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Distributed operating systemsOn the other hand, the network will be tied up longer with a larger transfer, blocking other faults caused by other processes. Also, too large an effective page size introduces a new problem, called false sharing, illustrated in Fig. 6-26. Here we have a page containing two unrelated shared variables, A and B. Processor 1 makes heavy use of A, reading and writing it. Similarly, process 2 uses B. Under these circumstances, the page containing both variables will constantly be traveling back and forth between the two machines. Fig. 6-26. False sharing of a page containing two unrelated variables. The problem here is that although the variables are unrelated, since they appear by accident on the same page, when a process uses one of them, it also gets the other. The larger the effective page size, the more often false sharing will occur, and conversely, the smaller the effective page size, the less often it will occur. Nothing analogous to this phenomenon is present in ordinary virtual memory systems. Clever compilers that understand the problem and place variables in the address space accordingly can help reduce false sharing and improve performance ...» |
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