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Multicore Processors: A Necessity
(Released September 2008)

 
  by Bryan Schauer  

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Multicore Challenges

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Having multiple cores on a single chip gives rise to some problems and challenges. Power and temperature management are two concerns that can increase exponentially with the addition of multiple cores. Memory/cache coherence is another challenge, since all designs discussed above have distributed L1 and in some cases L2 caches which must be coordinated. And finally, using a multicore processor to its full potential is another issue. If programmers don't write applications that take advantage of multiple cores there is no gain, and in some cases there is a loss of performance. Application need to be written so that different parts can be run concurrently (without any ties to another part of the application that is being run simultaneously).

Power and Temperature

If two cores were placed on a single chip without any modification, the chip would, in theory, consume twice as much power and generate a large amount of heat. In the extreme case, if a processor overheats your computer may even combust. To account for this each design above runs the multiple cores at a lower frequency to reduce power consumption.

To combat unnecessary power consumption many designs also incorporate a power control unit that has the authority to shut down unused cores or limit the amount of power. By powering off unused cores and using clock gating the amount of leakage in the chip is reduced.

CELL Thermal Diagram
Figure 7: CELL Thermal Diagram [6]

To lessen the heat generated by multiple cores on a single chip, the chip is architected so that the number of hot spots doesn't grow too large and the heat is spread out across the chip. As seen in Figure 7, the majority of the heat in the CELL processor is dissipated in the Power Processing Element and the rest is spread across the Synergistic Processing Elements. The CELL processor follows a common trend to build temperature monitoring into the system, with its one linear sensor and ten internal digital sensors.

Cache Coherence

Cache coherence is a concern in a multicore environment because of distributed L1 and L2 cache. Since each core has its own cache, the copy of the data in that cache may not always be the most up-to-date version. For example, imagine a dual-core processor where each core brought a block of memory into its private cache. One core writes a value to a specific location; when the second core attempts to read that value from its cache it won't have the updated copy unless its cache entry is invalidated and a cache miss occurs. This cache miss forces the second core's cache entry to be updated. If this coherence policy wasn't in place garbage data would be read and invalid results would be produced, possibly crashing the program or the entire computer.

In general there are two schemes for cache coherence, a snooping protocol and a directory-based protocol. The snooping protocol only works with a bus-based system, and uses a number of states to determine whether or not it needs to update cache entries and if it has control over writing to the block. The directory-based protocol can be used on an arbitrary network and is, therefore, scalable to many processors or cores, in contrast to snooping which isn't scalable. In this scheme a directory is used that holds information about which memory locations are being shared in multiple caches and which are used exclusively by one core's cache. The directory knows when a block needs to be updated or invalidated. [23]

Intel's Core 2 Duo tries to speed up cache coherence by being able to query the second core's L1 cache and the shared L2 cache simultaneously. Having a shared L2 cache also has an added benefit since a coherence protocol doesn't need to be set for this level. AMD's Athlon 64 X2, however, has to monitor cache coherence in both L1 and L2 caches. This is sped up using the HyperTransport connection, but still has more overhead than Intel's model.

Multithreading

The last, and most important, issue is using multithreading or other parallel processing techniques to get the most performance out of the multicore processor. "With the possible exception of Java, there are no widely used commercial development languages with [multithreaded] extensions." [12] Rebuilding applications to be multithreaded means a complete rework by programmers in most cases. Programmers have to write applications with subroutines able to be run in different cores, meaning that data dependencies will have to be resolved or accounted for (e.g. latency in communication or using a shared cache). Applications should be balanced. If one core is being used much more than another, the programmer is not taking full advantage of the multicore system. Some companies have heard the call and designed new products with multicore capabilities; Microsoft and Apple's newest operating systems can run on up to 4 cores, for example. [12, 11]

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