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e-Journal

 

Multicore Processors: A Necessity
(Released September 2008)

 
  by Bryan Schauer  

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Improved Memory System

With numerous cores on a single chip there is an enormous need for increased memory. 32-bit processors, such as the Pentium 4, can address up to 4GB of main memory. With cores now using 64-bit addresses the amount of addressable memory is almost infinite. An improved memory system is a necessity; more main memory and larger caches are needed for multithreaded multiprocessors.

UltraSparc T1 multicore processor
Source: Sun Microsystems

System Bus and Interconnection Networks

Extra memory will be useless if the amount of time required for memory requests doesn't improve as well. Redesigning the interconnection network between cores is a major focus of chip manufacturers. A faster network means a lower latency in inter-core communication and memory transactions. Intel is developing their Quickpath interconnect, which is a 20-bit wide bus running between 4.8 and 6.4 GHz; AMD's new HyperTransport 3.0 is a 32-bit wide bus and runs at 5.2 GHz [14]. A different kind of interconnect is seen in the TILE64's iMesh, which consists of five networks used to fulfill I/O and off-chip memory communication. Using five mesh networks gives the Tile architecture a per tile (or core) bandwidth of up to 1.28 Tbps (terabits per second). [27]

The question remains though, which type of interconnect is best suited for multicore processors? Is a bus-based approach better than an interconnection network? Or is there a hybrid like the mesh network that would work best?

Parallel Programming

To use multicore, you really have to use multiple threads. If you know how to do it, it's not bad. But the first time you do it there are lots of ways to shoot yourself in the foot. The bugs you introduce with multithreading are so much harder to find. [36]

In May 2007, Intel fellow Shekhar Borkar stated that "The software has to also start following Moore's Law, software has to double the amount of parallelism that it can support every two years." [35] Since the number of cores in a processor is set to double every 18 months, it only makes sense that the software running on these cores takes this into account. Ultimately, programmers need to learn how to write parallel programs that can be split up and run concurrently on multiple cores instead of trying to exploit single-core hardware to increase parallelism of sequential programs.

Developing software for multicore processors brings up some latent concerns. How does a programmer ensure that a high-priority task gets priority across the processor, not just a core? In theory even if a thread had the highest priority within the core on which it is running it might not have a high priority in the system as a whole. Another necessary tool for developers is debugging. However, how do we guarantee that the entire system stops and not just the core on which an application is running?

These issues need to be addressed along with teaching good parallel programming practices for developers. Once programmers have a basic grasp on how to multithread and program in parallel, instead of sequentially, ramping up to follow Moore's law will be easier.

Starvation

If a program isn't developed correctly for use in a multicore processor one or more of the cores may starve for data. This would be seen if a single-threaded application is run in a multicore system. The thread would simply run in one of the cores while the other cores sat idle. This is an extreme case, but illustrates the problem.

With a shared cache, for example Intel Core 2 Duo's shared L2 cache, if a proper replacement policy isn't in place one core may starve for cache usage and continually make costly calls out to main memory. The replacement policy should include stipulations for evicting cache entries that other cores have recently loaded. This becomes more difficult with an increased number of cores effectively reducing the amount of evictable cache space without increasing cache misses.

Homogeneous vs. Heterogeneous Cores

Architects have debated whether the cores in a multicore environment should be homogeneous or heterogeneous, and there is no definitive answer. . . yet. Homogenous cores are all exactly the same: equivalent frequencies, cache sizes, functions, etc. However, each core in a heterogeneous system may have a different function, frequency, memory model, etc. There is an apparent tradeoff between processor complexity and customization. All of the designs discussed above have used homogeneous cores except for the CELL processor, which has one Power Processing Element and eight Synergistic Processing Elements.

Homogeneous cores are easier to produce since the same instruction set is used across all cores and each core contains the same hardware. But are they the most efficient use of multicore technology?

Each core in a heterogeneous environment could have a specific function and run its own specialized instruction set. Building on the CELL example, a heterogeneous model could have a large centralized core built for generic processing and running an OS, a core for graphics, a communications core, an enhanced mathematics core, an audio core, a cryptographic core, and the list goes on. [33] This model is more complex, but may have efficiency, power, and thermal benefits that outweigh its complexity. With major manufacturers on both sides of this issue, this debate will stretch on for years to come; it will be interesting to see which side comes out on top.

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