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.
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jigglywiggly - Friday, October 5, 2012 - link
wish the onboard gpu was better =/woula been nice for a laptop
tipoo - Friday, October 5, 2012 - link
2x the HD4000 is pretty decent for integrated. I wonder if that's 2x with or without the eDRAM cache though.ElvenLemming - Friday, October 5, 2012 - link
It's been known for a while that Haswell was only going to have a moderate improvement in the iGPU and the next big overhaul would be coming with Broadwell.csroc - Friday, October 5, 2012 - link
This is impressive, it might convince me it's time for a new laptop. On the other hand I also need to build a new desktop workstation and Haswell so far hasn't impressed me in that space.mayankleoboy1 - Friday, October 5, 2012 - link
Is Intel sacrificing Desktop CPU performance to make an architecture that is geared to the mobile space ?csroc - Friday, October 5, 2012 - link
It feels that way to me. Mobile performance seems to be their big concern now, that and improving the GPU. Two things I generally can't be bothered to care about when I'm looking to build a new workstation. I suspect I'll build an Ivy Bridge system because I could use it now and see nothing worth getting excited about.dishayu - Friday, October 5, 2012 - link
I fully share your sentiment. TO be very crude, i don't mind at all, paying for power imporvements, because it will pay back for itself in the long term (by consuming less power AND needing lesser cooling). But i DO mind very much, paying for 40 EUs of GPU on my desktop build which i will not use even for a second. Me, you and many others do not care about on-die graphics and Intel should realize that.I don't know why intel can't offer us both GPU and GPU-less options, the way they did with motherboards back in the days? P965 had no graphics, G965 did. Pretty sure it's technologically not an issue.
DanNeely - Friday, October 5, 2012 - link
If it makes you feel any better; reports elsewhere are that GT3 will be mobile only, because desktops don't have the power/size constraints driving the need for premium IGPs.Intel's not IGP CPUs are the E series parts; unfortunately they've failed to execute on the enthusiast side in terms of price/launch date leaving them as mostly server parts.
There just aren't enough of us to justify Intel adding another die design for their mass market socket that doesn't have an IGP at all instead of just letting us turn it off and use the extra TDP headroom for more time at boost speeds.
Omoronovo - Friday, October 5, 2012 - link
I'm somewhat in disagreement with you both.Whilst I share a concern that Intel is no longer focusing on raw performance improvements in the purely desktop space, they are still delivering incremental updates to the architecture that will benefit all current software (even if only marginally). However, processor performance has been reaching more and more diminishing returns in recent years, namely that software is simply not able to take advantage of multiple cores and improved performance because of (primarily) locks and complexity in creating multi-threaded applications.
As such, Intel has been focusing on that area - to make it easier for software and software developers to take advantage of the performance that exists *now* rather than brute forcing the issue by simply delivering more raw performance (much of which will be wasted/remain idle due to current software constraints).
With this, Intel has been able to focus on keeping performance high whilst subsequently dropping power usage substantially - the fact the iGPU is oftentimes not being used in a desktop environment does not invalidate it's utility - QuickSync is a prime example of where the gpu can accelerate certain types of processing, and if more software takes advantage of this we should see even more gains in future.
For the last 6 years or so, Intel has shown that it knows what demands will be placed on future computing hardware, and they seem convinced that this is the way to go. We might not be there yet, but technologies like C++AMP, OpenCL and such make me hopeful that this will change in a few years.
cmrx64 - Friday, October 5, 2012 - link
I solved this problem by buying an Ivy Bridge Xeon (specifically, an E3-1230v2). No GPU, lower power consumption than the equivalent i5/i7, has hyperthreading, performs really good, and a lot cheaper than an i7.If you don't care about the GPU, look to the Xeon line.