|Red Hat Enterprise Linux 4: Introduction to System Administration
|Chapter 4. Physical and Virtual Memory
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 incapable of supporting virtual memory. This means that performance is never as good with virtual memory as it is when the same application 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 examine those system resources impacted by heavy use of the virtual memory subsystem.
For a moment, take what you have read in this chapter and consider what system resources are used by extremely heavy page fault and swapping activity:
RAM — It stands to reason that available RAM is low (otherwise there would be no need to page fault or swap).
Disk — While disk space might not be impacted, I/O bandwidth (due to heavy paging and swapping) would be.
CPU — The CPU is expending cycles doing the processing required 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 understand how resource shortages can lead to severe performance problems.
All it takes is a system with too little RAM, heavy page fault activity, and a system running near its limit in terms of CPU or disk I/O. At this point, the system is thrashing, with poor performance the inevitable result.
At best, the overhead from virtual memory support presents 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 are dedicated to actually running applications, instead of running the operating system's memory management code
From this, the overall point to keep in mind is that the performance impact of virtual memory is minimal when it is used as little as possible. This means the primary determinant of good virtual memory subsystem performance is having enough RAM.
Next in line (but much lower in relative importance) are sufficient disk I/O and CPU capacity. However, keep in mind that these resources only help the system performance degrade more gracefully from heavy faulting and swapping; they do little to help the virtual memory subsystem performance (although they obviously can play a major role in overall system performance).
A reasonably active system always experiences some level of page fault activity, due to page faults incurred as newly-launched applications are brought into memory.