Processor deallocation impacts to applications

Processor deallocation is transparent for the vast majority of applications, including drivers and kernel extensions. However, you can use the published interfaces to determine whether an application or kernel extension is running on a multiprocessor machine, find out how many processors there are, and bind threads to specific processors.

The bindprocessor interface for binding processes or threads to processors uses bind CPU numbers. The bind CPU numbers are in the range [0..N-1] where N is the total number of CPUs. To avoid breaking applications or kernel extensions that assume no "holes" in the CPU numbering, AIX® always makes it appear for applications as if it is the "last" (highest numbered) bind CPU to be deallocated. For instance, on an 8-way SMP, the bind CPU numbers are [0..7]. If one processor is deallocated, the total number of available CPUs becomes 7, and they are numbered [0..6]. Externally, it looks like CPU 7 has disappeared, regardless of which physical processor failed.

Potentially, applications or kernel extensions that are binding processes or threads could be broken if AIX silently terminated their bound threads or forcefully moved them to another CPU when one of the processors needs to be deallocated. Dynamic Processor Deallocation provides programming interfaces so that such applications and kernel extensions can be notified that a processor deallocation is about to happen. When these applications and kernel extensions receive notification, they are responsible for moving their bound threads and associated resources (such as timer request blocks) away from the last bind CPU ID and for adapting themselves to the new CPU configuration.

After notification, if some threads remain bound to the last bind CPU ID, the deallocation is aborted, the aborted deallocation is logged in the error log, and AIX continues using the ailing processor. When the processor ultimately fails, it causes a total system failure. Therefore, it is important that applications or kernel extensions receive notification of an impending processor deallocation and act on this notice.

Even in the rare cases that the deallocation cannot go through, Dynamic Processor Deallocation still gives advanced warning to system administrators. By recording the error in the error log, it gives them a chance to schedule a maintenance operation on the system to replace the ailing component before a global system failure occurs.