Derek's Conjecture Regarding SP Pipelining and TMT

Temporal Multithreading

We know that the execution of instructions in each SP is pipelined. We know that the throughput of SPs is one instruction per clock cycle and that rather than stalling the pipeline the Tesla architecture usually doesn't need to wait for computational results or memory operations because it is highly likely another thread will be ready to execute. Context switches happen every four clocks from the perspective of the SPs within an SM, and in these four clocks each of the 32 threads in the currently active and executing warp will be serviced.

Organization of the executing threads isn't something we know except SMs process warps in two groups of 16 threads. That this point was made is of interest, but there are too many possibilites for us to come up with any real guess as to how they are issuing instructions to 8 physical SPs in groups of 16. Off hand we asked them if the SPs supported hardware level SMT (simultaneous multithreading) like hyperthreading and our answer was no but with a curious twist: "... they are pipelined processors and support many threads in progress in the pipeline." This was the light switch that brought together the realization of many potential advantages of this architecture for us.

If throughput is really one instruction per clock per SP, and each SP handles 4 threads from a warp over 4 clock cycles, then the pipeline is actually working on executing one instruction from a different thread at every stage in the pipeline.

This is actually a multithreading technique known as fine-grained TMT or temporal multithreading and is different than SMT in that it doesn't expose virtual parallel processors to the software but processes multiple threads in different time slices. TMT isn't some hot new technology you missed when it hit the scene: TMT is what computers have made use of for decades to process the many threads running on a single CPU concurrently without starving them. On a desktop CPU we are very familiar with course-grained multithreading where a single thread is serviced for a while before a context switch happens and another thread starts running. This context switch will normally occur after a certain number of cycles or if a higher priority thread needs the processor or if the thread needs to wait on IO or memory for something.

The real interesting bit comes in the differences between fine and course-grained TMT. In course-grained implementations (what we are all used to) all the pipeline stages of a processor are servicing an instruction stream from a single context, whereas in fine-grained we can have multiple context switches happening within the pipeline down to a context switch every stage. Making such fine-grained implementations happen can be tough, but NVIDIA has used a couple tricks to make it easier to manage.

In G80 and GT200, because of the fact that context is stored per warp, even though the SPs are working on an instruction for a different thread in every pipeline stage, they are not working on a different context at every pipeline stage. Each SP processes four threads in a row from the same warp and thus from the same context. Because it is incredibly likely at 1.5GHz that the SPs have more than 4 pipeline stages, we will still see more than one context switch within the pipeline itself, but it still isn't down to a different context for every stage.

So what's the big deal? Latency insensitivity and a maximal avoidance of pipeline bubbles and stalls.

In a modern CPU architecture, we see many instructions from the same thread running one after the other. If everything is running as smoothly as possible we have as many instructions retiring per clock cycle as we are capable of issueing per clock cycle, but this isn't gauranteed. Data dependancies, memory operations, cache misses and the like cause instructions to wait in the pipeline which means clock cycles go by without as much work as possible being done. Techniques to reduce this sort of delay are many. Data forwarding between pipeline stages is necessary to accomodate cases where one instruction is dependant on the result of the previous. This works by forwarding the result from one stage of the pipeline back to a previous stage so that instructions needing that data won't have to wait for it. Hyperthreading is even a technology to help increase pipeline utilization in that it makes one pipeline look like two different processors in order to fill it with more independent instructions and increase utilization.

Fine-grained TMT eliminates the need for data forwarding because there are zero dependant instructions coming down the pipeline: warps are context switched out after issuing one instruction for each independant thread and if NVIDIA's scheduler does its job right then warps won't be rescheduled until their data is available. Techniques like Hyperthreading are unnecessary because the pipeline is already full of instructions from independant threads at every stage.

Managing a pool of warps that are from a mix of different shader programs and different types of shaders (vertex, geometry and pixel) means that the chance every warp being serviced by an SM is wating on the same data is minimized, but having multiple warps from the same shader program is also a good idea to make sure that once data arrives it enables the processing of more than one warp. Of course, since SMs within one TPC share texture address, filter and cache, it is also a good idea to load up similar warps across the SMs on a TPC so that texture look ups by one thread might also be useful to many others. The balance here would be interesting to know, but we'll probably have to wait for Intel to enter the graphics market before we start getting confirmation on the really cool architectural aspects of all this.

How Deep is an SP?

As for pipeline depth, NVIDIA isn't helping us out with this one either, but let's walk through a little reasoning and see what we can come up with. At the insane and stupid extreme, we know NVIDIA wouldn't build a machine with a pipeline longer than they have threads in flight to fill. We'll assume G80 and GT200 are equally pipelined as they are clocked very similarly and we'll use what we know about G80 to draw a baseline. With G80 having 24 warps in flight per SM and each warp taking up 4 pipeline stages per SP, SPs can't possibly have more than 96 stages. Sure, that's crazy anyway, but if we expect that any warp executing in the pipeline won't be rescheduled until completion, then we would expect a higher proportion of warps to be waiting than executing.

If we go on this assumption we've got less than 48 stages, and I'd think it'd be fair to guess that they'd want to have at least two thrids of their their in-flight thread not in the pipeline, so that brings us down to a potential 32 stage pipeline. On the minimal end, there are at least 4 stages because if there were any less, high priority warps wouldn't get context switched at every opportunity: the instructions form the first threads scheduled would be completed and ready to go. Having 8 stages would give maximum flexibility as warps could be scheduled every other opportunity if they were otherwise ready. This would also keep at most three contexts active at different points in the pipeline, and while this type of fine-grained TMT does offer advantages, it is not free to implement a pipeline with access to a high number of contexts. And it is possible to design a single precision FP unit that can do a MAD in 8 cycles at 1.5GHz, but using Itanium as an example is usually seen as extravagant.

It would be tough to put a finer point on it without some indication from NVIDIA, but at least 8 and at most 32 stages is as good as we can get looking at their architecture. But knowing that power and performance per watt are key concerns of NVIDIA we can be fairly certain of eliminating anything higher than say 16 pipeline stages. Everyone remembers the space heater that was the Pentium 4 in general (and Prescott in particular), and it just isn't power efficient to go too deep.

By now we are at a fairly reasonable minimum of 8 stages and taking both architecture and power into consideration 16 seems like the max we could believe. Of course, that's all the way from one end of the world to the next. Anand's original guess was 12-15, but Derek was able to sell him on 8 stages as the sweet spot because of the simplicity of the cores (there are no decode or scheduling stages in the SPs). So was all that guessing about pipeline stages useful? Not really. But it sure was fun!

Now let's blow your mind and suggest that all this combined with the other details of NVIDIA's architecture suggest that all SP operations have the same latency. This way the entire thing would just work like a clock: one in, one out, very little overhead, and as simple as possible. All the overhead is managed outside the SP and the compute core can just focus on what it does best (as long as the rest of the chip does its job and keeps it fed).

UPDATE: We got lots of response on this page, and many CUDA developers, graphics software designers and hardware enthusiasts emailed us links to many resources on these topics. We discovered some very useful info: instruction latency is actually about 22 cycles in G80, so Anand and I were both way off. This and a couple other things we learned are available in our quick update on the GT200 pipeline published a couple days after this article first went live.

Tweaks and Enahancements in GT200 NVIDIA's Dirty Dealing with DX10.1 and How GT200 Doesn't Support it
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  • elchanan - Monday, June 30, 2008 - link

    VERY eye-opening discussion on TMT. Thank you for it.
    I've been trying to understand how GPUs can be competitive for scientific applications which require lots of inter-process communication, and "local" memory, and this appears to be an elegant solution for both.

    I can identify the weak points of it being hard to program for, as well as requiring many parallel threads to make it practical.

    But are there other weak points?
    Is there some memory-usage profile, or inter-process data bandwidth, where the trick doesn't work?
    Perhaps some other algorithm characteristic which GPUs can't address well?



    Reply
  • Think - Friday, June 20, 2008 - link

    This card is a junk bond when taking into consideration cost/perfomance/power consumption.

    Reminds me of a 1976 Cadillac with a 7.7litre v8 with only 210 horsepower/3600 rpm.

    It's a PIG.
    Reply
  • Margalus - Tuesday, June 24, 2008 - link

    this shows how many people don't run a dual monitor setup. I would snatch up one of these 260/280's over the gx2's anyday, gladly!!

    The performance may not be quite as good as an sli setup, but it will be much better than a single card which is what a lot of us are stuck with since you CANNOT run a dual monitor setup with sli!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    Reply
  • iamgud - Wednesday, June 18, 2008 - link

    "I can has vertex data"


    LOL

    These look fine, but need to be moved to 55nm. By the time I save up for one they will .
    Reply
  • calyth - Tuesday, June 17, 2008 - link

    Well what the heck are they doing with 1.4B transistors, which is becoming the largest die that TMSC has been producing so far?
    The larger the core, the more likely that an blemish would take out the core. As far as I know, didn't Phenom (4 cores on die) suffered low-yield problems?
    Reply
  • gochichi - Tuesday, June 17, 2008 - link

    You know, when you consider the price and you look at the benchmarks, you start looking for features and NVIDIA just doesn't have the features going on at all.

    COD4 -- Ran perfect at 1920x1200 with last gen stuff (the HD3870 and 8800GT(S))so now the benchmarks have to be for outrageous resolutions that a handful of monitors can handle (and those customers already bought SLI or XFIRE, or GTX2 etc.)

    Crysis is a pig of a game, but it's not that great (it is a good technical preview though, I admit), and I don't think even these new cards really satisfy this system hog... so maybe this is a win, but I doubt too many people care... if you had an 8800GT or whatever, you're already played this game "well enough" on medium settings and are plenty tired of it. Though we'll surely fire it up in the future once our video cards "happen to be able to run it on high" very few people are going to go out of their way $500+ for this silly title.

    In any case, then you look at ATI, and they have the HDMI audio, the DX 10.1 support and all they have to do at this point is A) Get a good price out the door, B) Make a good profit (make them cheap, which these NVIDIA are expensive to make, no doubt) and C) handily beat the 8800GTS and many of us are going to be sold.

    These cards are what I would call a next gen preview. Some overheated prototypes of things to come. I doubt AMD will be as fast, and in fact I hope they aren't just as long as they keep the power consumption in check, the price, and the value (HDMI, DX10.1, etc).

    Today's release reminded me that NVIDIA is the underdog, they are the company that released the FX series (desperate technology, like these are). ATI has been around well before 3DFX made 3d-accelerators. They were down for a bit, and we all said it was over for ATI but this desperate release from NVIDIA makes me think that ATI is going to be quite tought to beat.

    Reply
  • Brazofuerte - Tuesday, June 17, 2008 - link

    Can I go somewhere to find the exact settings used for these benchmarks? I appreciate the tech side of the write up but when it comes to determining whether I want one of these for my gaming machine (I ordered mine at midnight), I find HardOCP's numbers much more useful. Reply
  • woofermazing - Tuesday, June 17, 2008 - link

    AMD/ATI isn't going to abandon the high end like your article implies. Their plan is to make a really good mid range chip, and ductape to cores together ala the X2's. Nvidia goes from the high-end down, ATI from the mid-end up. From the look of it, ATI might have the right idea, atleast this time around. I seriously doubt we'll see a two core version of this monster anytime soon. Reply
  • DerekWilson - Tuesday, June 17, 2008 - link

    they are abandoning the high end single GPU ...

    we did state that they are planning on competing in the high end space with multiGPU cards, but that there are drawbacks to that.

    we'll certainly have another article coming out sometime soon that looks a little more closely at AMD's strategy.
    Reply
  • KeypoX - Tuesday, June 17, 2008 - link

    i dont like it, not impressed either :(. Hopefully my 8800gt last for a while, far past this crap atleast Reply

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