Not Quite a Pentium, Not Quite an Atom: The Larrabee Core

Intel gave us enough information about Larrabee to begin a discussion of specifications, but not enough to even begin making any conclusions. We'll start with what we pretty much already know.

Intel's Larrabee is built out of a number of x86 cores that look, at a very high level, like this:

Each core is a dual-issue, in-order architecture loosely derived from the original Pentium microprocessor. The Pentium core was modified to include support for 64-bit operations, the updates to the x86 instruction set, larger caches, 4-way SMT/Hyper Threading and a 16-wide vector ALU.

While the team that ended up working on Atom may have originally worked on the Larrabee cores, there are some significant differences between Larrabee and Atom. Atom is geared towards much higher single threaded performance, with a deeper pipeline, a larger L2 cache and additional microarchitectural tweaks to improve general desktop performance.

  Intel Larrabee Core Intel Pentium Core (P54C) Intel Atom Core
Manufacturing Process 45nm 0.60µm 45nm
Simultaneous Multi-Threading 4-way 1-way 2-way
Issue Width dual-issue dual-issue dual-issue
Pipeline Depth 5-stages (?) 5-stages 16-stages
Scalar Execution Resources 2 x Integer ALUs (?)
1 x FPU (?)
2 x Integer ALUs
1 x FPU
2 x Integer ALUs
1 x FPU
Vector Execution Resources 16-wide Vector ALU None 1 x SIMD SSE
L1 Cache (I/D) 32KB/32KB 8KB/8KB 32KB/24KB
L2 Cache 256KB None (External) 512KB
ISA 64-bit x86
SSEn support?
Parallel/Graphics?
32-bit x86 64-bit x86
Full Merom ISA compatibility

 

Larrabee on the other hand is more Pentium-like to begin with; Intel states that Larrabee's execution pipeline is "short" and followed up with us by saying that it's closer to the 5-stage pipeline of the original Pentium than the 16-stage pipeline of Atom. While both Atom and Larrabee support SMT (Simultaneous Multi-Threading), Larrabee can work on four threads concurrently compared to two on Atom and one on the original Pentium.

L1 cache sizes are similar between Larrabee and Atom, but Larrabee gets a full 32KB data cache compared to 24KB on Atom. If you remember back to our architectural discussion of Atom, the smaller L1 D-cache was a side effect of going to a register file instead of a small signal array for the cache. Die size increased but operating voltage decreased, forcing Atom to have a smaller L1 D-cache but enabling it to reach lower power targets. Larrabee is a little less constrained and thus we have conventional balanced L1 caches, at 4x the size of that in the original Pentium.

The Pentium had no on-die L2 cache, it relied on external SRAM to be installed on the motherboard. In order to maintain good desktop performance Atom came equipped with a 512KB L2 cache, while each Larrabee core will feature a 256KB L2 cache. Larrabee's architecture does stress the importance of large, fast caches as you'll soon see, but 256KB is the right size for Intel's architecture at this point. Larrabee's default OpenGL/DirectX renderer is tile based and it turns out that most 64x64 or 128x128 tiles with 32-bit color/32-bit Z can fit in a 128KB space, leaving an additional 128KB left over for caching additional data. And remember, this is just on one Larrabee core - the whole GPU will be built out of many more.

The big difference between Larrabee, Pentium and Atom is in the vector execution side. The original Pentium had no SIMD units, Atom added support for SSE and Larrabee takes a giant leap with a massive 16-wide vector ALU. This unit is able to work on up to 16 32-bit floating point operations simultaneously, making it far wider than any of the aforementioned cores. Given the nature of the applications that Larrabee will be targeting, such a wide vector unit makes total sense.

Other changes to the Pentium core that made it into Larrabee are things like 64-bit x86 support and hardware prefetchers, although it is unknown as to how these compare to Atom's prefetchers. It is a fair guess to say that prefetching will include optimizations for data parallel situations, but whether this is in addition to other prefetch technology or a replacement for it is something we'll have to wait to find out.

The Design Experiment: Could Intel Build a GPU? Drilling Deeper and Making the AMD/NVIDIA Comparison
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  • Byte - Tuesday, August 05, 2008 - link

    Looks like Puma will have a hard prey to hunt. This should be pretty successful, even if it will be underpowered in DX games, but that shouldn't matter as even now Intel is selling lots of graphics just because they almost force it onto OEMs. Intel could similarly force these onto OEMs, but at least this time it won't be a huge pile of crap. Reply
  • ilkhan - Tuesday, August 05, 2008 - link

    So is the on package GPU we expect to see in Havendale & Auburndale chips going to be larrabee chips?

    If anything Id expect to see 8 or 16 core versions to be the onboard GPU for those. Probably 8 core, to keep costs down for onboard chips.
    Reply
  • steveyballmer - Tuesday, August 05, 2008 - link

    It only gets better on my blog!

    http://fakesteveballmer.blogspot.com">http://fakesteveballmer.blogspot.com
    Reply
  • DeepThought86 - Monday, August 04, 2008 - link

    Nice HPC platform, terrible idea for a graphics chip. Just look at the die allocation, it's optimized for instruction-heavy and data-poor tasks. Killer for BOINC and folding type stuff, but there's no way this general purpose use of transistor budget makes sense for graphics.

    Power consumption for the high-speed ringbus will be killer as well. In idle today's GPUs are quite efficient, Larrabee will burn watts doing nothing.

    This architecture will occasionally handle a particular game excellently, but completely fall down in others. In a way it's the opposite of Nvidia or AMD today.

    Ah well, they've had a good run since 2006, looks like they're headed for their next down cycle, just as AMD has started rising again...
    Reply
  • ltcommanderdata - Monday, August 04, 2008 - link

    From Intel's Siggraph paper, Larrabee's claimed performance is pretty decent.

    Intel's internal results are that Larrabee will only require about 10 cores running at 1GHz to maintain HL2 Episode 2 above 60fps at a 1600x1200 resolution. They estimate that a 25 core 1GHz Larrabee will be sufficient to maintain FEAR and Gears of War above 60fps at 1600x1200. FEAR is older than Gears of course, but FEAR had an occasional frame spike, probably on a more complicate frame, so 25 cores should guarantee a 60fps minimum fps. Of course, these are Intel's own benchmarks and they only tested a very small section of the game that they picked, but things do look promising. At the very least performance is better they trying to play the game on current Intel IGPs.
    Reply
  • iocedmyself - Tuesday, August 05, 2008 - link

    1ghz core x 10 to maintain HL2 above 60 fps in 1600x1200...wow...that's on par with a x1800xt? at absolute most.

    1ghz core x 25 for FEAR and gears of war @ 60 fps..that is the equivolent of a
    $180 ATi 4850 running in 1920x1200,@ 1600x1200 does 90 fps 50% better

    ...or

    the same frame rate as the $290 ATI 4870...in 2560x1600, in 1600x1200 it does 114 fps, nearly twice the performance.

    yes, they could scale it up to 50 cores, running 3ghz and it would still only equal about 2/3 the processing power as a single core 4870. Intel's 80 core terascale chip does 1 teraflop/sec at 3.2ghz.

    This is a horribly flawed design...they are doing the opposite of the logical step...in what twisted reality can someone say,

    "well if GPU's are capable of delivering x20-x40 the performance of a desktop cpu package running at 1/5th the clock speed (or more accurately x80-x110 the performance on a core by core basis) the logical solution is to put 48 cpu cores in a single package!"

    Intel couldn't manage to produce an IGP that ran the GUI of an operating system smoothly at all times, they took years longer than AMD to develope 2 core die dual core, years longer to be able to make a photocopy of thier IMC, and continues to fail in 64bit computations comparitively...

    but they think because they've developed a 32bit arch, built of a 10 year old design and gained market control for less than two years after producing complete and utter crap for the previous 7 straight...that they can take the video card market from 2 companies each having 13+ years expeirence in the market.

    AMD is already testing 40nm die 64bit dual/quad cpu with IMC supporting DDR2 AND DDR3, 1 or more gGpu's and a total of 6-10MB on die cache.

    Native dual core Gpu's, cpu's gpu's and a combination of both with built in memory...ya know, designs that actually have some promise...but they are going to nail an x86 in which developers will have to change the way they think, program and deploy ideas. We barely have software that will utilize 4 cores, let alone 40. Meanwhile all amd has to do is intergrate the 780G IGP into a cpu package and intel is screwed.

    But hey....i bet AMD could make a kick ass Gfx card if they took the r540 (x850xt PE core) gave it a die shrink down to 55nm and added SM4 support, then stuffed 50 or 60 into a single package it would do great.

    HELL why stop there, just give the r770 a die shrink down to 40nm, put 10 cores to a gpu die,
    make a dual gpu board,
    2x5 gigs of of GDDR5 memory clocked at 1250mhz (5ghz effective)

    they would have a single card capable of doing more than 20 teraflops/sec.

    BUT WAIT! THAN THEY HAVE CROSSFIREX THEY COULD HAVE 80 R770 CORES WITH 40 GIGS OF 5GHZ GDDR5 IN A SINGLE SYSTEM!!!!! 3DMARK06 WOULD BREAK 1,000,000 POINTS IN 4096x3200 WITH 1920 FSAA 1280 AF!!!

    EVERYONE WOULD BE ABLE TO RUN CRYSIS ON ULTRA HIGH SETTINGS USING A MOVIE THEATER SCREEN FOR A MONITOR WITH NO JAGGED EDGES!!!!!!!!!!!

    Then it would become aware, and improve the game code, crysis would spill over into Halo, halo would break into COD4, Fallout 3 would spill over into World of Warcraft where the characters would become self aware and program viruses to only infect intel based platforms...which would destroy Mac's completely,

    IT WOULD BE THE FIRST DIGITAL STD!!!!! ZOMG

    It would be sold with a 6000w PSU, and it would be Green because it would run on the power of internet porn, and have the power to heat your entire house....it would save the enviroment....ZOMFG!!

    But eventually....intel would come back from the wreckage...

    bringing with them the next revolutionary product...

    the octo-punmped Itanium 4...with Netburst 3.4 arch, featureing 127 Pentium MX cores, Each core could handle 3 threads, and it would scale to 50,000mhz, with 2 terabyte SATA 4 hard drives used for the L1 cache of each core...and testing has shown that each core will only have to run 4.7ghz to achieve 60fps in the human genome project...

    Sigh...sorry, i was pretending i worked at intel. It sure is fun to imagine what could be...isn't it?
    Reply
  • ZootyGray - Tuesday, August 05, 2008 - link

    Hey OC - I just had a flashback to the "jump to light speed" scene in StarWars. Dude, total nirvana, o yeh, thx for the ride. :)

    BTW - my GF says, she heard a rumour that the whole thing runs on 'corn'.

    I think it must be nextgen corn, cos that's a lotta teraflops. Does any of this convert to metric tonnes of refined bs? Anyway, I think I will wait for your next release.

    And you accomplished that in less than one page? nano shrink, huh!

    peace.
    Reply
  • ltcommanderdata - Tuesday, August 05, 2008 - link

    So your own estimates are:

    "..or

    the same frame rate as the $290 ATI 4870...in 2560x1600, in 1600x1200 it does 114 fps, nearly twice the performance. "

    So you are admitting that a 1GHz x 25 core Larrabee could be about 50% the performance of a HD 4870. But, Larrabee could be available in configurations up to 48 cores, so then a 48 core Larrabee at 1GHz could match a HD 4870. Of course, launch clocks will be better than 1GHz, since the Intel only clocked the Larrabee cores at 1GHz in their benchmarks because it's a convenient reference base. You say that Terascale clocked at 3.2GHz, but being more conservative, if Larrabee clocked in a 2GHz at launch with 48 cores, then it would be twice as fast as a HD 4870.

    This is of course based on preproduction drivers. Final performance may be higher. Admittedly, this is mainly hypothetical on early Intel provided data, but using your own figures for comparison, Larrabee may not be able to be able to overtake the fastest GPUs available in 2009/2010, but it'll likely be competitive in the mid-range $200-$300 segment. Which is really all Intel needs, since the point is to get a more general purpose x86 based accelerator card into as many computers as possible. Gaming is just the vehicle to do it, and the mid-range is far higher volume than the top-end.

    And in terms of flops, I believe it was in the SIGGRAPH paper somewhere that a baseline prototype Larrabee with 1 core at 1GHz gets about 32GFLOPS. Now no doubt scaling isn't perfectly linearly, but just assuming it is through clock speed and core count, a 48 core Larrabee at 2GHz could peak at 3072GFLOPS or 3 times that of a HD 4870. ATI and nVidia will obviously keep moving forward in the next year or two, just as Larrabee is still evolving, but for now, Larrabee isn't really in as bad a position as you make it out to be.
    Reply
  • JarredWalton - Tuesday, August 05, 2008 - link

    What's worse is that there are all these assumptions made with no knowledge of the settings. 1600x1200 in HL2 at absolute maximum detail settings is nothing to scoff at, and certainly 60FPS would surpass an X1950 XTX. Are we running 4xAA or not? No idea from Intel, so we've got no reference point other than to say that it should be able to generate playing performance.

    FEAR is even better: 25 cores at 1GHz to hit 60FPS. Okay, that doesn't sound like a lot, but is that with or without 4xAA, and is it with or without soft shadows? Both of those factors can make a HUGE difference in performance. If they are enabled, 60 FPS at 1600x1200 is very impressive for early hardware. Now go with the assumption that Intel will hit clocks of at least 2GHz at launch and will likely have 32 or 48 cores. That should compare quite favorably with NVIDIA and ATI hardware next year.

    Besides all of the above commentary on not knowing settings, we don't even know the scenes that were tested. Pretty much we have nothing to go on without a frame of reference. If Intel had said, "we achieve 60 FPS with 10 cores at 1GHz, and that compares to an 8800 GT running at 60 FPS with the same settings" we could start from a meaningful baseline. Which is probably why we didn't get that information.

    Finally - and this is really the key - I believe all of the stuff right now is merely theoretical. They have modeled the performance of Larrabee in the various tests, but they do not have hardware and thus have not actually run any true tests. Okay, the modeling of the hardware is probably sufficient in all honesty, but some of you are talking as though these chips are actually up and running, and they're not (yet). We'll know a lot more in another year; until then, it all sounds very interesting but the proof as always is in the pudding.
    Reply
  • The Preacher - Tuesday, August 05, 2008 - link

    Man, you must have OC'ed yourself way too high! :D Reply

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