ARM details its new high-end CPU core, Cortex A72

http://arstechnica.com/gadgets/2015/04/arm-details-its-new-high-end-cpu-core-cortex-a72/

by aroch • 11 years ago

62 51 11 years ago

ChuckMcM3• 11 years ago

Ok, this was a fun read. Two things stood out for me, one was that their partially out of order 8 issue pipeline and the other was their watts per instruction.

For a long time Intel has been top dog in the "instructions per clock" or IPC space. So if you wanted performance you used their chips, except you paid for that by consuming a lot of power. ARM on the other hand has always tried to be the 'low power' chip which you could embed and run on batteries, and ultimately slower than the Intel architecture.

But into this a couple of interesting market realities intruded, the most obvious was that at some point computers were "fast enough" for enough people to make a durable market for lower power machines. That really took off course in the smart phone and tablet market, but is inching into the "low end" server market.

If ARM can get better at performance faster than Intel can get better at "low power" they can really put a dent into Intels market dominance in the larger computer space. And as a competitor with a completely different ISA they limit Intels ability to compete with lawsuits and/or changes to the "standard."

stephencanon4• 11 years ago

"In more detailed terms, the Cortex A72 CPU pairs a three-wide, in-order front end with a five-wide, out-of-order back end (i.e. 8-issue)."

... That's not how it works. You don't add front-end and back-end width to get issue width. I know that Ars' architectural chops took a hit when Stokes left, but this is ridiculous.

You take the minimum of the two[1], except that (per AnandTech's far better article [2]) the decoder is actually three fused macro-ops wide, not instructions, so the truth is actually somewhere between three and five instructions wide, depending on workload.

[1] or maybe just the back-end width, since most modern CPUs can short-circuit decode in tight loops.

[2] http://www.anandtech.com/show/9184/arm-reveals-cortex-a72-ar...

DannyBee4• 11 years ago

I don't think you understand how this works at all.

It's like when you add a few cortex m0's to your phone, and now you have a quad-core phone.

(It's 8 issue. You can issue 8 instructions to it, it just doesn't process them all in a single cycle :P)

ajross• 11 years ago

I don't think note #1 is true for this core. Intel CPUs have had a micro-op cache only since Sandy Bridge. I don't think ARM is doing that yet; certainly this article doesn't hint that they are.

ChuckMcM3• 11 years ago

I've not looked at the actual micro-architecture yet (has ARM put it up on their site yet he wonders ... ohh goody something is up (http://infocenter.arm.com/help/topic/com.arm.doc.100095_0001...) but when I read that I figured the 3 wide front end could ingest instructions until the 5 wide out of order back end was full (caveat pipeline bubbles from dependencies). That should boost the IPC of the A72 relative to the A57. I'll have to spend a couple of train trips reading the TRM to figure out if we could expect double the IPC on a typical workload.

swalsh• 11 years ago

"but is inching into the "low end" server market."

I've seen people run reddit bots using a Raspberry pi. After a few months its cheaper then a VPS!

icebraining• 11 years ago

Still takes quite a few months, you can get a VPS for $10/year (granted, with only 128MB, but it should be enough for a reddit bot).

tedunangst2• 11 years ago

Isn't the "will arm get fast before Intel gets efficient?" question at least five years old? And don't we have some answers? http://www.theverge.com/2012/10/4/3447984/motorola-razr-i-re...

angersock2• 11 years ago

there's also "will either happen before amd goes bankrupt"

Nelson69• 11 years ago

There is a more interesting dimension to that question. ARM chips tend to come with more third party accelerators. And I think there are more silicon vendors that are willing to cram more cores on to a chip. In the server space I'd think that it will be Intel with furious single core performance and maybe 10 or 20 cores total vs ARM based with 32-64+cores and some hardware acceleration for some more operations. It will some how be an asymmetric comparison unless someone starts really knocking out high performance ARM cores.

sliverstorm5• 11 years ago

TCO is supposedly becoming an increasingly dominant metric in the sever space, of which FLOPS/watt is probably more important than IPC. So in many server segments a class-leading "WPI" (we need some kind of name) with decent IPC could actually lead ARM to victory.

white-flame• 11 years ago

Realistically, how important is FLOPS to most servers? File servers, web servers, database servers, authentication servers... none of them tend to be heavy on floating point.

Sure, HPC is all about FLOPS, and desktops tend to use quite a bit for presentation nowadays, but I would consider memory bandwidth, I/O bandwidth, thread scaling, and single-thread performance non-FP metrics such as branch latency to be much more important in the general server market.

ksec• 11 years ago

Well I dont think TCO really flavor ARM at all. If you look at Server as a whole, putting in the ECC RAM, HDD or SSD and Network, the CPU power/performance scaling work much better in Intel's flavor.

minthd• 11 years ago

Another element that's being added to the mix is security. From a country's perspective, open cores and and control on chip design is valuable here. And that's exactly what the Chinese government is doing: "encouraging" their private sector to move away from intel's architecture to IBM's open power (and they'll probably won't mind an open ARM).

And who knoww, maybe later we'll see other countries shift to that kind of strategy.

digi_owl• 11 years ago

Reminds me of the Z80 and how everyone and their dog was making it and computers based on it.

Then came the PC clones and most nations stopped making their own stuff and just imported parts, or whole computers, from Asia.

InclinedPlane• 11 years ago

That's largely immaterial in the short to medium term, in the non-mobile space anyway. Intel has superior fab technology and vastly superior fab capacity, attaining parity with Intel (which would be difficult due to being behind in process technology) would still put you at a disadvantage, you'd need to be significantly better than Intel to make up for Intel's other advantages.

higherpurpose• 11 years ago

If ARM can get better at performance faster than Intel can get better at "low power" they can really put a dent into Intels market dominance in the larger computer space.

Even if it remains slightly behind on that metric, ARM is is basically untouchable by Intel in the price/performance metric. Many people, including the author seem to say stuff like "the next Core-M generation will totally beat this anyway!"

Except, Core M is nowhere near the price/performance of Cortex A72. That makes Core M uncompetitive in whatever markets the two would have to co-exist (which they don't right now, because Core M is only used in $1,000+ Windows or Mac OS X ultrabooks).

Even Atom can't be competitive on price/performance. The real (unsubsidized) gap between the two is somewhere around 2x and has remained that way since Atom's existence. The only way Atom can pretend to be competitive on price in the mobile market is because it's subsidized by Intel while it overcharges for the new Atom-based Celerons and Pentiums (Intel charges for them as much as it used to charge for the Haswell versions, and what it would've charged this year for the Broadwell versions - but those don't exist anymore).

M8• 11 years ago

How soon will we have 16/32 core mobile processors? It would be nice if something will force the industry to investigate alternative parallel programming paradigms.

robert_foss• 11 years ago

The workloads don't support that many threads. 8-core A53 chips exist, but they don't perform anywhere near what a 2-core A57 does for typical mobile workloads.

DannyBee4• 11 years ago

Yes, it doesn't make a whole heck of a lot of sense to add cores when they aren't cpu bound at all.

The idea that "the slow cores can just handle the background stuff" makes little to no sense in practice.

The most power effective thing to do is not add more cores (and all that entails), but get done the work something wants done as fast as humanly possible so you can go back to sleep. Having slow cores handle this for you sounds great in theory. In practice, it's been a huge bust (The one exception being dedicated-function coprocessors, etc)

This is one reason that, for example, higher speed wifi/etc chipsets often become less power intensive than the old chipsets (past the initial spike from it being new hardware) - they can sleep more

Scaevolus• 11 years ago

Companies aren't going to sacrifice millions of dollars to encourage development of new parallel programming techniques.

Developers barely know what to do with 4 cores, especially on mobile. The usual outcome is that foreground apps use a core, maybe two if they have async rendering, and background apps can run as well.

Mobile OpenCL is the closest we'll come to massively parallel programming in people's pockets for the foreseeable future.

madez• 11 years ago

It seems like you are ignorant about Servo. "We are still evaluating plans to ship Servo as a standalone product, and are focusing on the mobile and embedded spaces rather than a full desktop browser experience in the next two years." https://github.com/servo/servo/wiki/Roadmap

CHY8722• 11 years ago

We probably won't for a very long time. Many of the architectural designs that sit behind current processors would have to be entirely rethought.

For example snoopy caches would I think stop scaling by that point, and in order to fit the cores onto the chip you'd probably have to get rid of a lot of the control flow logic (superscalar, branch predictor), so you'd lose a tonne of performance on singlethreaded code.

You really don't want to move onto 16 cores unless you have to. More likely is more powerful instructions (perhaps true vector instructions).

StillBored2• 11 years ago

A72 is nice, but to really compete in the server market I think they need another product. One that drops the performance/watt metric for the "as fast as it goes" metric. That is because while a lot of people are going to want efficient chips, if ARM really takes off there will definitely be people who want to share a binary with a CPU that is screaming fast.

duaneb• 11 years ago

A72 is nice, but to really compete in the server market I think they need another product. One that drops the performance/watt metric for the "as fast as it goes" metric.

That seems like vertical scaling instead of horizontal scaling. I'm not sure how much money there is in the former market—maybe look at mainframe markets.

Horizontal scaling is a much easier sell, especially for data centers. I will put money on the fact that Facebook will eventually invest heavily in this architecture design.

tw04• 11 years ago

They will literally NEVER beat Intel at the speed game in servers and they know it. If you want blazing fast, you've already got a market leader that's held the crown almost their entire existence.

StillBored2• 11 years ago

? I'm not sure that is accurate. Before the PPRO, there were the RISC vendors, which overwhelmingly were significantly faster. In the early 2000's there was AMD, for about 5 years was faster in nearly every regard. Still today, you have things like the POWER 8, which while not necessarily faster for every workload definitely can beat Intel's offerings for certain workloads. So at the moment it seems to be similar to the early 80's when it wasn't clear cut who had the fastest CPU because it depended on workload. Hence my expectation that ARM will get there, if they put in the effort.

faragon• 11 years ago

Issuing up to 8 instructions per clock (3 in order, 5 OoO) is single-thread serious performance. It looks like is going to be much better than the NVidia Denver CPU (also 8 instruction/cycle, but all of them in-order). It will be scary comparing 20-70$ ARM CPUs performing similar to Intel's in the 100-200$ price range.

chisleu• 11 years ago

"There's also a reworked 3-way L1 cache that's "almost as powerful as direct-mapped cache," and a much smaller (~10 percent) and reorganized dispatch unit."

What? I'm guessing it is a typo for a 2/4 way set associative cache, or something I know not what.

ARM details its new high-end CPU core, Cortex A72

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