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. Read the rest of this entry »

Understanding Memory: Page Fault Resolution

For all its virtues, virtual memory can raise some serious performance issues. Among them is execution delays encountered by programs whenever they reference virtual memory locations not in the current set of memory-resident pages. This is known as page fault. A program thread that incurs a page fault is halted during page fault resolution, the time it takes the OS to find the specific page on disk and restore it to real memory.
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Understanding Memory: Virtual Addressing

Virtual memory is a feature supported by most advanced processors. Hardware support for virtual memory includes hardware mechanism to map from logical (virtual) memory address that application programs reference to physical (real) memory hardware addresses.

When an executable program’s image file is first loaded into memory, the logical memory address range of the application is divided into fixed-size chunks called pages. These logical pages are them mapped to similar-sized physical pages that are resident in real memory. Read the rest of this entry »

Detecting memory bottlenecks

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. Read the rest of this entry »

Detecting memory leaks

Memory leaks are caused by programs that allocate virtual memory and fail to free after they are finished with it. Memory leaks are nasty program bugs that are sometimes difficult to find. A program containing memory leak bug often executes to completion and produces the correct results, so the presence of the bug is not always detected. In fact, as long as the program containing the memory leak does not impact other executing processes, the bug can go undetected for a long time. Read the rest of this entry »