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Intel's 128MB L4 Cache May Be Coming To Broadwell and Other Future CPUs
MojoKid writes "When Intel debuted Haswell this year, it launched its first mobile processor with a massive 128MB L4 cache. Dubbed "Crystal Well," this on-package (not on-die) pool of memory wasn't just a graphics frame buffer, but a giant pool of RAM for the entire core to utilize. The performance impact from doing so is significant, though the Haswell processors that utilize the L4 cache don't appear to account for very much of Intel's total CPU volume. Right now, the L4 cache pool is only available on mobile parts, but that could change next year. Apparently Broadwell-K will change that. The 14nm desktop chips aren't due until the tail end of next year but we should see a desktop refresh in the spring with a second-generation Haswell part. Still, it's a sign that Intel intends to integrate the large L4 as standard on a wider range of parts. Using EDRAM instead of SRAM allows Intel's architecture to dedicate just one transistor per cell instead of the 6T configurations commonly used for L1 or L2 cache. That means the memory isn't quite as fast but it saves an enormous amount of die space. At 1.6GHz, L4 latencies are 50-60ns which is significantly higher than the L3 but just half the speed of main memory."
At least as marketed, the main advantage is allowing the GPU some RAM that isn't DDR3 stolen from the main system a couple of hops away (which has traditionally been one of the things that make integrated graphics really suck, and cheap discrete parts that use DDR instead of GDDR, and/or an excessively narrow or slow memory bus kind of suck).
Given that even intel's marketing optimists don't say much about CPU performance (and also given that it's a mobile-only feature, you can't even buy an non-BGA part expensive enough to have it, which would be unusual if it actually improved CPU performance enough to get enthusiasts worked up; but is downright sensible if the target market is laptops sufficiently size/power constrained not to have discrete GPUs; but where pure shared memory was dragging GPU performance down.)
Photoshop? Considering that the adobe rgb or other color spaces combined with the file sizes of some of the larger images coming out of cameras, your gains in latency would really depend on Photoshop and the OS being able to handle the L4 cache and keep the right part of the image in the cache. Video editing, with file sizes into the gigabyte range would probably see no gains at all. Video conversion, with a program that keeps a reasonably sized buffer, should see a good performance gain; but it would require code that knows the L4 is available or the OS to predict that L4 is a good place to put a 10-50-100MB buffer. The real gain will be in common things: playing a video, browsing the web (seen how much memory a bit of javascript or the JRE can eat up lately? Or Silverlight/Flash?) and email clients (cache all your email in L4 for faster searching).
As for battery life, I have no idea. It might use more power, since DRAM requires constant power to refresh data where SRAM is pretty stable; but the lower leakage of using a single transistor instead of 6 might prove to be a benefit. It would take a good bit of time and some pretty good test code to figure the difference, I suspect.
Cache performance impact is very heavily dependant upon application characteristics. Specifically, active memory.
Best case, when you're working with an active set that's larger than L3 but under L4 - around 100MB or so - and you're accessing it on a repeating pattern, and the compiler hasn't found any tweaks to help, and you're not multitasking, and the OS isn't swapping you out every slice, and the stars are aligned in your favor... the theoretical maximum performance gain can be up to 2x. It's very rare you'll find a program that benefits that much, though. Closest I can think of is image processing.
So in the real world, anywhere from 'no benefit' to 'double the speed' depending on application.
128MB L4 cache. [...] on-package (not on-die) pool of memory
what this means is the memory is not on the same piece of silicon as the CPU, just stuffed in the same chip package. this means they have to be connected by a lot of tiny wires instead of being integrated directly. the downside to this is that there is bandwidth between the L4 memory and the CPU is very limited and it uses more power. like AMD's first APUs where just two ICs on the same chip, i dont not think this will result in a drastic performance improvement but i'm unsure of the power savings. If AMD gets wise, they will beat Intel to the punch but then again. though if AMD is really smart, they would put out ARMv8 chips not just for servers(/desktops?) but for smartphones/tablets and laptops.
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Broadwell represents a miniaturization step from 22 to 14 nm structures. Why do they keep the capacity of the Crystalwell L4 cache at 128 MB? They could put twice that memory onto a die with the same area as the 22 nm Crystalwell version. Is the Crystalwell die for the Haswell CPUs so large and expensive that they have to reduce its size?
From Anandtech's article on Crystalwell:
There's only a single size of eDRAM offered this generation: 128MB. Since it's a cache and not a buffer (and a giant one at that), Intel found that hit rate rarely dropped below 95%. It turns out that for current workloads, Intel didn't see much benefit beyond a 32MB eDRAM however it wanted the design to be future proof. Intel doubled the size to deal with any increases in game complexity, and doubled it again just to be sure. I believe the exact wording Intel's Tom Piazza used during his explanation of why 128MB was "go big or go home". It's very rare that we see Intel be so liberal with die area, which makes me think this 128MB design is going to stick around for a while.
I get the impression that the plan might be to keep the eDRAM on a n-1 process going forward. When Intel moves to 14nm with Broadwell, it's entirely possible that Crystalwell will remain at 22nm. Doing so would help Intel put older fabs to use, especially if there's no need for a near term increase in eDRAM size. I asked about the potential to integrate eDRAM on-die, but was told that it's far too early for that discussion. Given the size of the 128MB eDRAM on 22nm (~84mm^2), I can understand why. Intel did float an interesting idea by me though. In the future it could integrate 16 - 32MB of eDRAM on-die for specific use cases (e.g. storing the frame buffer).
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