22.8. Troubleshooting Memory Issues

As the operating system loads, it allocates available memory to itself and other components. When applications load, they get clearance to occupy desired memory from the operating system. Operating systems and applications also set aside memory for temporary usage, which can grow considerably under heavy load. At some point, lower-priority transactions are cached in a temporary file on the hard disk to optimize remaining memory. Not enough system memory can result in excessive temporary file access, which slows the system down because accessing the hard disk is slower than accessing RAM.

22.8.1 Evaluating the Memory Subsystem

Two useful counters in the Memory category of performance objects in the System Monitor are Available Bytes (or Kilobytes or Megabytes) and Pages/Sec.

The Available Bytes (or Kilobytes or Megabytes) counter indicates the amount of physical memory available to processes running on the computer, in bytes. It is calculated by summing the space on the Zeroed, Free, and Standby memory lists. Free memory is ready for use; Zeroed memory is pages of memory filled with 0s to prevent later processes from seeing data used by a previous process. Standby memory is memory removed from the working set of a process (its physical memory) on route to disk, but is still available to be recalled. This counter displays the last observed value only; it is not an average.

The Pages/Sec counter indicates the number of pages read from or written to disk to resolve hard page faults. (Hard page faults occur when a process requires code or data that is not in its working set or elsewhere in physical memory, and must be retrieved from disk.) This counter was designed as a primary indicator of the kinds of faults that cause system-wide delays. It is the sum of Memory: Pages Input/sec and Memory: Pages Output/sec. It is counted in numbers of pages, so it can be compared to other counts of pages, such as Memory: Page Faults/sec, without conversion. It includes pages retrieved to satisfy faults in the file system cache (usually requested by applications) and noncached mapped memory files. This counter displays the difference between the values observed in the last two samples, divided by the duration of the sample interval.