Tutorials

The sar command under Linux and Unix systems gives the system activity reports. As the system works, a record of activities is kept in certain counters in the kernel. The sar command gives statistics regarding the CPU, I/O, paging, devices, memory, swap space, network, run queue length and ...

The best way to use pidstat is to pass a list of process ids with the -p parameter. The -u option reports CPU utilization. The -l parameter displays the process command name and its arguments. And we can optionally put time interval in ...

The mpstat command in a Linux or Unix system gives the processor related statistics. mpstat is a part of the sysstat package for performance monitoring under Linux. The mpstat command gives the CPU utilization report and the hardware and ...

The iostat command is for getting the CPU and input-output devices' statistics for Linux and Unix systems. For example,

$ iostat
Linux 3.0.0-14-generic (hostname) 	Wednesday 06 June 2012 	_i686_	(2 CPU)  

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
       ...

The vmstat command prints the system virtual memory statistics for Linux and Unix systems. vmstat prints information about system processes, memory, swap, I/O blocks, interrupts and context switches and the CPU activity. The most common command format is,

vmstat 

The optional delay parameter tells the time ...

The uptime and w commands in Linux and Unix systems give pointed information about the system uptime, load average and about logged in users.

uptime

The uptime command is given as,

uptime

And the output is like,

07:47:08 up 25 days, ...

Using the free command, the free and used memory in a Linux or Unix system can be found. The free command can be given from the command line as,

free

And the system responds with an output like,

       ...

Linux commands like top, uptime and w report the system load average. For instance, the first line of top command output is something like,

top – 04:56:31 up 60 days, 23:18, 1 user, load average: 0.02, ...

The Unix top command is useful for monitoring systems continuously for processes that take more system resources like the CPU time and the memory. top periodically updates the display showing the high resource consuming processes at the top. top is an excellent aid in checking ...

The ps command gives information about processes on a Linux or Unix system. Our programs run as processes in the system. Also, for each installed package, there might be one or more processes. So, to know what is happening in the system, the first instinctive action is to find out about processes ...

Coordinated Universal Time

The Coordinated Universal Time (UTC) is the universally accepted time standard based on atomic clocks with leap seconds added judiciously to keep it in tune with the traditional solar time. The UTC is available as standard from designated national measurement institutes (NMIs) in most countries. It is used as the standard by computers ...

Alarm system call

The alarm system call arranges for the SIGALRM signal to be sent to the calling process after s seconds.

#include <unistd.h>

unsigned int alarm (unsigned int s);

Any previously set alarm is cleared. And if s is 0, no alarm is set. So, passing 0 as argument to alarm clears any previously set alarm. Alarm ...

Execution time

A process has an execution time associated with it. The execution time comprises of two parts, the user time and the system time. The user time is the time spent by the CPU in executing instructions in user mode. The system time is the time spent by the CPU in executing instructions on behalf ...

1.0 The system calendar time since the Epoch, 1970-01-01 00:00:00, UTC

The Linux kernel maintains a system calendar time, which is initialized from the hardware real time clock (RTC) at the boot time and is incremented with the timer interrupt. The system calendar time is the number of seconds passed since January 1, 1970 00:00:00 UTC. ...

The hardware clock

There are two clocks in a Linux system, the hardware real time clock (RTC) and the Linux system time maintained by the kernel. In this tutorial, we will look at the hardware real time
clock.

The hardware real time clock is an integrated circuit, a part of computer's hardware, powered by a battery and ...