Linux analysis of execution time with time

Under Linux, the time command can be used to measure the performance duration for a command line program. This article shows this with updatedb program as an example.

Call

The time command is simply specified before the program to be executed when it is called:

[root@tpw ~]# time updatedb

real	1m5.682s
user	0m0.543s
sys	0m2.823s
[root@tpw ~]# 

The updatedb program is a suitable example. It searches the "whole" file system for all files and writes the information into a small data base. With the help of this database, you can then use the locate command to search for files very quickly later on. This search is more quickly than a normal search with the find command. However, the result does not reflect the current state, but the state of the data base (last call of the updatedb command.

As updatedb searches the entire file system and thus very random I/O accesses occur, the CPU spends most of its time waiting for the I/O when executing updatedb.

Analysis

The single values have the following meaning:

• real: Elapsed real time - the whole actual execution time.
• user: Total number of CPU-seconds that the process spent in user mode - the time during which the CPU actually executes the program's (user) code
• sys: Total number of CPU-seconds that the process spent in kernel mode - the time during which the CPU is busy executing kernel code for the program.

The CPU spends the rest of its time either running other programs or waiting for I/O. This also shows a parallel executed vmstat (see also Linux Performance Measurements using vmstat:

[user@tpw ~]$ vmstat 1 10
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 3  1   3276  17544 109672 869900    0    0    62   180  812 1340 17  6 75  2  0	
 1  1   3276  17000 110144 869900    0    0   464    76  815 1594 17  5  0 78  0	
 2  1   3276  16556 110552 869892    0    0   408    20  670 1415  2  5  0 93  0	
 0  1   3276  15980 111060 869936    0    0   508     0  844 1775  9  6  0 85  0	
 1  1   3276  15408 111524 869944    0    0   464     0  717 1592  5  4  0 91  0	
 0  1   3276  15000 111904 869976    0    0   372    12  627 1461  2  5  0 93  0	
 0  1   3276  15700 112788 869956    0    0   876    40  960 2177  5  7  0 88  0	
 0  1   3276  14420 113968 870136    0    0  1180    28  850 2020  5 11  0 84  0	
 0  1   3276  14628 115560 868288    0    0  1600     0 1281 2602  3 17  0 80  0	
 0  1   3276  14744 116996 866704    0    0  1696     0 1426 2806  7 19  0 74  0	
[user@tpw ~]$

Author: Werner Fischer Werner Fischer, working in the Knowledge Transfer team at Thomas-Krenn, completed his studies of Computer and Media Security at FH Hagenberg in Austria. He is a regular speaker at many conferences like LinuxTag, OSMC, OSDC, LinuxCon, and author for various IT magazines. In his spare time he enjoys playing the piano and training for a good result at the annual Linz marathon relay.

Translator: Alina Ranzinger Alina has been working at Thomas-Krenn.AG since 2024. After her training as multilingual business assistant, she got her job as assistant of the Product Management and is responsible for the translation of texts and for the organisation of the department.