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Distributed operating systemsThe address word following the descriptor is changed to reflect the address at which the region is located in the receiver's address space. This mechanism provides a way to move blocks of data at extremely high speed, because no copying is required except for the message header and the two-word body (the descriptor and the address). Depending on a bit in the descriptor, the region is either removed from the sender's address space or kept there. Although this method is highly efficient for copies between processes on a single machine (or between CPUs in a multiprocessor), it is not as useful for communication over a network because the pages must be copied if they are used, even if they are only read. Thus the ability to transmit data logically without moving physically them is lost. Copy-on-write also requires that messages be aligned on page boundaries and be an integral number of pages in length for best results. Fractional pages allow the receiver to see data before or after the out-of-line data that it should not see. 8.4.3 ...» |
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