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
lmcd - Saturday, October 6, 2012 - link
Interestingly, this might be the first chance in forever AMD has at competing with Intel. If Haswell's sole goal is to hit lower power targets, and Piledriver hits its 15% and Steamroller its 15% over that, AMD is suddenly right up with Intel's i5 series with its GPU-less chips, and upper i3-range with their APUs, which is absolutely perfect positioning: most i5 purchases are for people planning to pair with discrete graphics, while most i3 series seem to go to the PC buyer looking for low price tags.The one downside is that the i7 series is Intel's money-maker: the clueless people who think they're getting maximum performance but are really just feeding the binning system and buying an unbalanced PC.
milkod2001 - Sunday, October 7, 2012 - link
u got it wrong bro, Intels money maker is not i7, it's i3 and i5(low end and a bit of mainstreem)as for Haswell, on paper it looks too good to be true as Ivy did last year and ended up everything but impressive.
Since Intel conroe core(2006) there actually were not any significant improvements worth mentioning.There's not much extra what todays CPUs can do and Pentium4 could not a decade ago.
I would love to see some innovations user could really benefit from(something like reattachable,thin, light, portable, firm solar panel hooked at the back of screen or even build in as last layer into screen itself) and not that crap Intel/AMD gives us year by year.
xeizo - Sunday, October 7, 2012 - link
Anand is very right, it's everything about power savings which in effect makes smaller and more portable form factors possible!As for mainstream perfomance, my Linux workstation still uses a Q9450 rev. C1 from 2008 clocked at 3.2GHz and a SSD of course. That box feels in every way as snappy as my Windows-box with Sandy Bridge at 4.8GHz. Which means, I really didn't need more performance than what C2Q already gave. Of course the SB-box benchmarks much faster, about twice as fast in most things, but the point is for myself I really don't need that perfromance except for some occasional game.
But I could use a smaller, cooler running device instead!
Teknobug - Tuesday, October 16, 2012 - link
LOL my Linux system still runs a Sempron and it's still fast.oomjcv - Sunday, October 7, 2012 - link
Very interesting article, enjoyed reading it.Something I would like to see is a decent comparison between Intel's and AMD's plans. Many might be able to outline the basics, but a thorough article on the subject should be rather enlightening... Comparing their design philosophies, architectures, possible pitfalls and successes etc, pretty much what's been done with this article only with both companies.
I know it might be time consuming but I imagine it could be quite a nice read.
zwillx - Monday, January 21, 2013 - link
agreed; it's difficult to find the common ground with so many different chip architectures. x86 is a big enough competition but now it's getting split wide open with ARM and BIG/litle etc etc so it's always helpful to have either more charts or real world examples lol.My take from this article though: Haswell still won't have the prowess to beat the GT650. I have GTX660 in my laptop w/ Optimus (TM). It works. Runs a game on HD4000 at 17 FPS. On the GTX660 I get 100+ fps, and am able to use higher anti-aliasing settings. So, clearly a 100% improvement over Ivy bridge is only putting the chip into "mediocre" category by the time its released.
alexandrio - Sunday, October 7, 2012 - link
"The bigger concern is whether or not the OEMs and ISVs will do their best to really take advantage of what Haswell offers. I know one will, but will the rest?"I am curious who is that one OME that will do their best to really take advantage of Haswell offers?
zwillx - Monday, January 21, 2013 - link
Apple. Or are you joking. I personally hate Apple and have since the original iMac but their engineering is top notch when it comes to getting ideal performance from silicon to user. So.. guessing that's the reference.Silma - Monday, October 8, 2012 - link
A fine read, technically very comprehensive, but still overly melodramatic.While it is true that it is crucial for Intel to step a foot in the byod market some things still hold true:
- In value and profit the PC processor market is much bigger than the byod processor market and will stay so for years because PCs, especially business PCs won't disappear anytime soon.
- Nobody can touch Intel in this market, it has been proved for decades. Not AMD at the height of its success, not mighty IBM, not Sun, nobody.
- Contrary to what you say Intel has a definitive production advantage and there are very few fabs able to compete. Note that Apple is incapable of producing processors, it is dependent on external manufacturers.
- What Apple does with its processor is interesting business wise for its iPods/Pads/Phones, but Apple doesn't have the research power Intel and others have in the chip space and I can't see how it will innovate better than Intel and other competitors.
- Intel is aware of its shortcomings, is pushing tremendously in the right direction. A competitor that doesn't rest on its laurels is a mighty threat, ARM beware.
- If Apple stops using Intel processors, it will of course wipe a few hundred millions of Intel's turnover but won't be anything remotely dangerous for Intel
- It remains to be seen that Apple users will accept yet another platform change.
- It remains to be seen that it would make sense business-wise for Apple
- I am quite sure many phone companies will be open about renewed chip competition and not letting a single platform become too powerful.
All in all it seems to me Intel is as dangerous as ever, executing very well in its core business and heading towards great things in the phone/pad space.
johnsmith9875 - Thursday, October 11, 2012 - link
Why couldn't they at least stick to LGA2011?