Decoupled 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.

Feeding the Beast: 2x Cache Bandwidth in Haswell TSX
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  • kylewat - Tuesday, February 12, 2013 - link

    Compare the amount of space devoted to iOS v. Android. Your critique is not valid. The iPhone launch had a new version of the software, new hardware, new processor, the first apple designed processor, scuff gate, etc.

    S III may be a great phone, but it didn't have anywhere near the amount to talk about given it is an android phone (not even Nexus!).

    Anyway, Apple if anything is under served when compared to market share, they have a focused product line that doesn't follow trends like the Android Eco system as a whole (krait, jelly bean, etc do not need multiple reviews).

    Apple is the biggest company in the world driven by the biggest product in the world. The iPhone. That is just fact.
    Reply
  • Kepe - Friday, October 05, 2012 - link

    Also look at any other Apple product review. They are all ridiculously in-depth with analysis about almost every single component in the product. Macbook Pro with Retina Display got 18 pages, the 3rd gen iPad got 21 pages. Don't get me wrong, I like a proper review with everything analyzed, but it's only the Apple products that get these huge reviews. But compared to those massive Apple reviews, it's like all other products are just glanced over in a hurry. The new Razer Blade got 9 pages. Asus Transformer Pad Infinity got 8 pages. Reply
  • Peanutsrevenge - Friday, October 05, 2012 - link

    What the hell are you guys bitching about?

    Of course the iPhone articles are going to be longer and more numerous than GS3 articles.

    iPhone releases come with new iOS releases and have their own eco-system.

    Android phone releases use a common OS across them and therefore much of what's in one article doesn't need repeating in another.

    Anand liking Apple is not our problem, I can see why people like them (not so much Anand) and that's fine, personally I dislike them (hate was originally typed, but was edited due to being incorrect), but still respect them and respect people who purchase their products (and pay for their litigation).

    An entire page of comments talking about how Anand isn't allowed to like or talk about Apple products because you guys don't like is ridiculous, they're a PC company and should exist on a PC website.

    Grow up.
    Reply
  • Kepe - Friday, October 05, 2012 - link

    Sure, but I'm talking about dedicating entire, long articles to such things as the iPhone display or why it doesn't have a certain feature and so on. The SGS III has a very interesting display, too. Still it didn't get nearly as much attention. Of course Anand is allowed to talk about Apple products. What I want, though, is Anand(tech) to be as thorough in reviewing other products, too, or then stop making those huge articles only about Apple products. Because that is biased.
    In the Macbook Pro Retina article Anand talked about the cooling system and the fan blades for one page. When I read any other laptop review on Anandtech, cooling is briefly described in a sentence or two.
    Dedicating so much attention to just one company's products makes it look like Anandtech is biased. And that is not good.
    Reply
  • Magik_Breezy - Sunday, October 14, 2012 - link

    Hopefully because of these comments they'll finally see what we want, not some Apple crap. Good engineering stupid management Reply
  • Spunjji - Thursday, October 18, 2012 - link

    Nailed it. Reply
  • vFunct - Saturday, October 06, 2012 - link

    Android products would get more coverage if they bothered to do any engineering on them. Since they don't push the technology the way Apple does, they don't need a more in-depth review. Reply
  • StevoLincolnite - Saturday, October 06, 2012 - link

    You're kidding right? Hardware wise Apple has always been behind the curve compared to the competition in every facet of it's product line-ups or very quickly beaten. Reply
  • lmcd - Saturday, October 06, 2012 - link

    Umm, I would disagree there. Apple has always been ahead of the curve in GPUs and this is the FIRST TIME SINCE BEFORE THE A-SERIES that Apple has had a GPU without an overwhelming lead on the competition for more than half a year.*

    While GPU selection isn't always huge, it's one of the biggest points of differentiation in mobile chips, along with power use.

    *excluding the A4 if you count from when it was first in a phone as opposed to in a tablet.
    Reply
  • Magik_Breezy - Sunday, October 14, 2012 - link

    The last time I played a game on my phone was about 8 months ago and I'm 15! To say that Apple pushes their hardware is naive as it gets.
    The Galaxy S III was the best purchase Ive made, even my mum doesn't like my iPhone 4.
    Reply

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