Further Power Managed Cache?

One new thing we learned about Nehalem at IDF was that Intel actually moved to a 8T (8 transistor) SRAM cell design for all of the core cache memory (L1 and L2 caches, but not L3 cache). By moving to a 8T design Intel was able to actually reduce the operating voltage of Nehalem, which in turn reduces power consumption. You may remember that Intel’s Atom team did something similar with its L1 cache:

“Instead of bumping up the voltage and sticking with a small signal array, Intel switched to a register file (1 read/1 write port). The cache now had a larger cell size (8 transistors per cell) which increased the area and footprint of the L1 instruction and data caches. The Atom floorplan had issues accommodating the larger sizes so the data cache had to be cut down from 32KB to 24KB in favor of the power benefits. We wondered why Atom had an asymmetrical L1 data and instruction cache (24KB and 32KB respectively, instead of 32KB/32KB) and it turns out that the cause was voltage.

A small signal array design based on a 6T cell has a certain minimum operating voltage, in other words it can retain state until a certain Vmin. In the L2 cache, Intel was able to use a 6T signal array design since it had inline ECC. There were other design decisions at work that prevented Intel from equipping the L1 cache with inline ECC, so the architects needed to go to a larger cell size in order to keep the operating voltage low.”

Intel states that Nehalem’s “Core memory converted from 6-T traditional SRAM to 8-T SRAM”. The only memory in the “core” of Nehalem is its L1 and L2 caches, which may help explain why the L2 cache per core is a very small 256KB. It would be costly to increase the transistor count of Nehalem’s 8MB L3 cache by 33%, but it’s also most likely not as necessary since the L3 cache and the rest of the uncore runs on its own voltage plane as I’ll get to shortly.

Cache Hierarchy Integrated Memory Controller
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  • pool1892 - Friday, August 22, 2008 - link

    first of all the enthusiast market is a very tiny niche, it would not kill intel if you were right.
    but you are not. the L2 of penryn (and banias) is much more like the nehalem L3 than the nehalem L2. and if you have a single threaded game it now has 8mb at similar latencies, but with a second buffer, the 256k L2, and a MUCH smaller cache miss penalty.
    concerning hyperthreading: please read the article first. nehalem switches off what it does not need, powerwise. and about fiftytwo other very vaild arguments.
    Reply
  • smilingcrow - Thursday, August 21, 2008 - link

    You do realise that you aren’t meant to drink the snakeoil as it can rot your brain…. Reply
  • JarredWalton - Friday, August 22, 2008 - link

    I don't think Nehalem will "fail" at running games... in fact I expect it to be faster than Penryn at equivalent clocks. I just don't expect it to be significantly faster. We'll know soon enough, though. Reply
  • chizow - Friday, August 22, 2008 - link

    I'd agree with the OP, maybe with a different choice of words. Also, Johan seems to disagree with you Jarred, citing the decreased L2 and slower L3 along with similar maximum clock speeds as to why Nehalem may perform worst than Penryn at the same clocks in games. We've already seen a very preliminary report substantiating this from Hexus:

    http://www.hexus.net/content/item.php?item=15015&a...">http://www.hexus.net/content/item.php?item=15015&a...

    There's still a lot of questions about overclockability as well without a FSB. It appears Nehalem will run slightly hotter and draw a bit more power under load than Penryn with similar maximum clockspeeds when overclocked.

    What's most disappointing is all of these new, hyped features seem to amount to squat with Nehalem. Where's the benefit of the IMC? Where's the benefit of the triple channel memory bus? Where's the benefit of HT? I know....I know...its for the server market/HPC crowd, but no one finds it odd that something like an IMC is resulting in negligible gaming gains when it was such a boon for AMD's Athlon 64?

    All in all, very disappointing for the Gamer/Enthusiast as we'll most likely have to wait for Westmere for a significant improvement over Penryn (or even Kentsfield) in games.
    Reply
  • cornelius785 - Sunday, August 24, 2008 - link

    From looking at that article, I have doubts that it could be worse than penryn in games. I also doubt that the performance increase is going to be the same as it was for Core 2. People are expecting this and since their figuring out that their unrealistic expectations can't be realized in Nehalem, they call Nehalem trash and say they'll go back to amd, ignoring the fact that Intel is still king in gaming with EXISTING processors. It is also not like 50 fps at extremely high setting and resolution isn't playable. Reply
  • JarredWalton - Friday, August 22, 2008 - link

    Given that is early hardware, most likely not fully optimized, and performance is still substantially better than the QX6800 in several gaming tests (remember I said clock-for-clock I expected it to be faster, not necessarily faster than a 3.2GHz part when it's running at 2.93GHz).... Well, it's going to be close, but overall I think the final platform will be faster. Johan hasn't tested any games on the platform, so that's just speculation. Anyway, it's probably going to end up being a minor speed bump for gaming, and a big boost everywhere else. Reply
  • Felofasofanz - Thursday, August 21, 2008 - link

    It was my understanding that tick was the new architecture and tock was the shrink. I thought Conroe was tick, Penryn tock, then Nehalem tick, and the 32nm shrink tock? Reply
  • npp - Thursday, August 21, 2008 - link

    I think we're facing a strange issue right now, and it's about the degree of usefulness of Core i7 for the average user. Enterprise clients will surely benefit a lot from all the improvements introduced, but I would have hard time convincing my friends to upgrade from their E8400...

    And it's not about badly-threaded applications, the ones an average user has installed simply don't require such immense computing power. Look at your browser, or e-mail client - the tasks they execute are pretty well split over different threads, but they simply don't need 8 logical CPUs to run fast, most of them simply sit idle. It's a completely different matter that many applications who happen to be resource-hungry can't be parallelized to a reasonable degree - think about encryption/decryption, or similar, for example.

    It seems to me that the Gustafson's law begins to speak here - if you can't escape your seqential code, make your task bigger. So expect something like UltraMegaHD soon, then try to transcode it in real time... But then again... who needs that? It seems like a cowardly manner to me.

    Is it that software is laging behing technology? We need new OSes, completely different ways of interaction with our computers in order to justify so much GFLOPs on the desktop, in my opinion. Or that's at least what I'm dreaming of. The level of man-machine interaction has barely changed since the mouse was invented, and modern OSes are very similar to the very first OSes with a GUI, maybe somewhere in the 80s, or even earlier. But for that time span CPUs have increased their capabilities almost exponentially, can you spot a problem here?

    The issue becomes clearer if we talk about 16-threaded octacores... God, I can't think of an application that would require that. (Let's be honest - most people don't transcode video or fold proteins all the time...). I think it would be great if a new Apple or Microsoft would emerge from some small garage, to come and change the old picture for good. The only way to justify technology would be to change the whole user-machine paradigm significantly, going the old way leads to nowhere, I suspect.
    Reply
  • gamerk2 - Tuesday, August 26, 2008 - link

    Actually, Windows is the biggest bottleneck there is. Windows doesn't SCALE. It does limited tasks well, but breaks under heavy loads.

    Even the M$ guys are realizing this. Midori will likely be the new post-windows OS after Vista SP2 (windows 7).
    Reply
  • smilingcrow - Thursday, August 21, 2008 - link

    So eventually we may end up with a separate class of systems that are akin to ‘proper’ sports cars with prices to match. Intel seemingly already sees this as a potential which is why they are releasing a separate high end platform for Nehalem. Unless a new killer app or two is found that appeals to the mainstream I can’t see many people wanting to pay a large premium for these systems since as the entry level performance continues to rise less and less people require anything more than what they offer.

    One thing mitigating against this is the current relatively low cost of workstation/server Dual Processor components which should continue to be affordable due to the strong demand for them within businesses. It’s foreseeable that it might eventually be more cost affective to build a DP workstation system than a UP high end desktop. This matches Apple’s current product range where they completely miss out on high end UP desktops and jump straight from mid range UP desktop to DP workstation.
    Reply

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