Linux CPU utilization

From Rosetta Code
Linux CPU utilization is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
Task

Display the current CPU utilization, as a percentage, calculated from /proc/stat.


Background

Most Linux kernels provide a virtual   /proc   filesystem, providing an interface to various internal data structures.  

One of these internal structures (/proc/stat) includes information on the amount of time (in USER_HZ) spent in various states.   From this information, we can, with a little effort, determine the current level of CPU utilization, as a percent of time spent in any states other than idle.

To do this:

  1.   read the first line of   /proc/stat
  2.   discard the first word of that first line   (it's always cpu)
  3.   sum all of the times found on that first line to get the total time
  4.   divide the fourth column ("idle") by the total time, to get the fraction of time spent being idle
  5.   subtract the previous fraction from 1.0 to get the time spent being   not   idle
  6.   multiple by   100   to get a percentage


The times in /proc/stat are monotonically increasing, and begin at some point during the kernel's initialization (ie during boot up).   So the steps above will give the total CPU utilization since boot, which may or may not be useful to some.   To get a more real-time utilization, we simply repeat the steps above with some small sleep interval in between, and instead of using the absolute total and idle times, we use use the total time delta and the idle time delta to compute the utilization.

This project is based on   this blog post   by Paul Colby,   and the   Bash   version comes from there.

C++[edit]

#include <fstream>
#include <iostream>
#include <numeric>
#include <unistd.h>
#include <vector>
 
std::vector<size_t> get_cpu_times() {
std::ifstream proc_stat("/proc/stat");
proc_stat.ignore(5, ' '); // Skip the 'cpu' prefix.
std::vector<size_t> times;
for (size_t time; proc_stat >> time; times.push_back(time));
return times;
}
 
bool get_cpu_times(size_t &idle_time, size_t &total_time) {
const std::vector<size_t> cpu_times = get_cpu_times();
if (cpu_times.size() < 4)
return false;
idle_time = cpu_times[3];
total_time = std::accumulate(cpu_times.begin(), cpu_times.end(), 0);
return true;
}
 
int main(int, char *[]) {
size_t previous_idle_time=0, previous_total_time=0;
for (size_t idle_time, total_time; get_cpu_times(idle_time, total_time); sleep(1)) {
const float idle_time_delta = idle_time - previous_idle_time;
const float total_time_delta = total_time - previous_total_time;
const float utilization = 100.0 * (1.0 - idle_time_delta / total_time_delta);
std::cout << utilization << '%' << std::endl;
previous_idle_time = idle_time;
previous_total_time = total_time;
}
}
Output:
5.77731%
2.24439%
1.50754%
1.50754%
2.50627%

Fortran[edit]

 
Program CPUusage
implicit none
integer :: ios, i
integer :: oldidle, oldsum, sumtimes = 0
real :: percent = 0.
character(len = 4) lineID ! 'cpu '
integer, dimension(9) :: times = 0
 
write(*, *) 'CPU Usage'
write(*, *) 'Press Ctrl<C> to end'
do while (.true.)
open(unit = 7, file = '/proc/stat', status = 'old', action = 'read', iostat = ios)
if (ios /= 0) then
print *, 'Error opening /proc/stat'
stop
else
read(unit = 7, fmt = *, iostat = ios) lineID, (times(i), i = 1, 9)
close(7)
if (lineID /= 'cpu ') then
print *, 'Error reading /proc/stat'
stop
end if
sumtimes = sum(times)
percent = (1. - real((times(4) - oldidle)) / real((sumtimes - oldsum))) * 100.
write(*, fmt = '(F6.2,A2)') percent, '%'
oldidle = times(4)
oldsum = sumtimes
call sleep(1)
end if
end do
end program CPUusage
 
Output:
CPU Usage
 Press Ctrl<C> to end
  7.51 %
 18.23 %
  4.60 %
  4.53 %
  3.53 %
  2.53 %

Haskell[edit]

import Data.List ( (!!) ) 
 
splitString :: Char -> String -> [String]
splitString c [] = []
splitString c s = let ( item , rest ) = break ( == c ) s
( _ , next ) = break ( /= c ) rest
in item : splitString c next
 
computeUsage :: String -> Double
computeUsage s = (1.0 - ((lineElements !! 3 ) / sum times)) * 100
where
lineElements = map (fromInteger . read ) $ tail $ splitString ' ' s
times = tail lineElements
 
main :: IO ( )
main = do
theTimes <- fmap lines $ readFile "/proc/stat"
putStr $ show $ computeUsage $ head theTimes
putStrLn " %"
Output:
1.6090321637810434 %


Icon and Unicon[edit]

#
# utilization.icn, percentage of running cpu time used versus idle time
#
# tectonics: unicon -s utilization.icn -x
#
procedure main()
local stats, n, total, newtotal, delta, idle, idled
stats := gather()
idle := stats[4]
total := newtotal := 0
every n := !stats do total +:= n
write("Overall utilization: ", (1 - (real(idle) / total)) * 100, "%")
 
idle := stats[4]
delay(100)
 
stats := gather()
every n := !stats do newtotal +:= n
delta := newtotal - total
idled := stats[4] - idle
write("Interval utilization: ", (1 - (real(idled) / delta)) * 100, "%")
end
 
procedure gather()
local f, line, stats
 
f := open("/proc/stat", "r") | stop("Cannot open /proc/stat")
line := read(f)
close(f)
 
stats := []
line ? {
tab(upto(' ')) & tab(many(' '))
while put(stats, tab(upto(' '))) do tab(many(' '))
put(stats, tab(0))
}
return stats
end
Output:
prompt$ unicon -s utilization.icn -x
Overall utilization:  4.995469613984859%
Interval utilization: 10.52631578947368%
prompt$ ./utilization
Overall utilization:  4.994924759070784%
Interval utilization: 0.0%
prompt$ ./utilization
Overall utilization:  4.994135168922565%
Interval utilization: 4.878048780487809%


J[edit]

cputpct=:3 :0
if. 0>nc<'PREVCPUTPCT' do. PREVCPUTPCT=:0 end.
old=. PREVCPUTPCT
PREVCPUTPCT=:0 { 0.1,~0&".;._2 fread '/proc/stat'
100*1-(4&{ % +/) PREVCPUTPCT - old
)

Example use:

   cputpct''
1.76237

Notes: this gives the average non-idle time since the last time this verb was used. If for some reason /proc/stat were not updated between calls, the result would be 100 (percent), which seems appropriate.

Kotlin[edit]

// version 1.1.3
 
import java.io.FileReader
import java.io.BufferedReader
 
fun main(args: Array<String>) {
println("CPU usage % at 1 second intervals:\n")
var prevIdleTime = 0L
var prevTotalTime = 0L
repeat(10) {
val br = BufferedReader(FileReader("/proc/stat"))
val firstLine = br.readLine().drop(5) // get rid of cpu plus 2 spaces
br.close()
val split = firstLine.split(' ')
val idleTime = split[3].toLong()
val totalTime = split.map { it.toLong() }.sum()
if (it > 0) {
val deltaIdleTime = idleTime - prevIdleTime
val deltaTotalTime = totalTime - prevTotalTime
val cpuUsage = (1.0 - deltaIdleTime.toDouble() / deltaTotalTime) * 100.0
println("$it : ${"%6.3f".format(cpuUsage)}")
}
prevIdleTime = idleTime
prevTotalTime = totalTime
Thread.sleep(1000)
}
}

Sample output:

CPU usage % at 1 second intervals:

1 : 13.043
2 :  8.491
3 :  4.478
4 :  3.553
5 :  2.500
6 :  4.975
7 :  1.015
8 :  4.040
9 :  2.525

Perl 6[edit]

my $last-total = 0;
my $last-idle = 0;
 
loop {
my-total = (my $this-total = [+] my @cpu = "/proc/stat".IO.lines[0].words[1..*]) - $last-total;
my-idle = (my $this-idle = @cpu[3]) - $last-idle;
$last-total = $this-total;
$last-idle = $this-idle;
print "\b" x 40, (100 * (1 --idle /-total)).fmt("Utilization: %0.1f%% ");
sleep(1);
}

Python[edit]

from __future__ import print_function
from time import sleep
 
 
last_idle = last_total = 0
while True:
with open('/proc/stat') as f:
fields = [float(column) for column in f.readline().strip().split()[1:]]
idle, total = fields[3], sum(fields)
idle_delta, total_delta = idle - last_idle, total - last_total
last_idle, last_total = idle, total
utilisation = 100.0 * (1.0 - idle_delta / total_delta)
print('%5.1f%%' % utilisation, end='\r')
sleep(5)
Output:

Lines end in \r which causes old values to be overwritten by new when \r is supported otherwise successive figures appear on separate lines.

 26.5%
 12.4%
 10.6%
 49.5%
 15.5%
 13.8%
  8.3%
 11.0%
 18.5%
 13.9%
 11.8%
 35.6%

Racket[edit]

#lang racket/base
 
(require racket/string)
 
(define (get-stats) ; returns total and idle times as two values
(define line (call-with-input-file* "/proc/stat" read-line))
(define numbers (map string->number (cdr (string-split line))))
(values (apply + numbers) (list-ref numbers 3)))
 
(define prev-stats #f)
 
(define (report-cpu-utilization)
 ;; lazy: fixed string instead of keeping the last time
(define prompt (if prev-stats "last second" "since boot"))
(define-values [cur-total cur-idle] (get-stats))
(define prev (or prev-stats '(0 0)))
(set! prev-stats (list cur-total cur-idle))
(define total (- cur-total (car prev)))
(define idle (- cur-idle (cadr prev)))
(printf "Utilization (~a): ~a%\n" prompt
(/ (round (* 10000 (- 1 (/ idle total)))) 100.0)))
 
(let loop ()
(report-cpu-utilization)
(sleep 1)
(loop))
 

Tcl[edit]

This is coded a little bit awkwardly in order to correspond neatly to the task description.

proc stat fulfills the task description in two ways:

  • if called normally, it returns aggregate CPU utilization since boot
  • if called within a coroutine, it prints on stdout and repeats every 1s until the coroutine is terminated

A more reusable implementation might take a command as an argument to stat, and allow that command to interrupt the loop with return false or even break.

# forgive the pun!
proc stat {} {
 
set fd [open /proc/stat r]
set prevtotal 0
set previdle 0
 
while {1} {
# read the first line of /proc/stat
set line [gets $fd]
seek $fd 0
# discard the first word of that first line (it's always cpu)
set line [lrange $line 1 end]
# sum all of the times found on that first line to get the total time
set total [::tcl::mathop::+ {*}$line]
 
# parse each field out of line (we only need $idle)
lassign $line user nice system idle iowait irq softirq steal guest guest_nice
 
# update against previous measurement
incr idle -$previdle
incr total -$prevtotal
incr previdle $idle
incr prevtotal $total
 
# divide the fourth column ("idle") by the total time, to get the fraction of time spent being idle
set frac [expr {$idle * 1.0 / $total}]
# subtract the previous fraction from 1.0 to get the time spent being not idle
set frac [expr {1 - $frac}]
# multiply by 100 to get a percentage
set frac [expr {round($frac*100)}]
 
if {[info coroutine] eq ""} {
return $frac ;# if we're called outside a coroutine, return a number
} else {
puts [format CPU:%3d%% $frac] ;# else print output
yieldto after 1000 [info coroutine]
}
}
}
 
coroutine watchstat stat
Output:
CPU: 10%
CPU:  3%
CPU:  1%

UNIX Shell[edit]

Works with: bash

Example taken, verbatim, from Github.

#!/bin/bash
# by Paul Colby (http://colby.id.au), no rights reserved ;)
 
PREV_TOTAL=0
PREV_IDLE=0
 
while true; do
# Get the total CPU statistics, discarding the 'cpu ' prefix.
CPU=(`sed -n 's/^cpu\s//p' /proc/stat`)
IDLE=${CPU[3]} # Just the idle CPU time.
 
# Calculate the total CPU time.
TOTAL=0
for VALUE in "${CPU[@]}"; do
let "TOTAL=$TOTAL+$VALUE"
done
 
# Calculate the CPU usage since we last checked.
let "DIFF_IDLE=$IDLE-$PREV_IDLE"
let "DIFF_TOTAL=$TOTAL-$PREV_TOTAL"
let "DIFF_USAGE=(1000*($DIFF_TOTAL-$DIFF_IDLE)/$DIFF_TOTAL+5)/10"
echo -en "\rCPU: $DIFF_USAGE% \b\b"
 
# Remember the total and idle CPU times for the next check.
PREV_TOTAL="$TOTAL"
PREV_IDLE="$IDLE"
 
# Wait before checking again.
sleep 1
done
Output:
Each successive output overwrites the previous output, so there is only ever one line, but that line keeps updating in-place.
CPU: 1%

Ursa[edit]

#
# linux cpu utilization
#
 
# define variables to hold last idle and last total
decl int last_idle last_total
 
# loop indefintely
while true
# read the first line from /proc/stat
decl string line
decl file f
f.open "/proc/stat"
set line (in string f)
 
# get the data from the first line
decl int i
decl int<> fields
fields.clear
for (set i 2) (< i (size (split (trim line) " "))) (inc i)
append (int (split (trim line) " ")<i>) fields
end for
 
# get idle stat and total
decl int idle total
set idle fields<3>
set total (int (+ fields))
 
# set idle_delta and total_delta
decl int idle_delta total_delta
set idle_delta (int (- idle last_idle))
set total_delta (int (- total last_total))
 
# set last_idle and last_total
set last_idle idle
set last_total total
 
# set percentage utilization
decl double utilization
set utilization (* 100 (- 1 (/ idle_delta total_delta)))
 
# output percentage
out utilization "%\r" console
 
# close f, it will be reopened at the top of the loop
f.close
 
# sleep 2.5 seconds
sleep 2500
 
# clear the line of output
for (set i 0) (< i (+ (size (string utilization)) 1.0)) (inc i)
out "\b" console
end for
for (set i 0) (< i (+ (size (string utilization)) 1.0)) (inc i)
out " " console
end for
out "\r" console
end while

zkl[edit]

Translation of: Python
last_idle,last_total:=0,0;
while(True){
f:=File("/proc/stat"); // open each time to get fresh data
fields:=f.readln().strip().split()[1,*];
f.close();
idle, total := fields[3].toFloat(),fields.sum(0.0);
idle_delta, total_delta := idle - last_idle, total - last_total;
last_idle, last_total = idle, total;
utilisation := 100.0 * (1.0 - idle_delta / total_delta);
print("%5.1f%%\r".fmt(utilisation)); // stay on this line, overwrite old value
Atomic.sleep(5);
}
Output:

Shown on multiple lines, in xterm output stays on one line, overwriting itself.

 33.3%
  1.2%
  1.5%
  1.1%
  1.6%
  1.1%
  1.8%
^CCntl C noted