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andrewwalker27
l33t
Joined: 27 Jun 2005
Posts: 660
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 Posted: Sun Apr 01, 2018 6:03 pm Post subject: Trouble reading CPU load |
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I'm running a minecraft server on a Raspberry Pi with a small oled screen which reads out memory, disk space, ip address and CPU load but the CPU load seems wrong.
The line of code that displays it is as follows
| Code: | cmd = "top -bn1 | grep load | awk '{printf \"CPU Load: %.2f\", $(NF-2)}'"
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If I run top via a terminal with ssh the actual cpu load is nearer 10 - 15% but the oled says 1.04%. I suspect top is correct but, as the code isn't mine and I don't know anything about awk, I can't work out what is going wrong.
I'm running a Pi 3 which I think has 4 cores, can anyone give me any pointers?
EDIT:
Just realised it is reading load average, what I want it to display is CPU load, any ideas? |
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mike155
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Joined: 17 Sep 2010
Posts: 4438
Location: Frankfurt, Germany
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| Posted: Sun Apr 01, 2018 6:50 pm Post subject: |
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| Quote: | the actual cpu load is nearer 10 - 15% but the oled says 1.04%.
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Please look at the output of 'top':
- The 'load average' is shown in the first line and it's given as an absolute number, not in percent. The 'load average' has a particular meaning and it's probably not what you think it is. See https://en.wikipedia.org/wiki/Load_(computing).
- The %CPU value (which is shown in percent) is a task's share of the elapsed CPU time since the last screen update, expressed as a percentage of total CPU time.
Please don't confuse the two values.
This code (you said it isn't yours, right?)
| Code: | top -bn1 | grep load | awk '{printf "CPU Load: %.2f", $(NF-2)}'
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is probably the most stupid code I've seen for a long time - and it really makes me angry! Everything is wrong:
- There's no need to call 'top' if you can read the value from /proc/loadavg.
- 'grep load' will return more that one line if a process runs whose name contains 'load'.
- If you start somewhat heavy programs like top or awk, it surely will affect load and %CPU values.
- The code is very fragile. As soon as the output format of 'top' changes, the code won't work any longer.
It would be better to use:
| Code: | echo "CPU Load: $( cut -d ' ' -f 1 /proc/loadavg )"
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Last edited by mike155 on Sun Apr 01, 2018 7:09 pm; edited 1 time in total |
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Ant P.
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Joined: 18 Apr 2009
Posts: 6920
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| Posted: Sun Apr 01, 2018 7:09 pm Post subject: |
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I'd do something like this instead:
| Code: | atop -P CPU 5 | awk -n '/^CPU/ { print $9 / $6; fflush() }' | while read pct_cpu; do
printf "CPU Load: %.1f" $pct_cpu > /tmp/cpuavg
done |
And then cat /tmp/cpuavg in the actual command. Better yet would be to let this loop drive the updates, but you probably can't do that. |
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mike155
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Joined: 17 Sep 2010
Posts: 4438
Location: Frankfurt, Germany
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| Posted: Sun Apr 01, 2018 7:32 pm Post subject: |
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Ant P.:
I like your code. It's a nice solution!
Minor nits:- On my machine, it works only if I use '$10 / $6', instead of '$9 / $6'.
- Please don't call it 'CPU Load' - it's not a CPU load. 'CPU Usage' would be better.
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Ant P.
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Joined: 18 Apr 2009
Posts: 6920
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| Posted: Sun Apr 01, 2018 7:49 pm Post subject: |
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| I probably should've read the manpage more closely. Make that ($9 + $10) / $6... |
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andrewwalker27
l33t
Joined: 27 Jun 2005
Posts: 660
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| Posted: Sun Apr 01, 2018 7:59 pm Post subject: |
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Thanks for the info, the code I used was just some demo code from adafruit for their oled library but it seemed to work so I'm not going to criticise it!
Just in case anyone is interested, I was building this
https://www.instructables.com/id/The-OreServer-a-Raspberry-Pi-Dedicated-Minecraft-S/
and was having a few lag issues. My idea was to put a small oled 128 x 32 pixel display on it to monitor performance, the load average was slightly over 1 so I wanted to see if an individual core was being maxed out.
Not strictly Gentoo I know but I was ssh'ing in from my Gentoo box!
I'll give your code a go but otherwise I may just try out the Pi 3+ in case it has that slight performance increase to run it smoothly.
Maybe I could try running Gentoo instead of Raspbian on the PI, now there's a thought......
Thanks again. |
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khayyam
Watchman
Joined: 07 Jun 2012
Posts: 6227
Location: Room 101
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| Posted: Sun Apr 01, 2018 11:45 pm Post subject: |
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| mike155 wrote: | | Code: | | echo "CPU Load: $( cut -d ' ' -f 1 /proc/loadavg )" |
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mike155, et al ... it's unlikely the OP has zsh but perhaps of interest to others:
| Code: | % echo "CPU Usage:" ${$(</proc/loadavg)[1]}
CPU Usage: 0.04 |
best ... khay |
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