Inside Out

How the System Properties dialog box reports RAM

As we note in this chapter, a computer running a 32-bit (x86) version of Windows can address a maximum of 3.5 GB of physical memory. In Windows 7, the System Properties dialog box displays two values when it encounters a discrepancy between the amount of addressable memory and the amount of physical RAM installed . Thus, on a system with 4 GB of physical RAM, the value reported under the System heading for Installed Memory (RAM) will show 4.00 GB (3.50 GB Usable); the second value might be less, depending on your hardware configuration.

Memory management is confusing, perhaps more so than any other aspect of PC performance. If you scour the web for information on this topic, you will surely run into misguided advice and technical errors. Knowing the meaning of the following specialized memory-measurement terms helps you make sense of it all:

• Physical memory refers to actual RAM chips or modules, typically installed on a computer's motherboard. The amount of physical RAM available to Windows might be less than the total physical amount if another system component is using that memory for its own purposes, as is the case with "shared memory" video subsystems on portable computers . Physical memory measurements (total and in use) are reported on the Performance tab of Windows Task Manager and on the Memory tab of Resource Monitor.

• Virtual memory consists of physical memory plus the amount of space in the page file, which is stored on the hard disk. We discuss virtual memory in more detail in the next section, "Ensuring That You Have an Adequate Virtual-Memory Configuration" on page 724

• Kernel memory is owned by Windows and is used to provide system services to applications. Paged memory can be backed up to the page file and replaced by application memory if necessary; nonpaged memory must remain in physical RAM at all times

• Cached memory holds data or program code that has been fetched into memory during the current session but is no longer in use now. If necessary, the Windows memory manager will flush the contents of cached memory to make room for newly summoned data.

• Free memory represents RAM that does not contain any data or program code and is free for use immediately

• Working Set is the term that defines the amount of memory currently in use for a process . Private Working Set is the amount of memory that is dedicated to that process and will not be given up for other programs to use; Shareable Working Set can be surrendered if physical RAM begins to run scarce. Peak Working Set is the highest value recorded for the current instance of this process.

• Commit Charge (also called commit size) is the total amount of virtual memory that a program has touched (committed) in the current session, including memory that has been paged out of physical memory to the disk-backed page file. The Memory and Physical Memory counters on Task Manager's Performance tab represent the sum of this value for all processes and the kernel. The Commit Charge Limit is the total amount of physical RAM and page file available—in other words, the maximum virtual memory

• Hard faults are also known as page faults. Despite the negative connotation of the name, this is not an error condition. Rather, it represents an instance where a block of memory needed by the operating system or an application has to be fetched from the page file on the hard disk instead of from physical memory. A consistently high number of hard faults per second indicates a large—perhaps excessive—reliance on virtual memory, with consequent adverse performance effects.

The best way to gauge the adequacy of your currently installed RAM is to keep an eye on the Memory graph in Resource Monitor (see Figure 20-5 and the accompanying text in "Using Resource Monitor" on page 716). The green portion of the bar indicates the percentage of your physical memory that's currently in use; shades of blue indicate cached memory that is available on demand (Standby). It's also important to watch the green line on the graphs to the right, which indicates the number of hard faults per second your system is generating; if you see it spike off the top of the graph for extended periods of time, you'll want to take a closer look at how memory is being used.

Pay special attention to these numbers when you're asking the most of your computer; you might even consider performing a stress test by successively opening the applications you use most often . Switch between programs, open and edit some data files, browse a couple dozen webpages, and generally try to use more system resources than you can imagine using at one time under normal circumstances . If you find yourself bumping up against the ceiling regularly, you might get a noticeable performance boost from additional RAM.

Don't get hung up on mere percentages, though . If you routinely hit a maximum of 85 percent memory usage on a machine running 64-bit Windows 7 with 6 GB of physical RAM, you have 900 MB of free RAM, which is plenty of headroom .

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