H. F. Wedde, S. Böhm und W. Freund
Concurrent Read/Write - Real-Time Theory and Practice
4th IEEE International Symposium on Object-Oriented Real-Time Distributed Computing, Magdeburg, Germany, 2001-01
Read/ write conflicts, be it on the task or transaction level, may have harmful consequences for the real-time performance, in particular 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 at times 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 a 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. Extensive experiments for different record-based object access models show a very clear advantage of our novel adaptive scheduling algorithms that utilize the shadow copy paradigm, over the traditional object access models, both in terms of deadline failure rates and system survivability (the latter being the most appropriate performance criterion for safety-critical systems).