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Multiprocessors in Software Integrating PDF 417 in Software Multiprocessors




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Multiprocessors generate, create none none on none projects barcode 128 0 1 2 3 4 5 6 7. To go from pro cessor i (xyz in binary) to processor j (uvw), start at i, and at each stage k follow either the high link if the k th bit of the destination address is 0, or the low link if it is 1. For example, the path to go from processor 4 (100) to processor 6 (110) is marked in bold lines..

Figure 7.6. Bu none none tter y network of eight processors (three stages).

. r a 2D torus, which is like a mesh but with links wrapping around at the edges, is also a two-dimensional network; r an m-dimensional hypercube, where each processing element is connected by a link of length 1 to each of its m neighbors, is an m-dimensional network. Many multiprocessors, and parallel processors using message passing, have been built using indirect networks. For example, a ring was used in the Kendall KSR machine and in the Sequent CC-Numa, where the ring connected shared-bus SMPs.

The Intel Paragon used 2D meshes to connect up to 512 nodes, where each node. Figure 7.7. Sc none for none hematic view of a 2D mesh.

Each box is a processing element connected by a link of length 1 to its four neighbors.. 7.2 Cache Coherence consisted of a none for none n Intel i860 processor for computing purposes and one for switching purposes, along with DRAM memory. The Cray 3TD and 3TE use a 2D torus interconnection. The Connection Machine CM-2, an SIMD processor, used a 12-dimensional hypercube where each node had 16 1-bit processors.

Routing of a message from one processing element to another can be done in several ways. Higher dimension rst is a common pattern. In a 2D mesh, this implies that the message travels rst vertically, then horizontally.

A more important question is control ow, that is, how messages make progress along the source destination path. Various techniques, such as wormhole routing and virtual cutthrough, have been developed. However, in the realm of CMPs, where messages are memory requests, control ow might simply require acknowledgments and retries.

Although no commercial CMP uses indirect networks at this point, a number of research machines that can be classi ed under the rubric of grid processors use 2D meshes. The investigation of interconnection networks, which was a topic of high interest in the 1970s and 1980s, has seen a renewal of interest now that CMPs with more than eight processors, grid processors, and special-purpose processors with hundreds of ALUs on a chip are on the horizon..

7.2 Cache Coherence The performanc none none e value of a well-managed uniprocessor cache hierarchy was documented in 6. In multiprocessors, main memory is even farther from each individual processor, for the time to traverse the interconnection network will be longer than that of access via the memory bus. Moreover, there will be more contention, both in the network and at the memory modules themselves.

Finally, a new problem arises in shared-memory multiprocessors, namely, the consistency of shared data in the various caches, which is referred to as the cache coherence problem. To illustrate the cache coherence problem, consider a three-processor system as abstracted in Figure 7.8.

Each rectangular box represents a processor and its cache hierarchy. We show a UMA system, but the example would work similarly for a NUMA architecture..

EXAMPLE 1:. Initially, all none none caches are empty. Then processor P1 reads some data A. After resolving the cache miss, the data are stored in P1 s cache.

In the next action involving A, let processor P2 read the same data A. On the cache miss, a copy of A coming from either memory or P1 s cache, depending on the cache coherence protocol is stored in P2 s cache. At this point, shown in Figure 7.

8(a), P1 (or rather its cache), P2, and memory all have a valid copy of A, represented by a hatched box in the gure. Now let P3 have a write miss on the same data A. Once a copy of A is in P3 s cache, there are two possible courses of action.

In the rst case, P3 writes a new value for A and sends it to all other caches that contain a copy of A as well as to main memory, so that all caches and main memory have the same value for A. Figure 7.8(b) shows the results.

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