Understanding your virtual machine’s CPU and RAM usage is important for monitoring performance and troubleshooting issues. Here is a comprehensive guide on how to check CPU and RAM utilization on Linux VMs.
Why check CPU and RAM usage?
Monitoring CPU and RAM usage can help you:
- Identify performance bottlenecks caused by insufficient resources
- Understand application and service resource requirements for capacity planning
- Detect abnormal resource usage that may indicate problems
Checking usage regularly allows you to optimize your VMs for cost, performance, and reliability.
Checking CPU Usage
The Linux top command provides a dynamic real-time view of running processes and resource utilization. Here are some useful variations for checking CPU usage:
top – Show overview of resource usage including CPU and RAM
top -b – Start top in batch mode to get static CPU usage snapshot
top -d 5 – Refresh display every 5 seconds for monitoring
top -n 1 – Get single snapshot then quit top
Some useful fields in top for examining CPU usage:
- %Cpu – Percentage of CPU used over the sampling period
- %Cpu(s) – CPU usage of each individual core
- us – CPU time spent in user space
- sy – CPU time spent in kernel space
- ni – CPU time handling nice/low priority processes
- id – Idle CPU time when system has no tasks to process
- wa – IO Wait time when CPU is waiting for IO operation
If the CPU consistently runs at or near 100% this indicates there may be an undersupply of compute resources.
Key things to review in top for high CPU:
- Which processes are consuming the most CPU?
- Is the high usage from a single process or distributed?
- Identify trends over time – is CPU spiking or sustained?
This can help narrow down the potential cause, whether it’s a runaway process, resource leak, or insufficient CPU cores.
Checking RAM Usage
Along with CPU, monitoring memory utilization is key to understanding VM performance. Here are some useful commands for checking RAM usage in Linux:
free -m – Show RAM usage in MB
vmstat -s – Summary overview including total RAM
top – Sort by resident memory usage with R key
/proc/meminfo – Additional memory usage statistics
Key RAM usage metrics:
- MemTotal – Total installed memory
- MemFree – Unused memory available
- MemAvailable – Estimate of memory available for new processes
- Cached – Memory used by the page cache for file IO
- Buffers – File system metadata cache
- SwapTotal – Total swap space available
- SwapFree – Unused swap space
If the MemAvailable value is consistently low even with free RAM, or swap usage grows continuously, this typically indicates an undersupply of memory.
Things to examine:
- Which processes are consuming the most memory?
- What is the trend in RAM usage – is it leaked slowly or spiking at times?
- Is swap being used – impact on performance?
This can help determine if certain apps/services need more memory, if there is a memory leak, or if total RAM needs to be increased.
Long Term Monitoring & Alerting
For ongoing monitoring and alerts based on usage thresholds, consider using a monitoring system like:
- Prometheus – Popular open-source monitoring system and time-series database
- Netdata – Lightweight open-source health monitoring and troubleshooting agent
- Nagios – Enterprise-grade monitoring solution
- Datadog / NewRelic – Featureful SaaS performance monitoring
These tools allow graphing utilization over time, setting alerts on thresholds, correlation between metrics, and long term storage for analysis.
Most Linux monitoring plugins for these tools gather the core usage statistics shown above from /proc/ and top. Gathering utilization trends is invaluable for right-sizing resources and planning capacity.
Optimizing Resource Usage
If facing resource shortages on a Linux VM there are also various optimization techniques that can help:
For CPU constrained VMs:
- Switch to a CPU optimized instance type
- Reduce or throttle extraneous processes
- Enable haproxy compression for web traffic
- Add CPU cores if vertical scaling is limited
- Distribute load across multiple VMs
For Memory constrained VMs:
- Switch to a memory optimized instance type
- Add RAM if vertical scaling allows
- Reduce memory footprint of processes
- Enable swap if not already
- Investigate memory leaks
Tuning resource usage allows maintaining performance and reliability even with increasing demands.
Effectively monitoring CPU and RAM usage is critical for optimizing Linux VM performance and reliability. top and free provide real time visibility into utilization for identifying issues and trends over time. Long term monitoring and alerting through dedicated tools allows setting usage policies and proactively planning capacity. CPU and RAM optimization techniques also provide paths for addressing constrained resources. Check usage regularly and consider these tools and techniques for keeping your Linux VMs running smoothly even under high demands.
- Regularly check CPU usage in top and RAM usage in free to monitor performance, identify issues, and understand growth trends
- Enable long-term monitoring, alerting, and graphing with tools like Prometheus, Datadog or Nagios
- Optimize resources by switching instance type, adding cores/RAM, reducing footprint, load balancing, fixing leaks
- Set utilization alerts at 70%+ sustained usage as indication more resources needed
Frequently Asked Questions
Q: What is a good RAM usage % for Linux VM?
A: 70% sustained RAM usage is a good threshold to aim for. Greater than 70% on a sustained basis indicates potential memory shortage needing optimization or more resources. Monitor MemAvailable and swap usage trends.
Q: How do I monitor CPU usage in Linux command line?
A: The top command provides realtime CPU usage monitoring from the Linux command line. Use top, top -d 5, and top -n 1 to get an interactive view, refreshed view, and one time output.
Q: What happens when RAM is full on Linux?
A: When RAM fills up, Linux will start aggressively swapping less used memory pages from RAM to disk swap space. This leads to severe performance impacts. Optimizing memory usage or adding more RAM is recommended before it reaches 100% usage.
Q: What is a good tool for monitoring Linux server performance?
A: Good open source tools include Prometheus and Grafana for metrics monitoring, graphing, and alerting. Netdata provides lightweight health monitoring and troubleshooting. Nagios offers enterprise alerting capabilities. Datadog and New Relic provide full-featured SaaS performance monitoring.
Q: Where are Linux performance metrics stored?
A: Key Linux performance metrics including CPU, memory, disk, and network usage statistics are stored in the /proc virtual file system. Many monitoring tools gather data from /proc to monitor resource usage.
Q: What is a good CPU usage percentage for Linux?
A: If the CPU usage percentage consistently exceeds 70% utilization on a sustained basis, it likely indicates the CPU cores are insufficient for the current workloads. Adding cores or optimizing processes can help for sustained heavy loads.
Q: How do I monitor CPU usage per process?
A: The ‘top’ command shows CPU usage per process and can be sorted to identify processes using the most CPU resources. The ‘ps’ command can also be used with flags to show CPU usage per process and thread.
Q: Can you upgrade RAM on Linux VM?
A: Yes, most cloud virtual machine instances allow upgrading the VM to a machine type with more RAM through vertical scaling. Resizing options depend on the cloud provider. For on-prem VMs, RAM upgrades may require physical hardware change.
Q: What happens if a Linux server runs out of disk space?
A: Running out of disk space can cause critical system failures as disk writes will start failing. Monitoring free disk space and setting alerts is important. Adding additional storage volumes can help address insufficient disk capacity.
Q: What tool provides real time Linux performance monitoring?
A: Tools like ‘top’, ‘vmstat’, ‘mpstat’, and ‘iostat’ allow real time monitoring by providing interactive output and refresh rates on critical resource usage statistics from CPU, memory, disk and network.
Q: Where can I find documentation on Linux performance monitoring?
A: The Linux Documentation Project (TLDP) has excellent guides on understanding and monitoring various resource metrics including CPU, memory, disk, networking, and overall system performance. The Tuning and Optimization section provides in-depth documentation.