Performance Testing, LoadRunner Tips&Tricks

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General: Understanding Memory - Available Bytes

You can watch real memory filling up by monitoring Available Bytes, which represents free, unallocated RAM. Available Bytes counts the number of free pages in RAM at any particular time; it is the all-important buffer of free pages the OS maintains in order to resolve page faults quickly. The Available Bytes counter, like all the real memory allocation counters, report the amount of RAM currently not allocated to any process in bytes.

When you launch Task Manager, on the Performance Tab, the MEM Usage and Memory Usage History Graphs refer to virtual memory Committed Bytes. The value corresponds to the Commit Charge (K) Total field in the lower-left quadrant. The Task Manager Commit Charge field reports on virtual memory allocations in 1024 kilobytes (KB) segments; the corresponding Committed Bytes Limit is also shown. The Memory Usage History graph charts Committed Bytes against the Commit Limit, which servers as the maximum value for the Y-axis.

In the upper-right text quadrant (immediately below the Memory Usage History timeline), the Task Manager shows Total system memory (the amount of installed RAM), Available, and System Cache, all denominated in 1000 bytes. The Task Manager Available field corresponds exactly to the System Monitor Available Kbytes counter, by the way.

As you open new applications on your desktop, watch as virtual memory Committed Bytes increases while Available Bytes decreases in tandem. As a general rule of thumb, a 5 – 10% cushion of Available Bytes is normally ample.

If you are able to start enough application programs, you will eventually observe that the number of Committed Bytes begins to exceed the amount of physical RAM installed. This is the first indication that real memory might be filling up. As long as Committed Bytes is less than the amount of RAM installed, every virtual memory page requested by application process can fit in RAM. That means the amount of RAM installed is sufficient for this workload. However, just because Committed Bytes is greater than the amount of RAM installed dos not mean RAM is completely allocated. Some applications committed pages that have not been referenced in a long time may not be currently resident in real memory. Idle pages are subject to being paged out of the system, so RAM may still not be completely allocated.

Windows continues to permit applications to allocate more virtual memory up to the Committed Bytes Limit. If your applications continue to add more Committed Bytes to the system, at some point all those active virtual memory pages will not fit easily into physical RAM and you will start to notice an impact on system performance. At this point of time, when you click on an application that you have not accessed in a while, you are likely to hear the hard disk grinding away. Windows is performing paging I/O to the disk, taking older pages in memory and writing them to the paging file to make room for current pages that t must read back into memory from the paging file disk. As you continue to open new applications and drive Committed Bytes higher, the slowdown in system performance will eventually become acute, which eventually might leads to trashing.

When the number of virtual memory Committed Bytes starts to exceed the amount of physical RAM installed, you can probably also observe that the number of Available Bytes begins to stabilize at approximately 4MB. Windows attempts to maintain a 4MB cushion of Available Bytes in order to service application requests for a new page quickly. If the working sets of active processes exceed the size of physical memory, there is contention for real memory. In Windows, you can often observe signs of this contention when the system’s pool of Available Bytes reaches about 4MB. At approximately 4MB of available RAM, Windows crosses a threshold where behavior of its virtual memory management policy changes. If Windows has difficulty maintaining a 4MB pool of available RAM and the demand for virtual memory continues to increase, you will likely observe telltale signs of trashing.

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