How to Monitor Virtual Memory with 'vm_stat' (with examples)

The vm_stat command is a vital tool for anyone interested in the state of virtual memory on their macOS device. Virtual memory effectively increases the amount of usable memory space your applications can use by diverting some data to disk. Monitoring virtual memory statistics helps in understanding memory-related performance issues, assessing whether your system has enough memory, and ensuring optimal use of your system’s resources.

Use case 1: Display Virtual Memory Statistics

Code:

vm_stat

Motivation:

The primary motivation behind using vm_stat without arguments is to obtain a snapshot of the current state of the virtual memory system. This basic check is invaluable for system administrators, developers, and power users who need to quickly assess how well their system is handling memory. It’s particularly useful in diagnosing whether your system memory is under pressure or if there are unexpected spikes in memory usage that could suggest potential performance bottlenecks or memory leaks.

Explanation:

The vm_stat command, when executed without any additional arguments, defaults to providing a single report of virtual memory statistics. This provides instantaneous visibility into various critical metrics including free memory, active memory, inactive memory, wired memory, etc., which are essential for gauging the system’s memory performance.

Example output:

Mach Virtual Memory Statistics: (page size of 4096 bytes)
Pages free:                          92516.
Pages active:                       146810.
Pages inactive:                     131456.
Pages speculative:                   48384.
Pages throttled:                         0.
Pages wired down:                   103162.

The output details are straightforward, providing the number of memory pages for various states, helping users to make informed decisions regarding their system’s memory status.

Use case 2: Display Reports Every 2 Seconds for 5 Times

Code:

vm_stat -c 5 2

Motivation:

For real-time monitoring, such as when troubleshooting memory issues or observing memory patterns during heavy application usage, generating multiple consecutive reports is essential. The -c flag followed by parameters allows for the repeated invocation of vm_stat at specified intervals. This is crucial to identify trends or anomalies over time, which cannot be observed with a single snapshot. For instance, watching how the active and free memory fluctuate during a memory-intensive task like video rendering or a heavy multi-tab browsing session can be insightful.

Explanation:

• -c 5: Here, -c specifies the number of times to repeat the report. The number 5 indicates that the report will be generated five times.
• 2: This is the interval, specified in seconds, between each report iteration. Thus, 2 signifies that there will be a two-second pause between each of the five reports.

Example output:

Mach Virtual Memory Statistics: (page size of 4096 bytes)

Pages free:                     92020.
Pages active:                  147234.
Pages inactive:                131889.
Pages speculative:              48201.
Pages throttled:                    0.
Pages wired down:              103829.

*output repeats 4 more times, capturing transient memory changes*

The above output will refresh every two seconds until it has displayed five reports. Each refresh provides a fresh set of memory statistics in real-time, allowing you to track how your system’s virtual memory usage evolves over the period of time you are monitoring.

Conclusion:

The vm_stat command is a robust utility for monitoring virtual memory on macOS systems. Whether you need a quick snapshot or a series of data points to analyze trends over time, vm_stat can adequately provide insights that are crucial for maintaining smooth system operations. Understanding and utilizing this command can help prevent memory issues, ensure efficient resource utilization, and optimize system performance.