Read/write conflicts, be it on the task or transaction level, may have harmful consequences for the real-time performance, particularly in distributed systems. Under safety-critical constraints (where task arrival is unpredictable) such conflicts would then impose an unpredictable overhead. Static solutions of conflict handling by utilizing shadow copies have been suggested, even under non-preemptive object access (where shadow copies are almost useless). This paper reports on research concerning adaptive preemptive methods for handling record-based read/write access to objects providing shadow copies. We present a new refined task and resource scheduling method based on measures that take task or transaction criticality into account. A potential advantage with shadow copies stems from the fact that shadow copies are still available to readers once a writer has been preempted whereas otherwise all scheduled (conflicting) readers have to wait until the writer has been finished (unless the write action is rolled back). In our model, readers that are preempted through writers (or writers that are preempted through readers) need not acquire their locks anew after preemption thus allowing for a higher scheduling flexibility and choice through keeping their locks. We study the trade-off between the higher flexibility of preemptive shadow copy models and the higher overhead they incur, due to refreshing shadow copies in due time.
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