Cache & Memory Hierarchy

Qualcomm has a three level exclusive cache hierarchy in Krait. The lower two levels are private per core, while the third level is shared among all cores. Qualcomm calls these caches L0, L1 and L2.

Each Krait core has an 8KB L0 cache (4KB instruction + 4KB data cache). The L0 cache is direct mapped and accessible in a single cycle. Qualcomm claims an 85% hit rate in this level 0 cache, which helps save power by not firing up the larger L1 cache. The hierarchy is exclusive so L0 data isn't necessarily duplicated in L1.

Each core also has a 32KB L1 cache (16KB instruction + 16KB data). The L1 4-way set associative and can also be accessed in a single cycle. There's no way prediction at work here. With 1 cycle latency to both L0 and L1, the primary advantage here is power.

Krait Cache Architecture
  Size Architecture Frequency
L0 4KB + 4KB Direct Mapped Core
L1 16KB + 16KB 4-way set associative Core
L2 1MB (dual core) or 2MB (quad core) 8-way set associative 1.3GHz max

The L2 cache is shared among all cores. In dual-core designs the L2 cache is sized at 1MB (up from 512KB in Scorpion), while quad-core Krait SoCs will have a 2MB L2. Krait's L2 cache is 8-way set associative.

While the L0 and L1 caches operate at core frequency and are on the same voltage plane as their associated core, the L2 cache is separate. To save power the L2 cache runs at its own frequency (up to 1.3GHz depending on the currently requested performance level). The L2 cache is on its own power plane and can be power gated if necessary.

Although Scorpion featured a dual-channel LPDDR2 memory controller, in a PoP configuration only one channel was available to any stacked DRAM. In order to get access to both 32-bit memory channels the OEM had to implement a DRAM on-package as well as an external DRAM on the PCB. Memory requests could be interleaved between the two DRAM, however Qualcomm seemed to prefer load balancing between the two with CPU/GPU accesses being directed to the lower latency PoP DRAM. Very few OEMs seemed to populate both channels and thus Scorpion based designs were effectively single-channel offerings.

Krait removes this limitation and now OEMs can utilize both memory channels in a PoP configuration (simply put two 32-bit DRAM die on the PoP stack) or in an external configuration. The split PoP/external DRAM organization is no longer supported. This change will hopefully mean we'll see more dual-channel Krait designs than we saw with Scorpion, which will in turn improve performance.

Process Technology and Clock Speeds

Krait will be the world's first smartphone CPU built on a 28nm process. Qualcomm is working with both TSMC and Global Foundries, although TSMC will produce the first chips. Krait will be built, at first, on TSMC's standard 28nm LP process. According to Qualcomm there's less risk associated with TSMC's non-HKMG process. Qualcomm was quick to point out that the entire MSM8960 SoC is built on a 28nm LP process compared to NVIDIA's 40nm LPG design in Kal-El. From Qualcomm's perspective, 40nm G transistors are only useful at reducing leakage at high temperatures but for the majority of the time a homogeneous LP design makes more sense.

Just like Scorpion, Krait places each core on its own voltage plane driven at its own clock frequency. Cores can be clocked independently of one another, which Qualcomm insists gives it a power advantage in many workloads.

The first implementation of Krait will be in a dual-core 1.5GHz MSM8960, however a second revision of the silicon will be introduced next year that increases clock speed to 1.7 - 2.0GHz. Qualcomm claims that at the same 1.05V core voltage, Krait can run at 1.7GHz vs. 1.55GHz for Scorpion. At these two clock speeds and at the same voltage, Qualcomm tells us that Krait consumes 265mW of power vs. 432mW running an undisclosed workload. Although it should be possible to draw more power than Scorpion under load, Krait should hopefully be able to improve overall power efficiency by completing tasks quicker and thus dropping down to idle faster than its predecessor. Smartphone and tablet battery life should remain the same at worst and improve at best, as a result.

Krait Architecture The Adreno 225 GPU
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  • Blaster1618 - Saturday, October 8, 2011 - link

    While Single instruction multiple data whether short or long vector is a great idea, sadly under utilized except in graphics processing, compressible signals and cryptography. Does the NEON technology just an additional graphics/compression engine? Does it require special Neon programming/compiling or does it enhance normal MIMD programming? Reply
  • happy medium - Sunday, October 9, 2011 - link

    I thought the new tegra 3 was 5 cores? Reply
  • ET - Monday, October 10, 2011 - link

    Thanks to Anandtech for covering mobile chips. I find it pretty exciting to read about these low power combinations of CPU and GPU. Reply
  • tech360 - Monday, October 10, 2011 - link

    Excellent analysis. I guess x86 will have an uphill battle against ARM for years in bridging the gap in low power.

    Just wondering what are your thoughts on the integrated WiFi/GPS/BT/FM in S4? Does it have potential to integrate away the currently separate combo-chips?

    Thanks.
    Reply
  • The0ne - Monday, October 10, 2011 - link

    I'm a fan of efficient coding and design and hope that Qualcomm is following that path. I think there's been too much "just get it done no matter what" in the programming business for far too long. That's not to say there aren't very good free open source apps out there by astounding programmers but the mainstream seems to have forgotten or don't care.

    This is good news because mobile is still in the early phases and if efficiency is priority things can only get better if not easier to debug, code, change, etc.

    The bad is of course increasing speeds, any speeds like in the PC industry of yester-years. Sure we want more powerful hardware but lets not make it because of shotty code and design architectures. Again, in the mobile industry I believe these two points should be highly consider by anyone.
    Reply
  • broccauley - Monday, October 10, 2011 - link

    You need to add the ST-Ericsson Thor / NovaThor series of SoCs to your table.

    Also, of the SoCs in your table only the Qualcomm ones are true telecom SoCs - the others are mere application engines without telecom features.
    Reply
  • ssiu - Tuesday, October 11, 2011 - link

    Nothing about Apple A6? (I guess reliable info is hard to come by, but surely it would be out by December 2010 and should be included in the comparison table?) Reply
  • ssiu - Tuesday, October 11, 2011 - link

    ... December 2012 (where is the Edit button ...) Reply
  • sarge78 - Wednesday, October 12, 2011 - link

    You could add the ZiiLabs ZMS-20 (and quad core ZMS-40) to the table. That'll probably see action in a Creative Android tablet/PMP and other OEMs might pick up the Jaguar reference tablet. (Could make a good review piece?)

    http://www.ziilabs.com/products/processors/zms20.a...

    Or how about Marvell's 628 ARMADA tri-core SoC? Marvell are getting pushed out by Qualcomm but it could get some design wins next year (It has two 1.5GHz cores and a low power 624 MHz core, similar to Tegra 3)

    http://www.marvell.com/company/news/pressDetail.do...
    Reply
  • lancedal - Thursday, October 13, 2011 - link

    Wouldn't the use of L0 impact performance?
    If L1 is shut-down, there will be penalty on a L0 miss. Powering up L1 on a L0 miss would cost thousand of cycles at 1.5GHz.
    If L1 is on, then what is the point?
    Reply

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