Книга: Distributed operating systems
6.8. SUMMARY
6.8. SUMMARY
Multiple CPU computer systems fall into two categories: those that have shared memory and those that do not. The shared memory machines (multiprocessors) are easier to program but harder to build, whereas the machines without shared memory (multicomputers) are harder to program but easier to build. Distributed shared memory is a technique for making multicomputers easier to program by simulating shared memory on them.
Small multiprocessors are often bus based, but large ones are switched. The protocols the large ones use require complex data structures and algorithms to keep the caches consistent. NUMA multiprocessors avoid this complexity by forcing the software to make all the decisions about which pages to place on which machine.
A straightforward implementation of DSM, as done in IVY, is possible, but often has substantially lower performance than a multiprocessor. For this reason, researchers have looked at various memory models in an attempt to obtain better performance. The standard against which all models are measured is sequential consistency, which means that all processes see all memory references in the same order. Causal consistency, PRAM consistency, and processor consistency all weaken the concept that processes see all memory references in the same order.
Another approach is that of weak consistency, release consistency, and entry consistency, in which memory is not consistent all the time, but the programmer can force it to become consistent by certain actions, such as entering or leaving a critical region.
Three general techniques have been used to provide DSM. The first simulates the multiprocessor memory model, giving each process a linear, paged memory. Pages are moved back and forth between machines as needed. The second uses ordinary programming languages with annotated shared variables. The third is based on higher-level programming models such as tuples and objects.
In this chapter we studied five different approaches to DSM. IVY operates essentially like a virtual memory, with pages moved from machine to machine when page faults occur. Munin uses multiple protocols and release consistency to allow individual variables to be shared. Midway is similar to Munin, except that it uses entry consistency instead of release consistency as the normal case. Linda represents the other end of the spectrum, with an abstract tuple space far removed from the details of paging. Orca supports a model in which data objects are replicated on multiple machines and accessed through protected methods that make the objects look sequentially consistent to the programmer.
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