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General: Understanding Memory - Performance Concerns

A pervasive concern of virtual memory schemes is that the performance of application programs may suffer when there is a shortage of real memory and too many page faults occur. Windows maintain a pool of available (free) real memory pages to resolve page faults quickly. Whenever the pool is depleted, Windows replenishes its buffer of available RAM by trimming older (less frequently referenced) pages of active processes and by swapping the pages of idle foreground applications to disk. If the supply of real memory is ample, executing programs seldom encounter page faults that delay their execution, and the OS has no difficulty maintaining a healthy supply of free pages. If the system is short on real memory (relative to the memory required to back concurrently referenced pages with actual physical memory pages), a high rate of page fault may occur, slowing the performance of executing programs considerably. Obviously, the OS may not be able to maintain an adequate pool of available real memory when there is not enough real memory to go around.

You want to avoid three types of performance problems that can occur when there is too little real memory:

Too many page faults: Having too many page faults leads to excessive program execution delays during page fault resolution. This is the most straightforward performance problem associated with virtual memory addressing.

Disk contention: Virtual memory systems that are subject to a high page fault rate may also encounter disk performance problems because of the extra disk I/O workload generated to resole page faults. In systems where the paging file (or files) is located on the same disk (or disks) as application data files, I/O to the paging file can limit the ability of applications to access their own data files. A possible secondary effect of disk contention is elongation of page fault resolution time, further delaying the programs affected when page faults do occur.

Competition for memory: Because real memory is allocated to executing programs on demand, there is another potential pitfall. In virtual memory systems, programs resident in memory compete for access to available physical memory. The inevitable result of competition when memory is scarce is that the memory access pattern of one program can unduly influence other running programs.

A severe shortage of real memory can seriously impact performance. Moving pages back and forth between disk and memory consumes both processing and disk capacity. A system forced to use too many CPU and disk resources on virtual memory management tasks and too few on the application workload is said to be thrashing.

Inevitably, the solution to a paging problem is to install more real memory capacity. Alternatively (and less definitely), it may help to improve page fault resolution time by focusing on disk I/O performance. This typically involves some combination of the following.

  1. Defining additional paging files across multiple (physical) disks,
  2. Reducing disk contention by removing other heavily accessed files from the paging file physical disks, or
  3. Upgrading to faster disks.

The above was extracted from the book, Windows 2000 Performance Guide by Mark Friedman & Odysseas Pentakalos.

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