ARM’s Mali Midgard Architecture Explored
by Ryan Smith on July 3, 2014 11:00 AM EST
So far this is shaping up to be a banner year for SoCs. From a market perspective the mobile hardware space is still in a period of significant growth, but more importantly from a hardware point of view these products and especially the GPUs in these products have made significant strides in performance and in features. SoC GPUs will approach feature parity with desktop GPUs this year, and from a performance perspective they’re nearing the performance of the last-generation game consoles, a long-aspired goal given the “good enough” status attached to those devices.
Meanwhile at the same time that these products are maturing at a technical level, we’ve seen the various SoC firms mature at a professional level. The “wild west” days of SoCs have given way to mature markets of longer product cycles, longer product lives, and a more stable market overall. This both good and bad news for the various players in the SoC market as firms get squeezed out – SoC integrators such as TI and STMicroelectronics have been the first of such victims – but it also means that as companies become better established and more deeply entrenched, they can be more open about their projects and their products, and discuss them in greater detail than before without needing to be concerned about getting scooped by a competitor.
Here at AnandTech we’re particularly fond of doing architectural deep dives; our chance to talk to the people behind various processors and learn from them about how their products work and how they came together. Thanks to the maturation of the SoC market we’re finally getting a level of access in the SoC market that we haven’t had before, and in turn for the first time we get to tell the stories of the people behind these mind-bogglingly complex devices while better learning how they operate and as such how they compare. It’s admittedly a level of access we take for granted in the PC space, where companies such as Intel, AMD, and NVIDIA are regularly open, but it’s hard to contain our excitement about gaining this kind of access to the myriad of players in the SoC space.
This year then has been especially productive in that regard, and as of today it’s going to get even better. After we took a look at Imagination’s PowerVR Rogue architecture earlier this year, ARM contacted us and indicated that they would like to do the same; that they would like to take a seat at the “open architecture” table. To give us the access we need to discover how their GPUs work, and in turn tell you what we’ve learned.
To that end we’ve gladly let ARM pull up a seat, and today we’ll be taking our first in-depth look at ARM’s newest Mali SoC GPU architecture: Midgard. Now as with Imagination what we’re seeing today is most, but not all of the picture, as ARM has their secrets and they wish to keep some of them. But today we get to learn all about Midgard’s shader cores while also learning a thing or two about its pixel rendering pipeline, power optimizations, and other aspects of what makes Midgard tick. In other words, more than enough to keep us busy for one day.
But before we dive in we’d also like to quickly call attention to an Ask The Experts session we held with ARM’s Jem Davies, an ARM Fellow and VP of Technology in the Media Processing Division. While our deep dive is focusing on Midgard’s architecture, Jem has been answering all sorts of additional Mali-related questions, including business strategy and ARM’s views on GPU computing.
Finally, as this is the second article in our continuing series on SoC GPUs, we will be picking up from where we left off after our last article. While all of our articles are meant to be accessible to some degree, if you haven’t caught any of our previous articles I’d highly recommend our primer on how GPUs work for a quick overview of the technology before we dive into the nuts and bolts of ARM’s Midgard architecture.
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seanlumly - Thursday, July 3, 2014 - link
Given the "exotic" the ILP and the 128-bit VLIW SIMD, the Mali looks like an impressive performer. If a Mali T760MP10 is indeed a part fit for smartphone-level power consumption, then a quick linear-scaling -- given GFXBench scores of a T760MP4 -- would imply that such a GPU is very competitive with something like the adreno 420 and certainly impressive if scaled up further to a tablet-level power consumption. If, however, an MP10 consumes roughly as much as a K1 or GX6650, then I'm very sceptical about its competitive performance.I find the distribution of ALUs to memory units strange given mobile bandwidth limits. The Mali T760 in an 8-core configuration clocked at 600MHz will allow for 19.2 GB/s of load/store access (256-bits/clock: source ARM). This is quite high memory bandwidth, and an increase in GPU clocks or cores will likely yield idling load/stores units doing nothing but taking up valuable die area. Operations with high varying access, cache reads, and tile read/writes will of course make good use of these additional units, but it still seems like overkill on all but very memory-access heavy apps. ARM would know best, though I'm suspicious that so many load/store units are needed for common workloads. I would guess that in common scenarios, bandwidth to main memory would be exhausted long before all of the memory units were fully utilized on a Mali T760 of high core count (eg. MP12-16).
Decoupling the load/stores and texture units as their own "core" may allow more appropriate scaling to fit the bandwidth of the target system. A system with an ultra-high resolution, could be endowed with more load/stores and/or texture units. A system with a lower resolution could use less and opt for more ALUs in the same space. This would be similar to big.LITTLE (different cores for different targeted workloads). In this scenario, the memory unit cores could be scaled independently of the ALU cores, perfectly tailored to the target system.
EdvardS - Friday, July 4, 2014 - link
Remember that there is a cache system between those units and the SoC memory controller. Bandwidth to the cores is quite different from bandwidth to the DDR memory.seanlumly - Friday, July 4, 2014 - link
Indeed! Bandwidth to cache, tile memory, varying data, or textures would likely be more relevant with a tile-based renderer that often exploits spatial locality when processing batches of pixels. This is especially true with modern screen-space effects that do multiple dependent reads per display pixel (eg. SSAO), but are strongly confined to buffer fragments surrounding the target pixel. Such situations would value having many LS/Tex units at little penalty.But I do still like the idea of an independently scalable "memory" core (containing load/store/texure pipes) to complement a "math" core (containing ALUs). A high-performance system targeting a 720p display will likely consume far less bandwidth than one targeting a 4K display, and as such, it would be nice to trade LS/Tex units for more ALUs in such a case.
Such an arrangement may also enable ARM more leeway when making predictions about a new architecture -- no doubt the Midgard arch was in development many years before it saw implementation in a retail product, which means that ARM would have had to guess trends (eg. resolution) far in advance to attain the right balance of on-chip units per core; independently scalable memory-cores would be more forgiving if the trends turned out not match the initial predictions.
seanlumly - Friday, July 4, 2014 - link
Actually, I am starting to understand the motivation behind the ratio of ALUs to Memory units in a Midgard core. I notice in GFXBench 3.0 "Manhattan", that the Mali T760MP4 (Rockchip rk3288) performs incredibly well at 720p, but it's performance drops off more than proportionately as the resolution increases. This may imply that in these higher than 720p scenarios, the 4-core variant of GPU may not be able to keep up with the memory demands, as computation should scale very close to proportionally.Thus the 1080p offscreen score for the Mail T760 MP4 in the GFXBench 3.0 (offline) database may be misleading as the MP4 may be a bit small for this resolution, and thus the performance may be low relative to its competition. A T760 MP8 would likely more than double the performance for a doubling of the resolution, pushing something like the Mali T760 MP8 well beyond the competition, at what I suspect are similar levels of power and die-size. I predict that a T760 MP8 would get slightly north of 16fps in GFXBench 3.0 (assuming adequate bandwidth to DDR). Even an MP6 variant of the GPU (as was the case with the T628 MP6 in the Exynos 5420) should put it more-or-less on-par with the competition!
The Mali performs even better in the GFXBench 2.7 "T-Rex" test, where a small 4-core Mali T760 MP4 surpasses the competition at 720p and even sub-720p resolutions in some instances! This is incredible. In this case, it seems that the test is more computation bound, as there is a more proportional scaling between performance and resolution.
I hope that future GPUs consider using the T760 in higher-core-count configurations. I still like the idea of a Memory-core, though I have little doubt that a Mali GPU of evenly matched size can go toe-to-toe with the competition.
Frenetic Pony - Friday, July 4, 2014 - link
Every time I read an overview of a SoC GPU I am so, so glad I don't do anything with mobile stuff. "We support tessellation! I mean, don't actually do it. Ever. But you know, it's supported."kkb - Friday, July 4, 2014 - link
How come there is no comparison with intel GPUs like the ones in Baytrail?darkich - Friday, July 4, 2014 - link
Because there is no comparison phrase.That GPU is completely inferior compared to latest Mali, PowerVR and Adreno architectures
Krysto - Friday, July 4, 2014 - link
Word.kkb - Monday, July 7, 2014 - link
well.. I don't really agree. Please look at the AT review from last week or so.. http://www.anandtech.com/show/8197/samsung-galaxy-...MEMO pad is a baytrail product and definitely performs better than MALI devices.
darkich - Monday, July 7, 2014 - link
Get your facts and reading skills in order.Firstly, the GPU in Memo Pad is definitely not definitely performing better than even the Mali T628, in fact those very tests show it trades blows with it, mostly due to much lower resolution screen.
Secondly, do you realize that the T760 is MUCH faster than T628?
You can see here that it is basically comparable to the Tegra K1 and even the intimidating Series 6XT doesn't trounce it.
Rest assured that any of these three, as well as the Adreno 420, is way above the ULP HD graphics chip