synthetic benchmarks might show an improvement but I'm interested to see what everyday workloads, like web browsing while streaming a video, will react.

adrian_b2• 2 years ago

I was surprised that disabling SMT has improved by a few percents the Geekbench 6 multi-threaded results on a Zen 3 (5900X) CPU.

While there are also other tasks where SMT does not bring advantages, for the compilation of a big software project SMT does bring an obvious performance improvement, of about 20% for the same Zen 3 CPU.

In any case, Intel has said that they have designed 2 versions of the Lion Cove core, one without SMT for laptop/desktop hybrid CPUs and one with SMT for server CPUs with P cores (i.e. for the successor of Granite Rapids, which will be launched later this year, using P-cores similar to those of Meteor Lake).

papichulo2023• 2 years ago

Probably because the benchmark is not using all cores so the cores hit the cache more often.

dagmx3• 2 years ago

Generally HT/SMT has never been favored for high utilization needs or low wattage needs.

On the high utilization end, stuff like offline rendering or even some realtime games, would have significant performance degradation when HT/SMT are enabled. It was incredibly noticeable when I worked in film.

And on the low wattage end, it ends up causing more overhead versus just dumping the jobs on an E core.

The_Colonel3• 2 years ago

And on the low wattage end, it ends up causing more overhead versus just dumping the jobs on an E core.

For most of the HT's existence there weren't any E cores which conflicts with your "never" in the first sentence.

jeffbee2• 2 years ago

Backend-bound workloads that amount to hours of endless multiplication are not that common. For workloads that are just grab-bags of hundreds of unrelated tasks on a machine, which describes the entire "cloud" thing and most internal crud at every company, HT significantly increases the apparent capacity of the machine.

mmaniac• 2 years ago

The need for hyperthreading has diminished with increasing core counts and shrinking power headroom. You can just run those tasks on E cores now and save energy.

pjmlp2• 2 years ago

Since side-channel attacks became a common thing, there is hardly a reason to keep hyperthreading around.

It was a product of its time, a way to get cheap multi-cores when getting real cores was too expensive for regular consumer products.

Besides the security issues, for high performance workloads they have always been an issue, stealing resources across shared CPU units.

sapiogram2• 2 years ago

there is hardly a reason to keep hyperthreading around.

Performance is still a reason. Anecdote: I have a pet project that involves searching for chess puzzles, and hyperthreading improves throughput 22%. Not massive, but definitely not nothing.

binkHN2• 2 years ago

For what it's worth, for security reasons, OpenBSD disables hyperthreading by default.

klooney• 2 years ago

The removal of Hyperthreading makes a lot of sense for Lunar Lake to both reduce the die size of the version of Lion Cove found in Lunar Lake along with simplifying the job of Thread Director.

And, you know, stop the security vulnerability bleeding.

andrewia4• 2 years ago

I don't think hyperthreading was the bulk of the attack surface. It definitely presented opportunities for processes to get out of bounds, but I think preemptive scheduling is the bulk of the issue. That genie not going back in the bottle another way to significantly improve processor performance for the same amount of instructions.

nextaccountic• 2 years ago

I think the real problem is cache sharing and hyperthreading kind of depends on it, so it was only ever secure to run two threads from the same security domain in the same core

andrewia4• 2 years ago

It's interesting to see that modern processor optimization still revolves around balancing hardware for specific tasks. In this case, the vector scheduler has been separated from the integer scheduler, and the integer pipeline has been made much wider. I'm sure it made sense for this revision, but I wonder if things will change in a few generations in the pendulum will swing back to simplifying and integrating more parts of the arithmrtic scheduler(s) and ALUs.

It's also interesting to see that FPGA integration hasn't gone far, and good vector performance is still important (if less important than integer). I wonder what percentage of consumer and professional workloads make significant use of vector operations, and how much GPU and FPGA offload would alleviate the need for good vector performance. I only know of vector operations in the context of multimedia processing, which is also suited for GPU acceleration.

dogma11384• 2 years ago

AMD tried that with HSA in the past it doesn’t really work. Unless your CPU can magically offload vector processing to the GPU or another sub-processor you are still reliant on new code to get this working which means you break backward compatibility with previously compiled code.

The best case scenario here is if you can have the compiler do all the heavy lifting but more realistically you’ll end up having to make developers switch to a whole new programming paradigm.

andrewia4• 2 years ago

I understand that you can't convince developers to rewrite/recompile their applications for a processor that breaks compatibility. I'm wondering how many existing applications would be negatively impacted by cutting down vector throughput. With some searching, I see that some applications make mild use of it like Firefox. However there are applications that would negatively affected, such as noise suppression in Microsoft Teams, and crypto acceleration in libssl and the Linux kernel. Acceleration of crypto functions seems essential enough to warrant not touching vector throughput, so it seems vector operations are here to stay in CPUs.

JonChesterfield3• 2 years ago

Persuading people to write their C++ as a graph for heterogeneous execution hasn't gone well. The machinery works though, and it's the right thing for heterogeneous compute, so should see adoption from XLA / pytorch etc.

hajile5• 2 years ago

I think the answer here is dedicated cores of different types on the same die.

Some cores will be high-performance, OoO CPU cores.

Now you make another core with the same ISA, but built for a different workload. It should be in-order. It should have a narrow ALU with fairly basic branch prediction. Most of the core will be occupied with two 1024-bit SIMD units and a 8-16x SMT implementation to hide the latency of the threads.

If your CPU and/or OS detects that a thread is packed with SIMD instructions, it will move the thread over to the wide, slow core with latency hiding. Normal threads with low SIMD instruction counts will be put through the high-performance CPU core.

gumby2• 2 years ago

good vector performance is still important (if less important than integer)

This is in part (major part IMHO) because few languages support vector operations as first class operators. We are still trapped in the tyranny that assumes a C abstract machine.

And so because so few languages support vectors, the instruction mix doesn’t emphasize it, therefore there’s less incentive to work on new language paradigms, and we remained tapped in a suboptimal loop.

I’m not claiming there are any villains here, we’re just stuck in a hill-climbing failure.

davedx2• 2 years ago

But isn’t that why we have things like CUDA? Who exactly is “we” here, people who only have access to CPU’s? :)

dan-robertson2• 2 years ago

It’s not obvious that that’s what’s happened here. Eg vector scheduling is separated but there are more units for actually doing certain vector operations. It may be that lots of vector workloads are more limited by memory bandwidth than ILP so adding another port to the scheduler mightn’t add much. Being able to run other parts of the cpu faster when vectorised instructions aren’t being used could be worth a lot.

Symmetry5• 2 years ago

As CPU cores get larger and larger it makes sense to always keep looking for opportunities to decouple things. AMD went with separate schedulers in the Athalon three architectural overhauls ago and hasn't reversed their decision.

frutiger• 2 years ago

It's interesting to see that modern processor optimization still revolves around balancing hardware for specific tasks

Asking sincerely: what’s specifically so interesting about that? That is what I would naively expect.

xw390112• 2 years ago

It's also important to note that in modern hardware the processor core proper is just one piece in a very large system.

Hardware designers are adding a lot of speciality hardware, they're just not putting it into the core, which also makes a lot of sense.

https://www.researchgate.net/figure/Architectural-specializa...

jandrewrogers2• 2 years ago

The CPU vector performance is important for throughput-oriented processing of data e.g. databases. A powerful vector implementation gives you most of the benefits of an FPGA for a tiny fraction of the effort but has fewer limitations than a GPU. This hits a price-performance sweet spot for a lot of workloads and the CPU companies have been increasingly making this a first-class "every day" feature of their processors.

KingOfCoders2• 2 years ago

Main thing for me, they take the Apple hint and again increase caches (and add another cache layer L0)

trynumber92• 2 years ago

I don't think the L0 is the "new" cache. It's still 48K like the old L1. The 9 cycle 192K cache about halfway between the old L1 and L2 is really the new cache level in size and latency. And that's now called L1.

hajile5• 2 years ago

L0 latency is 4 cycles while old L1 was 5 cycles, so it's definitely a new cache even though the size is the same.

Apple does a lot better. I'm not sure about newer chips, but M1 has the same 192kb of L1, but with the 4 cycle latency of Intel's tiny 48kb cache.

formerly_proven• 2 years ago

End of the unified scheduler is big news, Intel had this since Core. Both AMD and Apple have split the scheduling up for a long time.

bee_rider• 2 years ago

Now that the floating point vector units have gotten large enough to engage in mitosis, I wonder if there will be room in the future for little SIMD units stuck on the integer side. Bring back SSE, haha.

Symmetry5• 2 years ago

Removing a core's SMT aka "hyperthreading" has some modest hardware savings but but biggest cost is that it makes testing and validation much more complicated. Given the existence of Intel's p-cores I'm not surprised they're getting rid of it.

dralley• 2 years ago

From Intel's perspective, I doubt that's true, when taking into consideration the constant stream of side channel vulnerabilities they were needing to deal with.

Symmetry5• 2 years ago

It's exactly the potential for side channel vulnerabilities that makes SMT so hard to get right.

dan-robertson2• 2 years ago

I think you’re saying that testing smt makes it expensive, which sounds mostly right to me, though I can imagine some arguments that it isn’t much worse. When I first read your comment, I thought it said the opposite – that removing smt requires expensive testing and validation work.

trynumber92• 2 years ago

Lion Cove doesn't remove hyper-threading in general. Some variants will have it so Intel still has to validate it (eventually). But the first part shipping, Lunar Lake, removes hyper-threading. It may have saved them validation time for this particular part but they'll still have to do it.

adrian_b2• 2 years ago

Intel has said that they have designed two different versions of the Lion Cove core.

One with SMT, for server CPUs, and one in which all circuits related to SMT are completely removed, for smaller area and power consumption, to be used in hybrid CPUs for laptops and desktops, together with Skymont cores.

therealmarv2• 2 years ago

And the main question: Is it overall better (faster and less power hungry) than a reduced instruction set, powerful laptop ARM based CPU which is around the corner (Qualcomm)? Guessing not...

Rinzler89• 2 years ago

There's still an insurmountable amount of apps that are still X86 exclusive both new and legacy. So the chip not beating the best of ARM in benchmarks is largely irelevant.

A Ferrari will beat a tractor on every test bench numbers and every track, but I can't plow the field with a Ferrari, so any improvements in tractor technology is still welcome despite they'll never beat Ferraris.

hajile5• 2 years ago

I hear this argument, but I don't really believe it.

If you're talking apps before say 2015 (10 years ago), they can be emulated on ARM faster than they ran natively. That rules out 95% of the backward compatibility argument.

Most more recent apps are very portable. They were written in a managed language running on a cross-platform runtime. The source code is likely stored in git so it can be tracked down and recompiled.

Over 15 years of modern smartphones has ensured that most low-level libraries have support for ARM and other ISAs too as being ISA-agnostic has once again become important. Apple's 4 years of transition aren't to be underestimated either. Lots of devs/creatives use ARM machines and have ensured that pretty much all of the biggest pro software runs very well on non-x86 platforms.

Yes, some stuff remains, but I don't think the remaining stuff is as big a deal as some people claim.

jeffbee2• 2 years ago

Amazing that you can look at an ISA like ARM and say "reduced instruction set". It has 1300+ opcodes.

mhh__• 2 years ago

At best ARM is regular rather than reduced

JonChesterfield3• 2 years ago

Aarch64 looks a lot like x86-64 to me. Deep pipelines, loads of silicon spent on branch prediction, vector units.

aidenn0• 2 years ago

It seems that "RISC" has just become a synonym for "load-store architecture"

Non-embedded POWER implementations are around 1000 opcodes, depending on the features supported, and even MIPS eventually got a square-root instruction.

yjftsjthsd-h2• 2 years ago

I used to think this too, but apparently RISC isn't about the number of instructions, but the complexity or execution time of each; as https://en.wikipedia.org/wiki/Reduced_instruction_set_comput... puts it,

The key operational concept of the RISC computer is that each instruction performs only one function (e.g. copy a value from memory to a register).

and in fact that page even mentions at https://en.wikipedia.org/wiki/Reduced_instruction_set_comput... that

Some CPUs have been specifically designed to have a very small set of instructions—but these designs are very different from classic RISC designs, so they have been given other names such as minimal instruction set computer (MISC) or transport triggered architecture (TTA).

ffgjgf1• 2 years ago

IIRC they claimed that it supposedly is

therealmarv2• 2 years ago

Would be great if true! Competition always good for us consumers.

yjftsjthsd-h2• 2 years ago

What are you guessing from? Historically, generation for generation, x86 is good at performance and awful at power consumption. Even when Apple (not aarch64 in general, just Apple) briefly pulled ahead on both, subsequent x86 chips kept winning on raw performance, even as they got destroyed on perf per Watt.

dur-randir2• 2 years ago

kept winning on raw performance

13900K lost couple of % in single tread performance, which lead to 14900K being so overclocked/overvolted that it lead to it being useless for what it's made for - crunching numbers. See https://www.radgametools.com/oodleintel.htm.

SilverBirch• 2 years ago

I'm just wondering, how much of this analysis is real? I mean, how much of this analysis is a true weighing up of design decisions and performance, and how much is copy/pasted Intel marketing bumf?

antoniojtorres2• 2 years ago

That’s typically the question with all marketing (probably cherry-picked) benchmarks. Have to wait until independent reviewers get their hands on them.

dur-randir2• 2 years ago

Nothing real. They cherry-pick for what they're trying to sell you this time - advent of 2nd core, +HT, E-cores, -HT, you-name-it-next-time. Without independent benchmark + additional interpretation for your particular workload you can safely ignore all their claims.

JonChesterfield3• 2 years ago

The dropping hyperthreading is interesting. I'm hoping x64 will trend in the other direction - I'd like four or eight threads scheduled across a pool of execution units as a way of hiding memory latency. Sort of a step towards the GPU model.

jandrewrogers2• 2 years ago

This has not been successful historically because software developers struggle to design code that has mechanical sympathy with these architectural models. These types of processors will run idiomatic CPU code with mediocre efficiency, which provides an element of backward compatibility, but achieving their theoretical efficiency requires barrel-processing style data structures and algorithms, which isn't something widely taught or known to software developers.

I like these architectures but don't expect it to happen. Every attempt to produce a CPU that works this way has done poorly because software developers don't know how to use them properly. The hardware companies have done the "build it and they will come" thing multiple times and it has never panned out for them. You would need a killer app that would strongly incentivize software developers to learn how design efficient code for these architectures.

bri3d• 2 years ago

I think another huge issue is the combination of general-purpose/multi-tenant computing, especially with virtualization in the mix, and cache. Improvements gained from masking memory latency by issuing instructions from more thread execution contexts are suddenly lost when each of those thread contexts is accessing a totally different part of memory and evicting the cache line that the next thread wanted anyway.

In many ways barrel processing and hyperthreading are remarkably similar to VLIW - they're two very different sides of the same coin. At the end of the day, both work well for situations where multiple simultaneous instruction streams need to access relatively small amounts of adjacent data. Realtime signal processing, dedicated databases, and gaming are obvious applications here, which I think is why VLIW has done so well in DSP and hyperthreading did well in gaming and databases. Once the simultaneous instruction streams are accessing completely disparate parts of main memory, it all blows up.

Plus, in a multi-tenant environment, the security issues inherent to sharing even more execution state context (and therefore side channels) between domains also become untenable fairly quickly.

pradn• 2 years ago

Hyperthreading is also useful for simply making a little progress on more threads. Even if you're not pinning a core by making full use of two hyperthreads, you can handle double the threads at once. Now, I don't know how important it is, but I assume that for desktop applications, this could lead to a snappier experience. Most code on the desktop is just waiting for memory loads and context switching.

Of course, the big elephant in the room is security - timing attacks on shared cores is a big problem. Sharing anything is a big problem for security conscious customers.

Maybe it's the case of the server leading the client here.

Covzire• 2 years ago

I've been disabling HT for security reasons on all my machines ever since the first vulnerabilities appeared, with no discernible negative impact on daily usage.

groos• 2 years ago

The gain from HT for doing large builds has been 20% at most. Daily usage is indistinguishable, as you say.

binkHN2• 2 years ago

For security reasons, OpenBSD disables hyperthreading by default.

duffyjp• 2 years ago

I'm running a quite old CPU currently, a 6-core Haswell-E i7-5930K. Disabling HT gave me a huge boost in gaming workloads. I'm basically never doing anything that pegs the entire CPU so getting that extra 10-15% IPC for single threaded tasks is huge.

And like you said, the vulnerability consideration makes HT a hard sell for workloads that would benefit (hypervisors).

That i7 is a huge downgrade from what I was running before (long story) so I'm looking forward to Arrow Lake and I like everything I've read. In addition to removing HT, they're supporting Native JEDEC DDR5-6400 Memory so XMP won't be necessary. I've never liked XMP/Expo...

AzzyHN• 2 years ago

To summarize Intel's reasoning, the extra hardware (and I guess associated firmware or whatever it'd be called) required to manage hyperthreading in their P-cores takes up package space and power, meaning the core can't boost as high.

And since hyperthreading only increases IPC by 30% (according to Intel), they're planning on making up the loss of threads with more E-cores.

But we'll have to see how that turns out, especially since Intel's first chiplet design (the Core Ultra series 1) had performance degradations compared to their 13th Gen mobile counterparts

killerstorm• 2 years ago

That's what SPARC and Power architectures do now, with 8 threads per core.

The only issues with these architectures is that they are priced as "high-end server stuff" while x64 is priced like a commodity.

osigurdson• 2 years ago

From an investor perspective, I'd rather hear: "we pay more for talent than our competition". That is the first step toward greatness.

tambourine_man• 2 years ago

Moving to a more customizable design will allow Intel to better optimize their P-cores for specific designs moving forward

Amateur question: is that due to the recent advances in interconnect or are they designing multiple versions of this chip?

wmf• 2 years ago

Cheese is talking about different chips (e.g. laptop, desktop, and server) that contain Lion Cove cores. Intel doesn't really reuse chips between different segments.

ein0p• 2 years ago

Once (and if) Intel manages to get out of the woods with lithography, they will have an incredibly strong product lineup.

Intel's Lion Cove Architecture Preview

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