Virtual Memory and its influences on performance

Virtual Memory and its influences on performance

While virtual memory makes it possible for computers to more easily handle larger and more complex applications, as with any powerful tool, it comes at a price. The price in this case is one of performance — a virtual memory operating system has a lot more to do than an operating system that is not capable of virtual memory. This means that performance will never be as good with virtual memory than with the same application that is 100% memory-resident.

However, this is no reason to throw up one's hands and give up. The benefits of virtual memory are too great to do that. And, with a bit of effort, good performance is possible. The thing that must be done is to look at the system resources that are impacted by heavy use of the virtual memory subsystem.

Worst Case Performance Scenario

For a moment, take what you have read earlier, and consider what system resources are used by extremely heavy page fault and swapping activity:

· RAM -- It stands to reason that available RAM will be low (otherwise there would be no need to page fault or swap).

· Disk -- While disk space would not be impacted, I/O bandwidth would be.

· CPU -- The CPU will be expending cycles doing the necessary processing to support memory management and setting up the necessary I/O operations for paging and swapping.

The interrelated nature of these loads makes it easy to see how resource shortages can lead to severe performance problems. All it takes is:

· A system with too little RAM

· Heavy page fault activity

· A system running near its limit in terms of CPU or disk I/O

At this point, the system will be thrashing, with performance rapidly decreasing.

Best Case Performance Scenario

At best, system performance will present a minimal additional load to a well-configured system:

· RAM -- Sufficient RAM for all working sets with enough left over to handle any page faults

· Disk -- Because of the limited page fault activity, disk I/O bandwidth would be minimally impacted

· CPU -- The majority of CPU cycles will be dedicated to actually running applications, instead of memory management