I'm using tar to backup my server once every day. Even though CPU usage (of the tar command and the system overall) doesn't go over 10%, my system load is like 3.98 3.71 3.82 when the script is running. (on a 2 core 4 thread system)

Why does tar show so little CPU in practice, while producing such a high system load in the meanwhile. And, most importantly, should I be worried?

I don't use compression, just a pure tarball. I compress at the start of each new month.

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    Without more data it's hard to say with certainty but it's probably I/O load rather than CPU load. – DopeGhoti Oct 4 '18 at 19:49
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    It indeed turns out that system load includes the I/O (wait). I was not aware of this, but now I know. Thank you for your contribution. – MicroParsec Oct 4 '18 at 19:53

You see a high system load because tar spends a lot of time waiting for I/O. You see a low CPU usage because tar uses very little CPU time: it's mostly just copying some bytes when the disk delivers them. Linux includes time waiting for I/O in the load average (unlike many other Unix variants), but not in a process's CPU time. (Source: https://linuxtechsupport.blogspot.com/2008/10/what-exactly-is-load-average.html via Wikipedia)

There's nothing to be worried about. You asked the computer to do an I/O-bound operation and it's busy doing some I/O. Business as expected.

  • That explains a lot actually. I should not be worried then, right? The computer isn't warming up even, so it just seemed a bit fishy to be honest. I didn't know that I/O wait was included in system load, so that clears up a lot. Thank you very much. – MicroParsec Oct 4 '18 at 19:51
  • If the I/O times start increasing for unexplained reasons though, you may want to check dmesg to make sure that there aren't any disk problems. A headless server that I was monitoring had a failing drive and started getting slow because of retries. – ErikF Oct 4 '18 at 22:35

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