## Processor time and memory usage: `top`, `htop` etc
We can use [ps](/Programming_Languages/Shell_Scripting/Processes.md) to list the currently running processes but it does not provide much information about the resource metrics or how the process changes over time. We can use `top` to get more information.
`top` provides an interactive interface for the information that `ps` displays. It updates in real time and shows the most active processes based on the CPU time that they are utilising. You can also order by memory usage.
_Here I have pressed `u` to show only the processes associated with my user:_
### Main commands
| Command | Action |
| ------- | ------------------------------- |
| -u | Show processes by selected user |
| M | Sort by memory usage |
| P | Sort by cumulative CPU usage |
| ? | View key and explanation |
## Files being used by active processes: `lsof`
`lsof` stands for _list open files_. It lists opened files and the processes using them. Without modifiers it outputs a huge amount of data. The best way to use it is to execute it against a specific PID. For example the below output gives me some useful info about which files VS Code is using:
## System calls: `strace`
A system call is when a process requests a service from the [kernel](/Operating_Systems/The_Kernel.md), for instance an I/O operation to memory. We can trace these system calls with `strace`.
We can use the `uptime` program to assess overall CPU performance in the form of a load average.
> Load average is the number of active processes currently ready to run. It is an estimate of the number of processes that are capable of using the CPU at any given time.
- The three numbers are load averages for the past 1 minute, 5 minutes and 15 minutes respectively.
- A load average close to 0 is usually a good sign because it means that your processor isn't being challenged and you are conserving power. Anything equal to or above 1 means that a single process is using the CPU nearly all the time. You can identify that process with `htop` and it will obviously be near to the top. (This is often caused by Chrome and Electron-based software.)
