Evaluating Memory and Cache Usage

Low memory conditions can slow the operation of applications and services on your computer and impact the performance of other resources in your system. For example, when your computer is low on memory, paging — that is, the process of moving virtual memory back and forth between physical memory and the disk — can be prolonged, resulting in more work for your disks. Because it involves reading and writing to disk, this paging activity might have to compete with whatever other disk transactions are being performed, intensifying a disk bottleneck . (A disk bottleneck occurs when disk performance decreases to the extent that it affects overall system performance.) In turn, all this work by the disk can mean the processor is used less or is doing unnecessary work, processing numerous interrupts due to repeated page faults . (Page faults occur when the system cannot locate requested code or data in the physical memory available to the requesting process.) In the end, applications and services become less responsive.

Figure 28.1 illustrates the sequence in which you conduct the monitoring process. Memory has such an important influence on system performance that monitoring and analyzing memory usage is one of the first steps you take when assessing your systems performance.

Figure 28.1 Role of Memory Monitoring in Overall Monitoring Sequence

In the first phase of analyzing memory usage, you need to understand your current memory configuration and workload. To help you do this, use the steps in the following list:

• Determine the amount of physical memory that is currently installed, and compare it against minimum operating system requirements.

• Read Understanding Memory and the File System Cache later in this chapter for information about the relationship between system memory and the file system cache.

• Establish a baseline for memory usage on your computer by determining performance ranges for low or idle, average, and peak usage periods.

• Optimize the system memory configuration to the system workload, including verifying cache and paging file sizes.

Focus subsequent monitoring on how your system uses memory, and on identifying memory shortages or other problems, by taking the following steps:

• Look at memory characteristics of processes using the Process\Working Set and Process\Private Bytes counters. The Working Set counter reports the amount of committed memory allocated to the process. This might include shared and private bytes currently residing in physical memory. The Private Bytes counter reports memory allocated exclusively to the process. Working set monitoring is important because, when memory is in short supply, the operating system trims the working sets of processes and paging occurs. (The working set of a process is the amount of physical memory assigned to that process by the operating system.)

• Monitor the counters listed in Investigating Disk Paging later in this chapter to understand the relationship between the amount of paging and the amount of disk activity and their effect on overall performance. Excessive paging can burden the disk.

• Determine the effectiveness of the file system cache. Your system performs better when it can find requested data in the cache, rather than when it must read from the disk.

• Learn how to tune the working ** sets of applications, if you have access to source code. Efficient applications maintain a small working set without generating page faults.