Monitoring the system memory (RAM) is not usually helpful in identifying memory performance problems. A better indicator will be monitoring paging activities to the page/swap file. Memory paging activities will be covered in another article. Most current OS have virtual memory comprising of the actual (real) system memory using physical RAM chips, and one or more page/swap files on the system disks. Processes that are currently running are operating in real memory. The OS can take pages from any of the processes currently in real memory and swap them out to disk. This is known as paging. Paging leaves free space in real memory to allocate to other processes that need to bring in a page from disk. Obviously, if all the processes currently running can fit into real memory, there is no need for the system to swap out any pages. However, if there are too many processes to fit into real memory, paging allows the system to free up system memory to run more processes.
Paging affects system performance in many ways. One, is that if a process moved some some pages to disk and the process becomes runnable, the OS has to retrieve the pages from the disk before that process can be run which results to delays in performance. In addition, both CPU and the disk I/O spend time doing the paging, reducing available processing power and increasing the load on the disks. This cascading effect involving both the CPU and I/O can degrade the performance of the whole system in such a way that it maybe difficult to even recognize that paging is the problem. The extreme version of too much paging is thrashing, in which the system is spending too much time moving pages around that it fails to perform any other significant work. (The next step is likely to be a system crash).
A little paging of the system does not affect the performance enough to cause concern. In fact, some paging can be considered good which indicates that the system’s memory resources are fully utilized. But at the point where paging becomes a significant overhead, the system is overloaded. To monitor paging activities is easy. On UNIX, the utilities vmstat and iostat provide details as to the level of paging, disk activity and memory levels. On Windows, the performance monitor has categories to show these details, such as Pages/sec, Page faults/sec and % Page files usage.
If there is more paging than optimal, the system’s RAM is insufficient or processes are too big. To improve this situation, you need to reduce the memory being used by (a) reducing the number of processes or (b) the memory utilization of some processes. Alternatively, you (c) can add RAM. If the problem is caused by a combination of your application and others, you can partially address the situation by using process priorities. By issuing priorities to the processes, you need to be aware that using this option reduces the amount of RAM available to all other processes, which can make overall system performance worse. Therefore measure the trade-offs before deciding the resolution.
(Source: “Java Performance Tuning”)
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