Chapter 10 Optional Features for Improving I/O Performance

Two features of OpenVMS Alpha and Integrity servers provide dramatically improved I/O performance: Fast I/O and Fast Path. These features are designed to promote OpenVMS as a leading platform for database systems. Performance improvement results from reducing the CPU cost per I/O request and improving symmetric multiprocessing (SMP) scaling of I/O operations. The CPU cost per I/O is reduced by optimizing code for high-volume I/O and by using better SMP CPU memory cache. SMP scaling of I/O is increased by reducing the number of spinlocks taken per I/O and by substituting finer-granularity spinlocks for global spinlocks.

The improvements follow a natural division that already exists between the device-independent and device-dependent layers in the OpenVMS I/O subsystem. The device-independent overhead is addressed by Fast I/O, which is a set of lean system services that can substitute for certain $QIO operations. Using these services requires some coding changes in existing applications, but the changes are usually modest and well contained. The device-dependent overhead is addressed by Fast Path, which is an optional performance feature that creates a “fast path” to the device. It requires no application changes.

Fast I/O and Fast Path can be used independently; however, together they can provide a 45 percent reduction in CPU cost per I/O on uniprocessor systems and a 52 percent reduction on multiprocessor systems.