B.12.6 Interactions with OpenVMS

There are several interactions with the OpenVMS operating system that should be noted:

• Like system service calls, pagefault waits are also thread-synchronous. When a thread incurs a "hard" pagefault (one which involves reading the page from disk), an upcall is performed and the thread is placed in a blocked state. DECthreads schedules another thread to run in its place. When the pagefault resolution is complete, another upcall occurs and DECthreads schedules it to run at its next opportunity. It is possible for multiple threads to take faults on the same page at approximately the same time. Each of them is blocked in turn, and they are all unblocked when the page becomes valid.

• Most OpenVMS system services would not be able to tolerate being called by multiple threads concurrently. Therefore, calls to OpenVMS system services are serialized, by means of a mechanism called the inner mode semaphore. If a second thread attempts to call a system service while another thread is in the middle of calling a system service, the second thread will be blocked by an upcall until the first thread completes its service call.

• DECthreads timeslicing has changed slightly under OpenVMS Alpha Version 7.0. Prior to Version 7.0, the DECthreads timeslicer was implemented using an OpenVMS timer. This caused a DECthreads scheduler AST to be delivered to the process at regular wall-clock time intervals. While running on wall-clock time was a necessary evil (in order to be able to interrupt system service blocks), this timeslice mechanism had several drawbacks. In Version 7.0, timeslicing is implemented by means of an upcall that is delivered after the thread has consumed a sufficient amount of CPU time. Thus, when no threads are running, no timeslicing takes place.