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Design for Manycore Systems


Hiding Latency: Complex Cores vs. Hardware Threads

One of the major reasons today's modern CPU cores are so big and complex, to make single-threaded applications run faster, is that the complexity is used to hide the latency of accessing glacially slow RAM -- the "memory wall."

In general, how do you hide latency? Briefly, by adding concurrency: Pipelining, out-of-order execution, and most of the other tricks used inside complex CPUs inject various forms of concurrency within the chip itself, and that lets the CPU keep the pipeline to memory full and well-utilized and hide much of the latency of waiting for RAM. (That's a very brief summary. For more, see my machine architecture talk, available on Google video. [2])

So every chip needs to have a certain amount of concurrency available inside it to hide the memory wall. In 2006, the memory wall was higher than in 1997; so naturally, 2006 cores of any variety needed to contain more total concurrency than in 1997, in whatever form, just to avoid spending most of their time waiting for memory. If we just brought the 1997 core as-is into the 2006 world, running at 2006 clock speeds, we would find that it would spend most of its time doing something fairly unmotivating: just idling, waiting for memory.

But that doesn't mean a simpler 1997-style core can't make sense today. You just have to provide enough internal hardware concurrency to hide the memory wall. The squeezing-the-toothpaste-tube metaphor applies directly: When you squeeze to make one end smaller, some other part of the tube has to get bigger. If we take away some of a modern core's concurrency-providing complexity, such as removing out-of-order execution or some or all pipeline stages, we need to provide the missing concurrency in some other way.

But how? A popular answer is: Through hardware threads. (Don't stop reading if you've been burned by hardware threads in the past. See the sidebar "Hardware Threads Are Important, But Only For Simpler Cores.")


Hardware Threads Are Important, But Only For Simpler Cores

Hardware threads have acquired a tarnished reputation. Historically, for example, Pentium hyperthreading has been a mixed blessing in practice; it made some applications run something like 20% faster by hiding some remaining memory latency not already covered in other ways, but made other applications actually run slower because of increased cache contention and other effects. (For one example, see [3].)

But that's only because hardware threads are for hiding latency, and so they’re not nearly as useful on our familiar big, complex cores that already contain lots of other latency-hiding concurrency. If you've had mixed or negative results with hardware threads, you were probably just using them on complex chips where they don’t matter as much.

Don't let that turn you off the idea of hardware threading. Although hardware threads are a mixed bag on complex cores where there isn’t much remaining memory latency left to hide, they are absolutely essential on simpler cores that aren't hiding nearly enough memory latency in other ways, such as simpler in-order CPUs like Niagara and Larrabee. Modern GPUs take the extreme end of this design range, making each core very simple (typically not even a general-purpose core) and relying on lots of hardware threads to keep the core doing useful work even in the face of memory latency.

—HS


Toward Simpler, Threaded Cores

What are hardware threads all about? Here's the idea: Each core still has just one basic processing unit (arithmetic unit, floating-point unit, etc.) but can keep multiple threads of execution "hot" and ready to switch to quickly as others stall waiting for memory. The switching cost is just a few cycles; it's nothing remotely similar to the cost of an operating system-level context switch. For example, a core with four hardware threads can run the first thread until it encounters a memory operation that forces it to wait, and then keep doing useful work by immediately switching to the second thread and executing that until it also has to wait, and then switching to the third until it also waits, and then the fourth until it also waits -- and by then hopefully the first or second is ready to run again and the core can stay busy. For more details, see [4].

The next question is, How many hardware threads should there be per core? The answer is: As many as you need to hide the latency no longer hidden by other means. In practice, popular answers are four and eight hardware threads per core. For example, Sun's Niagara 1 and Niagara 2 processors are based on simpler cores, and provide four and eight hardware threads per core, respectively. The UltraSPARC T2 boasts 8 cores of 8 threads each, or 64 hardware threads, as well as other functions including networking and I/O that make it a "system on a chip." [5] Intel's new line of Larrabee chips is expected to range from 8 to 80 (eighty) x86-compatible cores, each with four or more hardware threads, for a total of 32 to 320 or more hardware threads per CPU chip. [6] [7]

Figure 3 shows a simplified view of possible CPU directions. The large cores are big, modern, complex cores with gobs of out-of-order execution, branch prediction, and so on.

Figure 3: A few possible future directions.

The left side of Figure 3 shows one possible future: We could just use Moore's transistor generosity to ship more of the same -- complex modern cores as we're used to in the mainstream today. Following that route gives us the projection we already saw in Figure 2.

But that's only one possible future, because there's more to the story. The right side of Figure 3 illustrates how chip vendors could swing the pendulum partway back and make moderately simpler chips, along the lines that Sun's Niagara and Intel's Larrabee processors are doing.

In this simple example for illustrative purposes only, the smaller cores are simpler cores that consume just one-quarter the number of transistors, so that four times as many can fit in the same area. However, they're simpler because they're missing some of the machinery used to hide memory latency; to make up the deficit, the small cores also have to provide four hardware threads per core. If CPU vendors were to switch to this model, for example, we would see a one-time jump of 16 times the hardware concurrency -- four times the number of cores, and at the same time four times as many hardware threads per core -- on top of the Moore's Law-based growth in Figure 2.

What makes smaller cores so appealing? In short, it turns out you can design a small-core device such that:

  • 4x cores = 4x FP performance: Each small, simple core can perform just as many floating-point operations per second as a big, complex core. After all, we're not changing the core execution logic (ALU, FPU, etc.); we're only changing the supporting machinery around it that hides the memory latency, to replace OoO and predictors and pipelines with some hardware threading.
  • Less total power: Each small, simple core occupies one-quarter of the transistors, but uses less than one-quarter the total power.

Who wouldn't want a CPU that has four times the total floating-point processing throughput and consumes less total power? If that's possible, why not just ship it tomorrow?

You might already have noticed the fly in the ointment. The key question is: Where does the CPU get the work to assign to those multiple hardware threads? The answer is, from the same place it gets the work for multiple cores: From you. Your application has to provide the software threads or other parallel work to run on those hardware threads. If it doesn't, then the core will be idle most of the time. So this plan only works if the software is scalably parallel.

Imagine for a moment that we live in a different world, one that contains several major scalably parallel "killer" applications -- applications that a lot of mainstream consumers want to use and that run better on highly parallel hardware. If we have such scalable parallel software, then the right-hand side of Figure 3 is incredibly attractive and a boon for everyone, including for end users who get much more processing clout as well as a smaller electricity bill.

In the medium term, it's quite possible that the future will hold something in be-tween, as shown in the middle of Figure 3: heterogeneous chips that contain both large and small cores. Even these will only be viable if there are scalable parallel ap-plications, but they offer a nice migration path from today's applications. The larger cores can run today's applications at full speed, with ongoing incremental improve-ments to sequential performance, while the smaller cores can run tomorrow's applica-tions with a reenabled "free lunch" of exponential improvements to CPU-bound per-formance (until the program becomes bound by some other factor, such as memory or network I/O). The larger cores can also be useful for faster execution of any unavoidably sequential parts of new parallel applications. [8]


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