Archive for Jun 30, 2011

Linux Boot Step


LINUX BOOT STEP ???


The following are the 6 high level stages of a typical Linux boot process:

1. BIOS

  • BIOS stands for Basic Input/Output System
  • Performs some system integrity checks
  • Searches, loads, and executes the boot loader program.
  • It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.
  • Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.
  • So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

  • MBR stands for Master Boot Record.
  • It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda
  • MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.
  • It contains information about GRUB (or LILO in old systems).
  • So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

  • GRUB stands for Grand Unified Bootloader.
  • If you have multiple kernel images installed on your system, you can choose which one to be executed.
  • GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.
  • GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).
  • Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.
  • #boot=/dev/sda default=0 timeout=5 splashimage=(hd0,0)/boot/grub/splash.xpm.gz hiddenmenu title CentOS (2.6.18-194.el5PAE) root (hd0,0) kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/ initrd /boot/initrd-2.6.18-194.el5PAE.img
  • As you notice from the above info, it contains kernel and initrd image.
  • So, in simple terms GRUB just loads and executes Kernel and initrd images.

4. Kernel

  • Mounts the root file system as specified in the “root=” in grub.conf
  • Kernel executes the /sbin/init program
  • Since init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.
  • initrd stands for Initial RAM Disk.
  • initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.

5. Init

  • Looks at the /etc/inittab file to decide the Linux run level.
  • Following are the available run levels
    • 0 – halt
    • 1 – Single user mode
    • 2 – Multiuser, without NFS
    • 3 – Full multiuser mode
    • 4 – unused
    • 5 – X11
    • 6 – reboot
  • Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.
  • Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level
  • If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.
  • Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

  • When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.
  • Depending on your default init level setting, the system will execute the programs from one of the following directories.
     
    • Run level 0 – /etc/rc.d/rc0.d/
    • Run level 1 – /etc/rc.d/rc1.d/
    • Run level 2 – /etc/rc.d/rc2.d/
    • Run level 3 – /etc/rc.d/rc3.d/
    • Run level 4 – /etc/rc.d/rc4.d/
    • Run level 5 – /etc/rc.d/rc5.d/
    • Run level 6 – /etc/rc.d/rc6.d/
     
  • Please note that there are also symbolic links available for these directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.
  • Under the /etc/rc.d/rc*.d/ direcotiries, you would see programs that start with S and K.
  • Programs starts with S are used during startup. S for startup.
  • Programs starts with K are used during shutdown. K for kill.
  • There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.
  • For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.




Linux Boot Step
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What is dwm.exe And Why Is It Running?

What is dwm.exe And Why Is It Running?


You are no doubt reading this article because you are wondering why this dwm.exe process is taking more memory than you think it should, and you are curious what it does. Thankfully for you, we have the answer.

So What Is It Anyway?

Desktop Window Manager (dwm.exe) is the compositing window manager that gives you all those pretty effects in Windows Vista: Transparent windows, live taskbar thumbnails (that you can resize now), and even the Flip3D switcher that you can disable and replace with Switcher.

image

What happens in Vista is that applications write the picture of their window to a specific place in memory, and then Windows creates one “composite” view of all the windows on the screen before sending it to your monitor. Because Vista is keeping track of the contents of each window, it can add effects when layering the windows such as the transparency we’re all used to, as well as the live preview thumbnails.
The benefit to using this approach is that Windows Vista can utilize the hardware acceleration features in your video card to create very smooth animations for minimizing and restoring, and even for the transparent effects.

What about Memory Usage? 

If you open up Task Manager, you can see the dwm.exe process in the list, typically taking somewhere between 30-50MB of memory in my testing:

image

The size of the DWM process is controlled by the number of windows that you have open, since each window requires a buffer in memory to store the contents of the window. If you have a large number of very large windows open, DWM will use more memory.
The benefits of using a compositing window manager are worth that relatively small amount of memory under most circumstances.

Does Switching to the Vista Basic Theme Turn it Off?

The short answer is no, just switching to the Vista Basic theme will not turn off DWM, but it will reduce the memory usage a great deal.

image

How do you switch to Vista Basic? Just right-click on the desktop, choose Personalize, and then Window Color and Appearance:

image

Click on classic appearance properties at the bottom:

image 

Then choose Windows Vista Basic and click the Apply button:

image

Note that I’m not recommending switching to the Basic theme, just explaining how to do it.

So How Do I Turn dwm.exe Off Then?

Note that I don’t recommend turning this off unless you are playing games in fullscreen mode, and even then it likely won’t help increase speed.
The only way to get rid of the dwm.exe process is to stop the service from running. Open up Services from Control Panel or the start menu, and then find the “Desktop Window Manager Session Manager” service in the list, and click the stop button. Note that you will be switched to the Vista Basic theme when you click the button.

image

To permanently disable it, double click on the item and select Disabled from the “Startup type” drop-down:

image

If you want to use the command line instead, you can open an administrator mode command prompt and use one of these commands:
Stop Service net stop uxsms
Start Service net start uxsms
Disable Service sc config uxsms start= disabled
Enable Service sc config uxsms start= auto

By now you should understand what the process does… do you really still want to get rid of it?





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