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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stop-a - Saturday, October 6, 2012 - link
100% agree on the well engineered part especially on the antenna gate when Steve God was saying "you're holding wrong", plus the recently ingeniously designed sapphire glass lens camera when Tim Schmok was saying "stay away from bright light source". Boy, Apple products must be engineered straight from the heaven; they are just too perfect for a mere earthling to use.Paer0 - Saturday, October 6, 2012 - link
@stop-a. Since you are a 100% Apple hater, let me ask you this what computer do you use? And what OS do you use on it? I hope it doesn't crash several times a day. I use a MacBook Pro 2012 and I don't see anything come close.Urizane - Saturday, October 6, 2012 - link
You really shouldn't use the 'crash several times a day' piece anymore. I'm annoyed every time I see this. My Windows 7 machine has an uptime of 20 days and counting. Most of the time, it waits for me to connect to it via SplashTop or FTP, or it's recording TV shows, but when I play games, I stress it bigtime. Seriously, stop with the Windows constantly crashes crap. It's just plain false now.P.S. - 20 days ago, I brought it to another house, thus the interruption in uptime.
StevoLincolnite - Saturday, October 6, 2012 - link
I have a Dual socket 2011 motherboard with dual Core i7 3930K's both chips clocked to 4.6ghz, 32gb of ram, Triple Radeon 7970 3gb cards powering 3x 27" Dell U2711 monitors in Eyefinity.Kay go. Lets see if your Mac can keep up or a Mac workstation at the same price. (Hint: Not going to happen.)
Besides, Mac's look ugly, I prefer the whole she-bang of a side window with a nice water cooling loop and having the whole thing light up, not some dull silver box.
Plus, my system is completely stable. Never had a crash yet with Windows 7 and... I have access to the last couple of decades worth of software and games, not to mention emulation of other platforms.
I can also pretty much find software and hardware easily and it will "just work" I never have to ask the question of: Will it work on a Mac?
lmcd - Saturday, October 6, 2012 - link
I don't think you're in their target audience, for some reason. They're the best preconfigured system out there, especially once you ignore price.Magik_Breezy - Sunday, October 14, 2012 - link
You'd hope a manufacturer can "configure" a system for an extra $1,400Hardware: $400
I'm Apple: $1,400
Total: $1,800
With PCs manufacturers almost always loose money selling motherboards
vt1hun - Tuesday, October 9, 2012 - link
Two Core i7s working on a dual 2011 socket motherboard? You need QPI links for that which only certain Xeons have. Sounds like your system will just NOT work !FunBunny2 - Tuesday, October 9, 2012 - link
If Steve hadn't done what Apple does best (according to Steve) "steal" BSD unix, would you still be crowing?Magik_Breezy - Sunday, October 14, 2012 - link
His operating system doesn't crash 7 times a day because he doesn't run OS X, I'll rephrase that, because he isn't a retardHisDivineOrder - Sunday, October 7, 2012 - link
Let's not forget the obscenely high failure rates due of Macbook Pro's because they are huge, metallic, and yet refuse to have vents ruin the smooth awesomeness of their aesthetic.Whoops, for many it won't last more than two years, if that. Hell, if you're lucky, your battery will give out before your laptop cooks. Regardless, look up what Apple suggests and you'll get:
Buy another one. Yours is old. ;)