PROCESSOR INVOLVEMENT TYPES AND CHARACTERISTICS in Visual Studio .NET

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PROCESSOR INVOLVEMENT TYPES AND CHARACTERISTICS
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shared-memory communication via a common buffer Both processors are only involved in communication during one-half of the network activity: the source processor while writing into the common buffer and the destination processor while reading from the buffer Another case of partial involvement can be given on a system with a communication device or network adapter that performs the data transfer over the network, but the processors have to copy the data to and from it The time the processor is involved is a function of the data size, but the processor is not engaged during the entire communication Note that the communication device is considered part of the communication network Packet Based Communication The clear separation of overhead and involvement is not so obvious for packet based communication For instance, consider two-sided interprocessor communication as in the last example and assume that the communication is packet based At the beginning of a communication the sending processor goes through the communication layer, which is represented by the overhead If the data has at most the size of one packet, it is copied to the network adapter, which then sends the message to its destination This behavior can very well be captured with the notions of overhead and involvement However, a long message must be split into several packets During the transfer of each packet on the network, the processor, is not involved and can continue computation But the processor experiences a new, although smaller, overhead and involvement for submitting the next packet to the adapter, after the transfer of the previous one nished The situation is similar on the receiving processor, which participates in the reception and assembling of the packets Like the treatment of packet based communication in edge scheduling (Section 731), the packet based communication can be approximated with a single overhead and a single involvement Figure 84 illustrates this for the previously described example The various overheads and involvements (Figure 84(a)) of all packets are uni ed into one overhead and one involvement (Figure 84(b)) Of course, the period of network activity is identical in both views Note that the single overhead in Figure 84(b) only corresponds to the initial overhead in the packet view (Figure 84(a)) The small incremental overheads are accredited to the involvement This is sensible, as those overheads happen concurrently with the network activity and can thus be considered involvement If instead they were accredited to the initial overhead, the network activity would be shifted to a later time interval, since its start would be delayed correspondingly The notions of overhead and involvement are very exible as they allow the uni ed description of all three types of interprocessor communication, even when the processors participation is only partial At the same time they are intuitive and, as will be seen in the next sections, easily integrate with edge scheduling The separation into overhead and involvement is also more general than the approach taken by the LogP model, as will be shown next After that, the notions of overhead and involvement are employed in the task scheduling strategy that considers the involvement of the processor in communication
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PROCESSOR INVOLVEMENT IN COMMUNICATION
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Figure 84 Overhead and involvement for a packet based communication: (a) packet view and (b) the approximation as one overhead and one involvement
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813 Relation to LogP and Its Variants The LogP model (Section 213) was motivated by a problem similar to the one investigated in this chapter: the popular PRAM model for algorithm design and complexity analysis is not suf ciently realistic for modern parallel systems Therefore, it is interesting to compare the discussed notions of overhead and scheduling with the approach of LogP, and some of its variants, since LogP also recognizes that the processor often participates in communication LogP LogP (Culler et al [46, 47]) differs in one important aspect from the discussed notions of overhead and involvement The three parameters L, o, and g are speci ed assuming a message of some xed short size M (Section 213) So while LogP s approach is equivalent to the notions of overhead and involvement for such a short message M o corresponds to the overhead and the involvement is negligible or can be considered part of the overhead it is quite different for a long message In LogP, a long message of size N must be modeled as a series of N/M short messages Figure 85 illustrates the sending of a message of size 6 M; the reception on the destination processor works correspondingly Since LogP does not distinguish between overhead and involvement, both are represented by the several o s of the long message Thus, the communication cost imposed on the processor is modeled as being linear in the message size Consequently, the LogP model does not capture the nature of the large overhead of communication layers such as MPI or even the smaller, but still signi cant, overhead (Culler et al [47]) of communication environments such as Active Messages This overhead is typically only paid for once for every communication, independent of the message size This is especially an issue for third-party communication, because the overhead is typically of constant length independent of message size
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