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Unix: ps vs top

post #1 of 11
Thread Starter 
I need to monitor CPU usage over the course of the day.

What is the difference between top and ps when looking at the %CPU values per process? I realize one is a snapshot while the other is real-time.
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post #2 of 11
you can use PS to get more detailed information about a single process/thread, where top is more of a "summary" of what is running and the resources it is using.
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post #3 of 11
PS will give you more details per process.
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post #4 of 11
Thread Starter 
Sorry, I was not clear. I understand what the two commands do. However, the %CPU values are different on the same process and I want to understand why.
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post #5 of 11
ps can give you a lot more detail on a process then top, actual cmd run, executable location, etc. There are plenty of switches in the ps command to find out anything you need to about a process.
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post #6 of 11
Quote:
Originally Posted by DuckieHo;13022789 
Sorry, I was not clear. I understand what the two commands do. However, the %CPU values are different on the same process and I want to understand why.

i would imagine the appropriate analogy here would be a picture taken of an event vs a video of the event....

since "PS" is giving you a "glimpse" of what was running and going on at that particular second, i don't imagine it would give you the same readings unless you were looking at top at the exact Ms that "PS" took its "snapshot".

i guess it is in how you would use the two programs, that separates their "meanings", i use top to get an overall idea what is using the most resources (i think its default is the "top" 20 process) where PS you can get a complete of all that is running...sorry i can't be of more help, as i use these two programs sparingly, and never in a "critical" way.
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post #7 of 11
Thread Starter 
Quote:
Originally Posted by transhour;13022974 
i would imagine the appropriate analogy here would be a picture taken of an event vs a video of the event....

since "PS" is giving you a "glimpse" of what was running and going on at that particular second, i don't imagine it would give you the same readings unless you were looking at top at the exact Ms that "PS" took its "snapshot".

i guess it is in how you would use the two programs, that separates their "meanings", i use top to get an overall idea what is using the most resources (i think its default is the "top" 20 process) where PS you can get a complete of all that is running...sorry i can't be of more help, as i use these two programs sparingly, and never in a "critical" way.

We can have processes that will utilize 100% of the CPUs. During this time, will summation of the %CPU in 100%? I want to understand the difference in how the values are derived just in case.

I know there are better monitoring tools but we do not have root/admin access and our SA are not very good (to say the least).

Code:
ps -A -o pcpu,pmem,rss,vsz,pid,args | sort -k 2 | nawk -v cpu_pct=$cpu_pct -v mem_pct=$mem_pct '$1 > cpu_pct || $2 > mem_pct' \
| nawk -v rundate=$rundate -v runtime=$runtime -v run_cnt=$run_cnt '{print run_cnt"|"rundate"|"runtime"|"$1"|"$2"|"$3"|"$4"|"$5"|"$6" "$7" "$8" "$9" "$10}' \
| sed -e '/|%CPU|%MEM|RSS|VSZ|PID|COMMAND/d' -e '/<defunct>/d' -e'/<exiting>/d' >> $PERFFILE || exit_sh 4
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post #8 of 11
Quote:
Originally Posted by DuckieHo;13023019 
We can have processes that will utilize 100% of the CPUs. During this time, will summation of the %CPU in 100%? I want to understand the difference in how the values are derived just in case.

I know there are better monitoring tools but we do not have root/admin access and our SA are not very good (to say the least).

this is taken from the PS manpage:
Quote:
CPU usage is currently expressed as the percentage of time spent running during the entire lifetime of a process. This is not ideal, and it does not conform to the standards that ps otherwise conforms to. CPU usage is unlikely to add up to exactly 100%.

and from the TOP manpage:
Quote:
The task's share of the elapsed CPU time since the last screen update, expressed as a percentage of total CPU time. In a true SMP environment, if 'Irix mode' is Off, top will operate in 'Solaris mode' where a task's cpu usage will be divided by the total number of CPUs. You toggle 'Irix/Solaris' modes with the 'I' interactive command.

further reading indicates that TOP is more accurate in determining the cpu usage than PS is.

it sucks to have morons for system admins, honestly if i was you duckie, i would go create an account over at linuxquestions.org, hit up the slackware or your specific OS distro section, and feel up those guys for information, they are a treasure trove most of the time smile.gif we got knowledgeable guys here about the stuff, but most there, live and breath this sutff smile.gif
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post #9 of 11
Thread Starter 
Quote:
Originally Posted by transhour;13023209 
CPU usage is currently expressed as the percentage of time spent running during the entire lifetime of a process. This is not ideal, and it does not conform to the standards that ps otherwise conforms to. CPU usage is unlikely to add up to exactly 100%. CPU usage is currently expressed as the percentage of time spent running during the entire lifetime of a process. This is not ideal, and it does not conform to the standards that ps otherwise conforms to. CPU usage is unlikely to add up to exactly 100%. CPU usage is currently expressed as the percentage of time spent running during the entire lifetime of a process. This is not ideal, and it does not conform to the standards that ps otherwise conforms to. CPU usage is unlikely to add up to exactly 100%.

I read this too.... but I am not sure if it is true. If it is, what is the polling rate and how is the history accessed?

What's a quick algorithm that will use cause load a CPU? A quick test would be to write a script to load CPU for 1min and then sleep for 1min.

I could do a PS of that and see what the %CPU at different intervals.
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post #10 of 11
i actually don't know how to do this tbh with you, well not an easy way to do this.

if i was to do something like this i would grab something like mprime (the linux version to prime95), extract it into your home folder, run it then terminate it manually...i did find this program:

http://weather.ou.edu/~apw/projects/stress/

you do have to compile it, but it seems to do what you want it to do, including having a "timeout" feature.
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