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What's Bogging Down Your Linux PC? Tracking Down Resource Hogs

top Has Many Useful Options

  • July 21, 2009
  • By Akkana Peck

Hardware is getting amazingly fast. But sometimes it seems like software gets slower faster than hardware speeds up. So why do so many apps feel poky?

In this series, I'll look at some of the ways you can examine your system to see what's taking up resources, and offer some tips on slimming your system down so it runs faster.

I'll start with two of the basic command-line applications for measuring performance: top and ps.

top

When you suspect something is running away with system resources, top should be your first recourse. It gives you a screen that updates every couple of seconds (Figure 1), showing you a list of the processes that are hogging the most CPU.

figure 1
figure 1

The fields, in order, are:

PID
process ID -- you can use this for killing the process or finding out more about it
USER
the user who started the process, most often either you or root
PR
The priority of the process. This usually doesn't mean much on Linux
NI
How "nice" the process is -- whether it will step aside when other processes need to do some work. Most normal programs will be at zero; a positive number up to 20 means the process will give way to other processes, while a negative number means an important process (often part of the kernel) which will take priority over normal programs.
VIRT
The process's virtual memory size. This is virtually meaningless -- I'll write more about memory in a future article.
RES
The amount of memory the process is currently using in memory. Again, there are caveats and this may not mean quite what you think it does.
SHR
Shared memory: how many shared libraries the process is using.
S
A single letter code representing the process's status. R means it's actively running. S means sleeping, but you'll also see it for lots of running processes, if top happens to check in between updates. T means it's temporarily stopped (you can stop a process by typing Control-Z in the terminal where you started it). D means it's in uninterruptible sleep, perhaps because it's waiting for a device to respond. Finally, Z means it's a zombie process: one that has exited but its parent process isn't paying attention any more. You can't get rid of zombie processes, but they don't take up any significant system resources.
%CPU
What percentage of the CPU the process is currently taking
%MEM
What percentage of memory the process is using
TIME+
How much CPU time the process has accumulated, in hundredths of a second.
COMMAND
The name of the process

Top has some useful options, too. For instance, if you want to run top just once and save its output into a file, you can do it like this:

top -b -n 1
When you're running top interactively, you can change the way it sorts: type M to sort by memory use rather than CPU, T to sort by total time, or P to go back to sorting by CPU. Those letters are all capitalized.
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