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Tips for top: Monitoring CPU load on Linux
Tips for top: Monitoring CPU load on Linux
Get a real-time view of the processes running on Linux to fix slowdowns and other problems.
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This article is excerpted from chapter 13 of the book Linux in Action, published by Manning.
Has the performance of your Linux machine been erratic or unusually slow? Do you suspect that growing demand might be outstripping your available resources? Here are some questions you should be asking yourself:
- How close are you to exhausting your CPU and memory resources?
- Is there something running unnecessarily that could be shut down?
- Is there something that's been running rogue without your knowledge?
The figure below shows a typical screen of
top data. The first line provides the current time, the elapsed time since the most recent system boot, the number of users currently logged in, and load averages for the last minute, five minutes, and 15 minutes. This information can also be returned by running
Since we're trying to resolve performance problems, the columns of data that should interest us the most are
%CPU (percentage of CPU capacity currently used by a given process) and
%MEM (percentage of memory capacity). You'll especially want to note the processes showing up at the top of the list.
In this case, you can see that the MySQL daemon is using 4.3% of the server's CPU and, from the next column, 13% of its memory. If you follow that row over to the left, you'll see that the process ID (PID) is
1367 and the process is "owned" by the
Perhaps you'll conclude that this process was taking more resources than can be justified and will have to be sacrificed (for the greater good, you understand). That
top display gave you everything you'll need to kill it. Since MySQL is a service managed by systemd (on those distros using systemd), your first choice should be to use
systemctl to bring the process down gently without putting any application data at risk.
systemctl stop mysqld
If the process you want to shut down is not managed by systemd, or if something's gone wrong and
systemctl failed to stop it, then you can use either
killall to eliminate your process. Some systems require you to install
killall as part of the
psmisc package. You pass the PID to
kill this way:
killall, on the other hand, uses the process name rather than its ID.
To kill or to killall, that is the question. Actually, the answer is kind of obvious.
kill will shut down a single process, based as it is on the PID, while
killall will kill as many instances of a particular program as are running. So if there were two or three separate MySQL instances—perhaps belonging to separate users—all would be stopped. Before launching
killall, make sure there aren't any similarly named processes you still want running that could become "collateral damage."
Of course, you'll also have to run
systemctl disable to make sure the process doesn't restart the next time you boot.
systemctl disable mysqld
In case you ever need them, the third line of
top output you saw a bit earlier gives us time values (as percentages) for a number of other CPU metrics. Here's a quick rundown of the jumble of acronyms you'll see there:
||Time running high-priority (un-niced) processes|
||Time running kernel processes|
||Time running low-priority (nice) processes|
||Time spent idling|
||Time waiting for I/O events to complete|
||Time spent managing hardware interrupts|
||Time spent managing software interrupts|
||Time stolen from this VM by its hypervisor (host)|
top display can be customized in real time through keyboard input. Type
h to learn more.
Making trouble (simulating CPU load)
Dying to see
top in action but, wouldn't you know it, everything is running smoothly?
Why not simulate crisis-level CPU overload? Much like children,
yes will output (digital) noise continuously until told to stop. On second thoughts, that's not at all like children.
This command will redirect that noise to the disposable
/dev/null file and the
& character will push the process into the background, giving control of the command line back to you. To ramp up the pressure, launch the command a few more times.
$ yes > /dev/null &
That should keep 'em busy. While all that's running, watch
top to see what's happening. You could also try to run other applications to see how much it will take to slow them down. When you're done, run
killall to knock off all your
yes sessions in one go.
$ killall yes