Multi-Core
From MediaCoderWiki
A multi-core CPU (or chip-level multiprocessor, CMP) combines two or more independent cores into a single package comprised of a single piece silicon integrated circuit (IC), called die, or more dies packaged together. A dual-core device contains two independent microprocessors and a quad-core device contains four microprocessors. A multi-core microprocessor implements multiprocessing in a single physical package. Cores in a multicore device may share a single coherent cache at the highest on-device cache level (e.g. L2 for the Intel Core 2) or may have separate caches (e.g. current AMD dual-core processors). The processors also share the same interconnect to the rest of the system. Each "core" independently implements optimizations such as superscalar execution, pipelining, and multithreading. A system with N cores is effective when it is presented with N or more threads concurrently. The most commercially significant (or at least the most 'obvious') multi-core processors are those used in computers (primarily from Intel & AMD) and games consoles (eg the Cell processor in the PS3). In this context, "multi" typically means a relatively small number of cores. However, the technology is widely used in other technology areas, especially those of embedded processors such as network processors and digital signal processors, and in GPUs. In these applications, multi-core processors with higher numbers of processing elements (hundreds on one die) now exist.
Contents |
Terminology
There is some discrepancy in the semantics by which the terms "multi-core" and "dual-core" are defined. Most commonly they are used to refer to some sort of central processing unit (CPU), but are sometimes also applied to DSPs and SoCs. Additionally, some use these terms only to refer to multi-core microprocessors that are manufactured on the same integrated circuit die. These people generally prefer to refer to separate microprocessor dies in the same package by another name, such as "multi-chip module", "double core", or even "twin core". This article uses both the terms "multi-core" and "dual-core" to reference microelectronic CPUs manufactured on the same integrated circuit, unless otherwise noted.
Development
While manufacturing technology continues to improve, reducing the size of single gates, physical limits of semiconductor-based microelectronics have become a major design concern. Some effects of these physical limitations can cause significant heat dissipation and data synchronization problems. The demand for more capable microprocessors causes CPU designers to use various methods of increasing performance. Some instruction-level parallelism (ILP) methods like superscalar pipelining are suitable for many applications, but are inefficient for others that tend to contain difficult-to-predict code. Many applications are better suited to thread level parallelism(TLP) methods, and multiple independent CPUs is one common method used to increase a system's overall TLP. A combination of increased available space due to refined manufacturing processes and the demand for increased TLP is the logic behind the creation of multi-core CPUs.
Advantages
The close proximity of multiple CPU cores on the same die has the advantage of allowing the cache coherency circuitry to operate at a much higher clock rate than is possible if the signals have to travel off-chip. Combining equivalent CPUs on a single die significantly improves the performance of cache snoop (alternative: Bus snooping) operations. But simply, this means that signals between different CPUs travel shorter distances, and therefore those signals degrade less. These higher quality signals allow more data to be sent in a given time period since individual signals can be shorter and do not need to be repeated as often.
Assuming that the die can fit into the package, physically, the multi-core CPU designs require much less Printed Circuit Board (PCB) space than multi-chip SMP designs. Also, a dual-core processor uses slightly less power than two coupled single-core processors, principally because of the increased power required to drive signals external to the chip and because the smaller silicon process geometry allows the cores to operate at lower voltages; such reduction reduces latency. Furthermore, the cores share some circuitry, like the L2 cache and the interface to the front side bus (FSB). In terms of competing technologies for the available silicon die area, multi-core design can make use of proven CPU core library designs and produce a product with lower risk of design error than devising a new wider core design. Also, adding more cache suffers from diminishing returns.
Disadvantages
In addition to operating system (OS) support, adjustments to existing software are required to maximize utilization of the computing resources provided by multi-core processors. Also, the ability of multi-core processors to increase application performance depends on the use of multiple threads within applications. For example, most current (as of 2006) video games will run faster on a 3 GHz single-core processor than on a 2GHz dual-core processor (of the same core architecture), despite the dual-core theoretically having more processing power, because they are incapable of efficiently using more than one core at a time. The situation is improving: for example the American video game developer Valve Corporation has stated that it will use multi core optimizations for the next version of its Source engine, shipped with Half-Life 2: Episode Two, the next installment of its Half-Life franchise.[1][2] See Dynamic Acceleration Technology for the Santa Rosa platform for an example of a technique to improve single-thread performance on dual-core processors.
Integration of a multi-core chip drives production yields down and they are more difficult to manage thermally than lower-density single-chip designs. From an architectural point of view, ultimately, single CPU designs may make better use of the silicon surface area than multiprocessing cores, so a development commitment to this architecture may carry the risk of obsolescence. Finally, raw processing power is not the only constraint on system performance. Two processing cores sharing the same system bus and memory bandwidth limits the real-world performance advantage. If a single core is close to being memory bandwidth limited, going to dual-core might only give 30% to 70% improvement. If memory bandwidth is not a problem, a 90% improvement can be expected. It would be possible for an application that used 2 CPUs to end up running faster on one dual-core if communication between the CPUs was the limiting factor, which would count as more than 100% improvement.
Hardware trend
The general trend in processor development has been from multi-core to many-core: from dual-, quad-, eight-core chips to ones with tens or even hundreds of cores; see manycore processing unit. In addition, multi-core chips mixed with simultaneous multithreading, memory-on-chip, and special-purpose "heterogeneous" cores promise further performance and efficiency gains, especially in processing multimedia, recognition and networking applications. There is also a trend of improving energy efficiency by focusing on performance-per-watt with advanced fine-grain or ultra fine-grain power management and dynamic voltage and frequency scaling (DVFS).
Software impact
Software benefits from multicore architectures where code can be executed in parallel. Under most common operating systems this requires code to execute in separate threads or processes. Each application running on a system runs in its own process so multiple applications will benefit from multicore architectures. Each application may also have multiple threads but, in most cases, it must be specifically written to utilize multiple threads. Operating system software also tends to run many threads as a part of its normal operation. Running virtual machines will benefit from adoption of multiple core architectures since each virtual machine runs independently of others and can be executed in parallel.
Most application software is not written to use multiple concurrent threads intensively because of the challenge of doing so. A frequent pattern in multithreaded application design is where a single thread does the intensive work while other threads do much less. For example, a virus scan application may create a new thread for the scan process, while the GUI thread waits for commands from the user (e.g. cancel the scan). In such cases, multicore architecture is of little benefit for the application itself due to the single thread doing all heavy lifting and the inability to balance the work evenly across multiple cores. Programming truly multithreaded code often requires complex co-ordination of threads and can easily introduce subtle and difficult-to-find bugs due to the interleaving of processing on data shared between threads (thread-safety). Consequently, such code is much more difficult to debug than single-threaded code when it breaks. There has been a perceived lack of motivation for writing consumer-level threaded applications because of the relative rarity of consumer-level multiprocessor hardware. Although threaded applications incur little additional performance penalty on single-processor machines, the extra overhead of development has been difficult to justify due to the preponderance of single-processor machines.
As of September 2006, with the typical mix of mass-market applications the main benefit to an ordinary user from a multi-core CPU will be improved multitasking performance, which may apply more often than expected. Ordinary users are already running many threads; operating systems utilize multiple threads, as well as antivirus programs and other 'background processes' including audio and video controls. The largest boost in performance will likely be noticed in improved response time while running CPU-intensive processes, like antivirus scans, defragmenting, ripping/burning media (requiring file conversion), or searching for folders. Example: if the automatic virus scan initiates while a movie is being watched, the movie is far less likely to lag, as the antivirus program will be assigned to a different processor than the processor running the movie playback.
Given the increasing emphasis on multicore chip design, stemming from the grave thermal and power consumption problems posed by any further significant increase in processor clock speeds, the extent to which software can be multithreaded to take advantage of these new chips is likely to be the single greatest constraint on computer performance in the future. If developers are unable to design software to fully exploit the resources provided by multiple cores, then they will ultimately reach an insurmountable performance ceiling.
The telecommuncation markets had been one of the first that needed a new design of parallel datapath packet processing because there were a very quick adoption of these multiple core processors for the datapath and the control plane. These MPUs are going to replace the traditional Network Processors that were based on proprietary micro- or pico- code. 6WIND was the first company to provide embedded software for these applications.
