This is linked to this question.
When I run top I get the following result:
pid 3038 is using 18% cpu, however when running
the result is 5.5%. And this number does not appear to be changing with time (i.e. when running the same command a bit later)…
Is the ps command somehow averaging the cpu usage?
Answers:
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Method 1
man ps in NOTES section.
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, guess you know, but you can also do:
top -p <PID>
Edit: as to your comment on other answer;
“Hmm yeah i Wonder how to get that (the instant CPU percentage) from ps“
Short answer: you can’t.
Why is it so?
It is like asking someone to calculate the speed of a car from a picture.
While top is a monitoring tool, ps is a snapshot tool. Think of it like this: At any given moment a process either uses the CPU or not. Thus you have either 0% or 100% load in that exact moment.
Giving: If ps should give instant CPU usage it would be either 0% or 100%.
top on the other hand keep polling numbers and calculate load over time.
ps could have given current usage – but that would require it to read data multiple times and sleep between each read. It doesn’t.
Calculation for ps %cpu
ps calculates CPU usage in the following manner:
uptime = total time system has been running. ps_time = process start time measured in seconds from boot. pu_time = total time process has been using the CPU. ;; Seconds process has been running: seconds = uptime - ps_time ;; Usage: cpu_usage = pu_time * 1000 / seconds print: cpu_usage / 10 "." cpu_usage % 10
Example: uptime = 344,545 ps_time = 322,462 pu_time = 3,383 seconds = 344,545 - 322,462 = 22,083 cpu_usage = 3,383 * 1,000 / 22,083 = 153 print: 153 / 10 "." 153 % 10 => 15.3
So the number printed is: time the process has been using the CPU during it’s lifetime. As in the example above. It has done so in 15.3% of its lifetime. In 84,7% of the time it has not been bugging on the CPU.
Data retrieval
ps, as well as top, uses data from files stored under /proc/ – or the process information pseudo-file system.
You have some files in root of /proc/ that have various information about the overall state of the system. In addition each process has its own sub folder /proc/<PID>/ where process specific data is stored. So, for example the process from your question had a folder at /proc/3038/.
When ps calculates CPU usage it uses two files:
/proc/uptime The uptime of the system (seconds), and the amount of time spent in idle process (seconds). /proc/[PID]/stat Status information about the process.
- From
uptimeit uses the first value (uptime). - From
[PID]/statit uses the following:
# Name Description 14 utime CPU time spent in user code, measured in jiffies 15 stime CPU time spent in kernel code, measured in jiffies 16 cutime CPU time spent in user code, including time from children 17 cstime CPU time spent in kernel code, including time from children 22 starttime Time when the process started, measured in jiffies
A jiffie is clock tick. So in addition it uses various methods, ie., sysconf(_SC_CLK_TCK), to get system’s Hertz (number of ticks per second) – ultimately using 100 as a fall-back after exhausting other options.
So if utime is 1234 and Hertz is 100 then:
seconds = utime / Hertz = 1234 / 100 = 12.34
The actual calculation is done by:
total_time = utime + stime
IF include_dead_children
total_time = total_time + cutime + cstime
ENDIF
seconds = uptime - starttime / Hertz
pcpu = (total_time * 1000 / Hertz) / seconds
print: "%CPU" pcpu / 10 "." pcpu % 10
Example (Output from a custom Bash script):
$ ./psw2 30894
System information
uptime : 353,512 seconds
idle : 0
Process information
PID : 30894
filename : plugin-containe
utime : 421,951 jiffies 4,219 seconds
stime : 63,334 jiffies 633 seconds
cutime : 0 jiffies 0 seconds
cstime : 1 jiffies 0 seconds
starttime : 32,246,240 jiffies 322,462 seconds
Process run time : 31,050
Process CPU time : 485,286 jiffies 4,852 seconds
CPU usage since birth: 15.6%
Calculating “current” load with ps
This is a (bit?) shady endeavour but OK. Lets have a go.
One could use times provided by ps and calculate CPU usage from this. When thinking about it it could actually be rather useful, with some limitations.
This could be useful to calculate CPU usage over a longer period. I.e. say you want to monitor the average CPU load of plugin-container in Firefox while doing some Firefox-related task.
By using output from:
$ ps -p -o cputime,etimes
CODE HEADER DESCRIPTION cputime TIME cumulative CPU time, "[DD-]hh:mm:ss" format. (alias time). etime ELAPSED elapsed time since the process was started, [DD-]hh:]mm:ss. etimes ELAPSED elapsed time since the process was started, in seconds.
I use etime over etimes in this sample, on calculations, only to be a bit more clear. Also I add %cpu for “fun”. In i.e. a bash script one would obviously use etimes – or better read from /proc/<PID>/ etc.
Start:
$ ps -p 30894 -o %cpu,cputime,etime,etimes
%CPU TIME ELAPSED ELAPSED
5.9 00:13:55 03:53:56 14036
End:
%CPU TIME ELAPSED ELAPSED
6.2 00:14:45 03:56:07 14167
Calculate times:
13 * 60 + 55 = 835 (cputime this far)
3 * 3,600 + 53 * 60 + 56 = 14,036 (time running this far)
14 * 60 + 45 = 885 (cputime at end)
3 * 3,600 + 56 * 60 + 7 = 14,167 (time running at end)
Calculate percent load:
((885 - 835) / (14,167 - 14,036)) * 100 = 38
Process was using the CPU 38% of the time during this period.
Look at the code
If you want to know how ps does it, and know a little C, do (looks like you run Gnome Debain deriavnt) – nice attitude in the code regarding comments etc.:
apt-get source procps cd procps*/ps vim HACKING
Method 2
man top %CPU -- CPU usage 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. man ps %cpu %CPU cpu utilization of the process in "##.#" format. Currently, it is the CPU time used divided by the time the process has been running (cputime/realtime ratio), expressed as a percentage. It will not add up to 100% unless you are lucky. (alias pcpu).
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