Intel's Haswell Architecture Analyzed: Building a New PC and a New Intel
by Anand Lal Shimpi on October 5, 2012 2:45 AM ESTDecoupled L3 Cache
With Nehalem Intel introduced an on-die L3 cache behind a smaller, low latency private L2 cache. At the time, Intel maintained two separate clock domains for the CPU (core + uncore) and a third for what was, at the time, an off-die integrated graphics core. The core clock referred to the CPU cores, while the uncore clock controlled the speed of the L3 cache. Intel believed that its L3 cache wasn't incredibly latency sensitive and could run at a lower frequency and burn less power. Core CPU performance typically mattered more to most workloads than L3 cache performance, so Intel was ok with the tradeoff.
In Sandy Bridge, Intel revised its beliefs and moved to a single clock domain for the core and uncore, while keeping a separate clock for the now on-die processor graphics core. Intel now felt that race to sleep was a better philosophy for dealing with the L3 cache and it would rather keep things simple by running everything at the same frequency. Obviously there are performance benefits, but there was one major downside: with the CPU cores and L3 cache running in lockstep, there was concern over what would happen if the GPU ever needed to access the L3 cache while the CPU (and thus L3 cache) was in a low frequency state. The options were either to force the CPU and L3 cache into a higher frequency state together, or to keep the L3 cache at a low frequency even when it was in demand to prevent waking up the CPU cores. Ivy Bridge saw the addition of a small graphics L3 cache to mitigate this situation, but ultimately giving the on-die GPU independent access to the big, primary L3 cache without worrying about power concerns was a big issue for the design team.
When it came time to define Haswell, the engineers once again went to Nehalem's three clock domains. Ronak (Nehalem & Haswell architect, insanely smart guy) tells me that the switching between designs is simply a product of the team learning more about the architecture and understanding the best balance. I think it tells me that these guys are still human and don't always have the right answer for the long term without some trial and error.
The three clock domains in Haswell are roughly the same as what they were in Nehalem, they just all happen to be on the same die. The CPU cores all run at the same frequency, the on-die GPU runs at a separate frequency and now the L3 + ring bus are in their own independent frequency domain.
Now that CPU requests to L3 cache have to cross a frequency boundary there will be a latency impact to L3 cache accesses. Sandy Bridge had an amazingly fast L3 cache, Haswell's L3 accesses will be slower.
The benefit is obviously power. If the GPU needs to fire up the ring bus to give/get data, it no longer has to drive up the CPU core frequency as well. Furthermore, Haswell's power control unit can dynamically allocate budget between all areas of the chip when power limited.
Although L3 latency is up in Haswell, there's more access bandwidth offered to each slice of the L3 cache. There are now dedicated pipes for data and non-data accesses to the last level cache.
Haswell's memory controller is also improved, with better write throughput to DRAM. Intel has been quietly telling the memory makers to push for even higher DDR3 frequencies in anticipation of Haswell.
Facebook
Twitter
RSS
245 Comments
View All Comments
tim851 - Friday, October 5, 2012 - link
This is a perfect demonstration of the power of competition.With AMD struggling badly, Intel was content in pushing Atom. They didn't want to innovate in that sector, they sold 10 year old technology with horribly outdated chipsets. Yes, they were relatively cheap, but I was appalled.
Step in ARM, suddenly becoming a viable competitor. Now Intel moves its fat ass and tries to actually build something worthwhile.
Sadly, free markets are an illusion. Intel should pay dearly for the Atom fiasco, but they won't. Just as they didn't pay for the Pentium 4 debacle. They will come 5 years late to the party, but with all their might, they will crush ARM. ARM will fall behind, they can't keep up with that viscious tick-tock-cycle. Who can?
In 8 years, ARM will have been bought by some company, perhaps Apple. ARM will then no longer be a competitor, it will be just a different architecture, like X86. I don't see Apple having any long-term interest in designing their own hardware, it's way too unsexy. They will just cross-licence ARM with Intel and in 10 years time, Intel will rule supremely again.
UpSpin - Friday, October 5, 2012 - link
You forget that Intel vs. ARM is something bigger than AMD vs. Intel.Behind ARM stand Qualcomm, Samsung, Apple, ...
All new software is written for ARM, not Intel (x86) any longer. Microsoft releases a rewritten ARM Windows RT with a rewritten Office for ARM. Android runs on ARM and everyone supports the ARM version, while only Intel has to keep it compatible with x86.
Haswell will get released, when exactly? In a year, ARM A15 in maybe two months. Haswell has nice power savings, but it's still a Ultrabook design. The current Atom SoCs are much worse than current A9/Krait SoCs. Intel heavily optimized the software to make it look not that bad (excellent Sunspider results), but they are.
If Windows 8 is a success, Intel can be lucky. If it's not, what many expect, Intel has a real problem.
Intel is a single company building and developing their CPU/SoC. ARM SoCs get build and developed by a magnitude of companies.
If Apple can design their own ARM based SoC which has the same performance as a Haswell CPU (which is easy in the GPU area (the iPad has a faster GPU than the Intel CPUs most probably already, and with A15 and Apples A6 it's possible to get as fast with the CPU, too), they will be able to move Mac OS to ARM. This allows them to build a very very power efficient, lightweight, silent MacBook. They can port apps from iOS to MacOS and vice versa. Because they designed their SoC in-house, they don't have to fear competition the near term.
Apple always wants a monopoly, so it doesn't make sense for them to cross-license anything.
tuxRoller - Friday, October 5, 2012 - link
Unless your app is doing some serious math you can get by with just using a cross platform key chain.Frankly, the hard part is targeting the different apis that are, currently, predominating on each arch. However, assuming those don't change , and the form factor doesn't either, your new app should just be a compile away.
Kidster3001 - Monday, October 15, 2012 - link
Current ATOM SOC's are not "much worse" than A9/Krait. Most CPU benchmarks running in native code will favor the Intel SoC. It's the addition of Android/Dalvik that leans the favor back to ARM. Android has been on ARM for a lot longer and is more optimized for ARM code. Android needs to be tweaked more yet to run optimally on x86.Kidster3001 - Monday, October 15, 2012 - link
" with A15 and Apples A6 it's possible to get as fast with the CPU, too"say what? A15 and A6 are a full order of magnitude slower than Haswell. omg
Dalamar6 - Sunday, May 12, 2013 - link
Nearly all of the software on Android is junk.Apple blocks everything at a whim and gives no control.
I don't know about Windows RT, but I suspect it will suffer the same manner of crap programs Android does if it's not already.
Even if people are more focused on developing for ARM, the ARM OSes are still way behind in program availability(especially quality). And it's downright sad seeing people charging money for simple, poorly coded programs that can't even compare to existing open source x86 software.
jacobdrj - Friday, October 5, 2012 - link
I agree competition is good/great. However, how you categorize Atom is just not true! Atom filled a very real niche. Cheap mobile computing. Not powerful, but x86 and fast enough to do basic tasks. I loved my Atom netbook and used it until it bit the dust last week. Would I have liked more power? Sure, but not at the expense of (at the time) battery life. Besides, once I maxed it out by putting in a SSD and 2 GB RAM, my netbook often outpaced many peoples' newer more powerful Core based laptops for basic tasks like word processing and web browsing.Just because power users were unhappy does not mean Atom was a 'fiasco'. Those old chipsets allowed Atom netbooks to regularly sell, fully functional, for under $200, a price point that Tablets of similar capability are only just starting to hit almost 4 years later...
Don't bash Atom just because you don't fit into it's niche and don't blame Intel for HP trying to oversell Atom to the wrong customers...
Peanutsrevenge - Friday, October 5, 2012 - link
If competition is 'good/great' what does that make cooperation?Imagine the possibility of Intel and AMD working together along with Qualcomm, Imagination etc.....
Zeitgeist Movement.
Kidster3001 - Monday, October 15, 2012 - link
Intel is not going this way because "ARM stepped in". Intel is going this way because it decided to go play in ARMs playground.krumme - Friday, October 5, 2012 - link
My Samsung 9 series x3c (ivy bridge), have a usage looking on this page with wifi at bt on ranging from 4.9W to 9.9W from lowest to higest screen brightness, with a normal usage of screen of 7.2W with good brightness (using samsung own measuring tool).So screen is by far the most important component on a modern machine. In the complete ecosystem i wonder if it matter how efficient Haswell is. The benefit of 10W tdp for say the same performance is nice, but does it really matter for the market effect. And the idle power is already plenty low.
I doubt Haswell will have an significant impact - as nice as it is. This is just to late and way to expensive for the mass market. Those days are over.
At the time it hits market dirt cheap TSMC 28nm A15 and bobcat successor hits the market for next to nothing, and will give 99% of the consumers the same benefits.