vmstat command in Linux

The vmstat (virtual memory statistics) command is a powerful tool in Linux systems that provides detailed information about system processes, memory, paging, block I/O, traps, and CPU activity. This article will explore the vmstat command in depth, covering its usage, output interpretation, and practical applications.

Introduction to vmstat

vmstat is a versatile command-line utility that reports various statistics about system resources. It’s particularly useful for system administrators and developers who need to monitor system performance and diagnose issues related to memory usage, CPU utilization, and I/O operations.

The basic syntax of the vmstat command is:

vmstat [options] [delay [count]]

• options: Various flags to customize the output
• delay: Time interval between updates (in seconds)
• count: Number of updates to display

Let’s start with an example output of the vmstat command, and then we’ll break it down in detail.

procs -----------memory----------  --swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff   cache   si   so    bi   bo   in   cs us sy id wa st
 2  0      0 1882748 191312 3975436   0    0     8   32  227  456  5  2 93  0  0

Now, let’s examine each column in detail:

• Procs (Processes):
  r: The number of runnable processes (running or waiting for run time). This indicates the number of processes competing for CPU time. Values consistently above the number of CPU cores may indicate CPU saturation.
  b: The number of processes in uninterruptible sleep. High values can indicate I/O problems or processes stuck in the kernel.
• Memory:
  swpd: Amount of virtual memory used (in KB). Zero indicates no swapping, which is good for performance.
  free: Amount of idle memory (in KB). This shows available memory not used by the system.
  buff: Memory used for buffers (in KB). This is memory used for temporary storage of raw disk blocks.
  cache: Memory used as cache (in KB). This is memory used to cache disk data for faster access.
• Swap:
  si: Amount of memory swapped in from disk (KB/s). Zero indicates no swapping in, which is good.
  so: Amount of memory swapped out to disk (KB/s). Zero indicates no swapping out, which is good.
• I/O:
  bi: Blocks received from a block device (blocks/s). This shows the rate of data read from disks.
  bo: Blocks sent to a block device (blocks/s). This shows the rate of data written to disks.
• System:
  in: The number of interrupts per second, including the clock. This can indicate how busy the system is handling hardware interrupts.
  cs: The number of context switches per second. High numbers can indicate a lot of task switching, potentially impacting performance.
• CPU:
  us: Percentage of CPU time spent running non-kernel code (user time, including nice time). This represents application processing time.
  sy: Percentage of CPU time spent running kernel code (system time). High values can indicate excessive system calls or driver issues.
  id: Percentage of CPU time spent idle. High values indicate low CPU utilization.
  wa: Percentage of CPU time spent waiting for I/O. High values can indicate I/O bottlenecks.
  st: Percentage of CPU time stolen from a virtual machine. Always 0 for non-virtualized environments.

Common Usage Examples

a) Display vmstat output every 2 seconds for 5 updates:

vmstat 2 5

b) Show memory statistics in megabytes:

vmstat -S M

c) Display timestamp with each update:

vmstat -t 1

d) Show detailed statistics about disk activity:

vmstat -d

e) Display wide output for easier reading:

vmstat -w

Interpreting vmstat Output

• High ‘r’ values: Indicates CPU contention; processes are waiting for CPU time 
• High ‘b’ values: Suggests I/O bottlenecks; processes are waiting for I/O operations 
• High ‘si’ and ‘so’ values: Indicates excessive swapping, which can severely impact performance 
• High ‘bi’ and ‘bo’ values: Shows heavy disk I/O activity  
• High ‘in’ and ‘cs’ values: May indicate high system load or inefficient application design 
• Low ‘id’ values: Suggests high CPU utilization; the system is busy

Where to use vmstat

• Performance Monitoring: Regularly running vmstat can help identify performance bottlenecks and resource constraints.
• Troubleshooting: When system performance degrades, vmstat can pinpoint whether the issue is related to CPU, memory, or I/O.
• Capacity Planning: By analyzing vmstat data over time, administrators can make informed decisions about resource allocation and hardware upgrades.
• Application Optimization: Developers can use vmstat to understand how their applications interact with system resources and optimize accordingly.

Limitations

While vmstat is a powerful tool, it has some limitations:

• It provides system-wide statistics and doesn’t break down data by individual processes
• The CPU utilization data is an average across all CPUs
• Some fields may not be available or accurate on all systems or kernel versions

Complementary Tools

To get a more comprehensive view of system performance, consider using vmstat in conjunction with other tools:

• top or htop: For process-specific information
• iostat: For detailed I/O statistics
• sar: For collecting, reporting, and saving system activity information
• free: For a quick overview of memory usage

Conclusion:

The vmstat command is an essential tool in the Linux system administrator’s toolkit. Its ability to provide a concise yet comprehensive overview of system performance makes it invaluable for monitoring, troubleshooting, and optimizing Linux systems. By understanding how to use vmstat and interpret its output, you can gain deep insights into your system’s behavior and make informed decisions to maintain optimal performance.

That’s all for now.
Thank you for reading!!

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