Java Performance Tuning

RAM

Maintaining watch directly on the system memory (RAM) is not usually that helpful in identifying performance problems. A better indication that memory might be affecting performance can be gained by watching for paging of data from memory to the swap files. To clarify the term paging: most current operating systems have a virtual memory that is made up of the actual (real) system memory using RAM chips, and one or more swap files on the system disks. Processes that are currently running are operating in real memory. The operating system 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.^[103]

Obviously, if all the processes currently running can fit into the real memory, there is no need for the system to page 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 further processes. Paging affects system performance in many ways. One obvious way is that if a process has had some pages moved to disk and the process becomes runnable, the operating system has to pull back the pages from the disk before that process can be run. This leads to delays in performance. In addition, both the CPU and the disk I/O subsystem spend time doing the paging, reducing available processing power and increasing the load on the disks. This ...