Interview questions about OS arch
Segmentation fault:
----------------------
If you ever see a Unix message that says "Segmentation fault", "core dumped" or something similar, this is exactly what has happened; an attempt by the running program to access memory (core) outside its segment has raised a fatal interrupt. This indicates a bug in the program code; the core dump it leaves behind is diagnostic information intended to help a programmer track it down.
Signal 11, or officially know as "segmentation fault", means that the program accessed a memory location that was not assigned. That's usually a bug in the program.
What is 32-bit" or "64-bit" computers:
---------------------------------------
Before we can go into this, you need to understand about the word size of your computer. The word size is the computer's preferred size for moving units of information around; technically it's the width of your processor's registers, which are the holding areas your processor uses to do arithmetic and logical calculations. When people write about computers having bit sizes (calling them, say, "32-bit" or "64-bit" computers), this is what they mean.
File names and directories:
---------------------------
Within each file system, the mapping from names to blocks is handled through a structure called an i-node. There's a pool of these things near the "bottom" (lowest-numbered blocks) of each file system (the very lowest ones are used for housekeeping and labeling purposes we won't describe here). Each i-node describes one file. File data blocks (including directories) live above the i-nodes (in higher-numbered blocks).
Every i-node contains a list of the disk block numbers in the file it describes. (Actually this is a half-truth, only correct for small files, but the rest of the details aren't important here.) Note that the i-node does not contain the name of the file.
Names of files live in directory structures. A directory structure just maps names to i-node numbers. This is why, in Unix, a file can have multiple true names (or hard links); they're just multiple directory entries that happen to point to the same i-node.
How a file gets looked up:
-----------------------------
Now we can look at the file system from the top down. When you open a file (such as, say, /home/esr/WWW/ldp/fundamentals.xml) here is what happens:
Your kernel starts at the root of your Unix file system (in the root partition). It looks for a directory there called ‘home’. Usually ‘home’ is a mount point to a large user partition elsewhere, so it will go there. In the top-level directory structure of that user partition, it will look for a entry called ‘esr’ and extract an i-node number. It will go to that i-node, notice that its associated file data blocks are a directory structure, and look up ‘WWW’. Extracting that i-node, it will go to the corresponding subdirectory and look up ‘ldp’. That will take it to yet another directory i-node. Opening that one, it will find an i-node number for ‘fundamentals.xml’. That i-node is not a directory, but instead holds the list of disk blocks associated with the file.
The Domain Name System:
------------------------
When you query for the IP address of www.tldp.org, what actually happens is this: First, your nameserver asks a root server to tell it where it can find a nameserver for .org. Once it knows that, it then asks the .org server to tell it the IP address of a .tldp.org nameserver. Once it has that, it asks the .tldp.org nameserver to tell it the address of the host www.tldp.org.
TCP and IP:
---------------
To understand how multiple-packet transmissions are handled, you need to know that the Internet actually uses two protocols, stacked one on top of the other.
The lower level, IP (Internet Protocol), is responsible for labeling individual packets with the source address and destination address of two computers exchanging information over a network. For example, when you access http://www.tldp.org, the packets you send will have your computer's IP address, such as 192.168.1.101, and the IP address of the www.tldp.org computer, 152.2.210.81. These addresses work in much the same way that your home address works when someone sends you a letter. The post office can read the address and determine where you are and how best to route the letter to you, much like a router does for Internet traffic.
The upper level, TCP (Transmission Control Protocol), gives you reliability. When two machines negotiate a TCP connection (which they do using IP), the receiver knows to send acknowledgements of the packets it sees back to the sender. If the sender doesn't see an acknowledgement for a packet within some timeout period, it resends that packet. Furthermore, the sender gives each TCP packet a sequence number, which the receiver can use to reassemble packets in case they show up out of order. (This can easily happen if network links go up or down during a connection.)
TCP/IP packets also contain a checksum to enable detection of data corrupted by bad links.
Kernel Panic:
---------------
the kernel will try to mount the root filesystem. The place is configurable at compilation time, or any time with rdev or the bootloader. The filesystem type is detected automatically. If the mounting of the root filesystem fails, for example because you didn't remember to include the corresponding filesystem driver in the kernel, the kernel panics and halts the system (there isn't much it can do, anyway).
Getty[tty]:
--------------
getty is the program which lets people log in via virtual consoles and serial terminals. init may also start some other programs, depending on how it is configured.
After kernel loaded into memory[booting process]:
--------------------------------------------------
After the Linux kernel has been read into the memory, by whatever means, and is started for real, roughly the following things happen:
The Linux kernel is installed compressed, so it will first uncompress itself. The beginning of the kernel image contains a small program that does this.
If you have a super-VGA card that Linux recognizes and that has some special text modes (such as 100 columns by 40 rows), Linux asks you which mode you want to use. During the kernel compilation, it is possible to preset a video mode, so that this is never asked. This can also be done with LILO, GRUB or rdev.
After this, the kernel checks what other hardware there is (hard disks, floppies, network adapters, etc), and configures some of its device drivers appropriately; while it does this, it outputs messages about its findings. For example, when I boot, I it looks like this:
The exact texts are different on different systems, depending on the hardware, the version of Linux being used, and how it has been configured.
Then the kernel will try to mount the root filesystem. The place is configurable at compilation time, or any time with rdev or the bootloader. The filesystem type is detected automatically. If the mounting of the root filesystem fails, for example because you didn't remember to include the corresponding filesystem driver in the kernel, the kernel panics and halts the system (there isn't much it can do, anyway).
The root filesystem is usually mounted read-only (this can be set in the same way as the place). This makes it possible to check the filesystem while it is mounted; it is not a good idea to check a filesystem that is mounted read-write.
After this, the kernel starts the program init (located in /sbin/init) in the background (this will always become process number 1). init does various startup chores. The exact things it does depends on how it is configured; see Section 2.3.1 for more information (not yet written). It will at least start some essential background daemons.
init then switches to multi-user mode, and starts a getty for virtual consoles and serial lines. getty is the program which lets people log in via virtual consoles and serial terminals. init may also start some other programs, depending on how it is configured.
After this, the boot is complete, and the system is up and running normally.
Init works:
-----------
When init starts, it finishes the boot process by doing a number of administrative tasks, such as checking filesystems, cleaning up /tmp, starting various services, and starting a getty for each terminal and virtual console where users should be able to log in.
fork and exec:
--------------
the only new process is the one created by init (using the fork system call); getty and login only replace the program running in the process (using the exec system call).
Date and Clock command:
------------------------
date only shows or sets the software clock. The clock commands synchronizes the hardware and software clocks. It is used when the system boots, to read the hardware clock and set the software clock. If you need to set both clocks, you first set the software clock with date, and then the hardware clock with clock -w.
NTP Network Time Protocol:
--------------------------
A networked computer (even if just over a modem) can check its own clock automatically by comparing it to the time on another computer known to keep accurate time. Network Time Protocol (or NTP) does exactly that. It is a method of verifying and correcting your computer's time by synchronizing it with a another system. With NTP your system's time can be maintained to within milliseconds of Coordinated Universal Time.
boot sector:
-------------
Usually the first sector on any given partition. It contains a very short program[boot loader---->grub] (on the order of a few hundred bytes) which will load and start running the operating system proper.
filesystem:
------------
A term which is used for two purposes and which can have two subtly different meanings. It is either the collection of files and directories on a drive (whether hard drive, floppy, Cd-ROM, etc). Or it is the markers put onto the disk media which the OS uses to decide where to write files to (inodes, blocks, superblocks etc). The actual meaning can almost always be inferred from context.
A filesystem is the methods and data structures that an operating system uses to keep track of files on a disk or partition; that is, the way the files are organized on the disk. The word is also used to refer to a partition or disk that is used to store the files or the type of the filesystem.
inode:
-------
A data structure holding information about files in a Unix file system. There is an inode for each file and a file is uniquely identified by the file system on which it resides and its inode number on that system. Each inode contains the following information: the device where the inode resides, locking information, mode and type of file, the number of links to the file, the owner's user and group ids, the number of bytes in the file, access and modification times, the time the inode itself was last modified and the addresses of the file's blocks on disk. A Unix directory is an association between file leafnames and inode numbers. A file's inode number can be found using the "-i" switch to ls.
master boot record:
--------------------
(MBR) The first logical sector on a disk, this is (usually) where the BIOS looks to load a small program that will boot the computer.
Superblock and inode:
----------------------
The superblock contains information about the filesystem as a whole, such as its size (the exact information here depends on the filesystem). An inode contains all information about a file, except its name. The name is stored in the directory, together with the number of the inode. A directory entry consists of a filename and the number of the inode which represents the file. The inode contains the numbers of several data blocks, which are used to store the data in the file.
Filesystem block size:
----------------------
The block size specifies size that the filesystem will use to read and write data. Larger block sizes will help improve disk I/O performance when using large files, such as databases. This happens because the disk can read or write data for a longer period of time before having to search for the next block.
On the downside, if you are going to have a lot of smaller files on that filesystem, like the /etc, there the potential for a lot of wasted disk space.
For example, if you set your block size to 4096, or 4K, and you create a file that is 256 bytes in size, it will still consume 4K of space on your harddrive. For one file that may seem trivial, but when your filesystem contains hundreds or thousands of files, this can add up.
Mounting:
---------
-attaching the new file system to existing file system.
-Since all files in UNIX are in a single directory tree, the mount operation will make it look like the contents of the new filesystem are the contents of an existing subdirectory in some already mounted filesystem.
NFS:
------
The Network File System (NFS) is probably the most prominent network service using RPC. It allows you to access files on remote hosts in exactly the same way you would access local files. A mixture of kernel support and user-space daemons on the client side, along with an NFS server on the server side, makes this possible. This file access is completely transparent to the client and works across a variety of server and host architectures.
NFS offers a number of useful features:
Data accessed by all users can be kept on a central host, with clients mounting this directory at boot time. For example, you can keep all user accounts on one host and have all hosts on your network mount /home from that host. If NFS is installed beside NIS, users can log into any system and still work on one set of files.
Data consuming large amounts of disk space can be kept on a single host. For example, all files and programs relating to LaTeX and METAFONT can be kept and maintained in one place.
Administrative data can be kept on a single host. There is no need to use rcp to install the same stupid file on 20 different machines.
NIS:
-----
This is why Sun developed the Network Information System (NIS). NIS provides generic database access facilities that can be used to distribute, for example, information contained in the passwd and groups files to all hosts on your network. This makes the network appear as a single system, with the same accounts on all hosts. Similarly, you can use NIS to distribute the hostname information from /etc/hosts to all machines on the network.
Add loop device to swap to expand the mamory:
----------------------------------------------
You can add a loop device to swap to expand your memory;
this will make compilation much slower, but at least it will work on such
devices that have insufficient memory:
dd if=/dev/zero of=/tmp/swapspace bs=1M count=128
losetup /dev/loop0 /tmp/swapspace
mkswap /dev/loop0
swapon /dev/loop0
will set up 128MB of swap space (or virtual memory). If it still fails,
increase the amount of disk space used (count=256; count=512; count=XXX). If
you are done compiling or want to increase the size, remove the added swapspace
with:
swapoff /dev/loop0
losetup -d /dev/loop0
rm /tmp/swapspace
Kernel:
------------
The kernel is the heart of the operating system. In fact, it is often mistakenly considered to be the operating system itself, but it is not. An operating system provides provides many more services than a plain kernel.
It keeps track of files on the disk, starts programs and runs them concurrently, assigns memory and other resources to various processes, receives packets from and sends packets to the network, and so on. The kernel does very little by itself, but it provides tools with which all services can be built. It also prevents anyone from accessing the hardware directly, forcing everyone to use the tools it provides. This way the kernel provides some protection for users from each other.
----------------------
If you ever see a Unix message that says "Segmentation fault", "core dumped" or something similar, this is exactly what has happened; an attempt by the running program to access memory (core) outside its segment has raised a fatal interrupt. This indicates a bug in the program code; the core dump it leaves behind is diagnostic information intended to help a programmer track it down.
Signal 11, or officially know as "segmentation fault", means that the program accessed a memory location that was not assigned. That's usually a bug in the program.
What is 32-bit" or "64-bit" computers:
---------------------------------------
Before we can go into this, you need to understand about the word size of your computer. The word size is the computer's preferred size for moving units of information around; technically it's the width of your processor's registers, which are the holding areas your processor uses to do arithmetic and logical calculations. When people write about computers having bit sizes (calling them, say, "32-bit" or "64-bit" computers), this is what they mean.
File names and directories:
---------------------------
Within each file system, the mapping from names to blocks is handled through a structure called an i-node. There's a pool of these things near the "bottom" (lowest-numbered blocks) of each file system (the very lowest ones are used for housekeeping and labeling purposes we won't describe here). Each i-node describes one file. File data blocks (including directories) live above the i-nodes (in higher-numbered blocks).
Every i-node contains a list of the disk block numbers in the file it describes. (Actually this is a half-truth, only correct for small files, but the rest of the details aren't important here.) Note that the i-node does not contain the name of the file.
Names of files live in directory structures. A directory structure just maps names to i-node numbers. This is why, in Unix, a file can have multiple true names (or hard links); they're just multiple directory entries that happen to point to the same i-node.
How a file gets looked up:
-----------------------------
Now we can look at the file system from the top down. When you open a file (such as, say, /home/esr/WWW/ldp/fundamentals.xml) here is what happens:
Your kernel starts at the root of your Unix file system (in the root partition). It looks for a directory there called ‘home’. Usually ‘home’ is a mount point to a large user partition elsewhere, so it will go there. In the top-level directory structure of that user partition, it will look for a entry called ‘esr’ and extract an i-node number. It will go to that i-node, notice that its associated file data blocks are a directory structure, and look up ‘WWW’. Extracting that i-node, it will go to the corresponding subdirectory and look up ‘ldp’. That will take it to yet another directory i-node. Opening that one, it will find an i-node number for ‘fundamentals.xml’. That i-node is not a directory, but instead holds the list of disk blocks associated with the file.
The Domain Name System:
------------------------
When you query for the IP address of www.tldp.org, what actually happens is this: First, your nameserver asks a root server to tell it where it can find a nameserver for .org. Once it knows that, it then asks the .org server to tell it the IP address of a .tldp.org nameserver. Once it has that, it asks the .tldp.org nameserver to tell it the address of the host www.tldp.org.
TCP and IP:
---------------
To understand how multiple-packet transmissions are handled, you need to know that the Internet actually uses two protocols, stacked one on top of the other.
The lower level, IP (Internet Protocol), is responsible for labeling individual packets with the source address and destination address of two computers exchanging information over a network. For example, when you access http://www.tldp.org, the packets you send will have your computer's IP address, such as 192.168.1.101, and the IP address of the www.tldp.org computer, 152.2.210.81. These addresses work in much the same way that your home address works when someone sends you a letter. The post office can read the address and determine where you are and how best to route the letter to you, much like a router does for Internet traffic.
The upper level, TCP (Transmission Control Protocol), gives you reliability. When two machines negotiate a TCP connection (which they do using IP), the receiver knows to send acknowledgements of the packets it sees back to the sender. If the sender doesn't see an acknowledgement for a packet within some timeout period, it resends that packet. Furthermore, the sender gives each TCP packet a sequence number, which the receiver can use to reassemble packets in case they show up out of order. (This can easily happen if network links go up or down during a connection.)
TCP/IP packets also contain a checksum to enable detection of data corrupted by bad links.
Kernel Panic:
---------------
the kernel will try to mount the root filesystem. The place is configurable at compilation time, or any time with rdev or the bootloader. The filesystem type is detected automatically. If the mounting of the root filesystem fails, for example because you didn't remember to include the corresponding filesystem driver in the kernel, the kernel panics and halts the system (there isn't much it can do, anyway).
Getty[tty]:
--------------
getty is the program which lets people log in via virtual consoles and serial terminals. init may also start some other programs, depending on how it is configured.
After kernel loaded into memory[booting process]:
--------------------------------------------------
After the Linux kernel has been read into the memory, by whatever means, and is started for real, roughly the following things happen:
The Linux kernel is installed compressed, so it will first uncompress itself. The beginning of the kernel image contains a small program that does this.
If you have a super-VGA card that Linux recognizes and that has some special text modes (such as 100 columns by 40 rows), Linux asks you which mode you want to use. During the kernel compilation, it is possible to preset a video mode, so that this is never asked. This can also be done with LILO, GRUB or rdev.
After this, the kernel checks what other hardware there is (hard disks, floppies, network adapters, etc), and configures some of its device drivers appropriately; while it does this, it outputs messages about its findings. For example, when I boot, I it looks like this:
The exact texts are different on different systems, depending on the hardware, the version of Linux being used, and how it has been configured.
Then the kernel will try to mount the root filesystem. The place is configurable at compilation time, or any time with rdev or the bootloader. The filesystem type is detected automatically. If the mounting of the root filesystem fails, for example because you didn't remember to include the corresponding filesystem driver in the kernel, the kernel panics and halts the system (there isn't much it can do, anyway).
The root filesystem is usually mounted read-only (this can be set in the same way as the place). This makes it possible to check the filesystem while it is mounted; it is not a good idea to check a filesystem that is mounted read-write.
After this, the kernel starts the program init (located in /sbin/init) in the background (this will always become process number 1). init does various startup chores. The exact things it does depends on how it is configured; see Section 2.3.1 for more information (not yet written). It will at least start some essential background daemons.
init then switches to multi-user mode, and starts a getty for virtual consoles and serial lines. getty is the program which lets people log in via virtual consoles and serial terminals. init may also start some other programs, depending on how it is configured.
After this, the boot is complete, and the system is up and running normally.
Init works:
-----------
When init starts, it finishes the boot process by doing a number of administrative tasks, such as checking filesystems, cleaning up /tmp, starting various services, and starting a getty for each terminal and virtual console where users should be able to log in.
fork and exec:
--------------
the only new process is the one created by init (using the fork system call); getty and login only replace the program running in the process (using the exec system call).
Date and Clock command:
------------------------
date only shows or sets the software clock. The clock commands synchronizes the hardware and software clocks. It is used when the system boots, to read the hardware clock and set the software clock. If you need to set both clocks, you first set the software clock with date, and then the hardware clock with clock -w.
NTP Network Time Protocol:
--------------------------
A networked computer (even if just over a modem) can check its own clock automatically by comparing it to the time on another computer known to keep accurate time. Network Time Protocol (or NTP) does exactly that. It is a method of verifying and correcting your computer's time by synchronizing it with a another system. With NTP your system's time can be maintained to within milliseconds of Coordinated Universal Time.
boot sector:
-------------
Usually the first sector on any given partition. It contains a very short program[boot loader---->grub] (on the order of a few hundred bytes) which will load and start running the operating system proper.
filesystem:
------------
A term which is used for two purposes and which can have two subtly different meanings. It is either the collection of files and directories on a drive (whether hard drive, floppy, Cd-ROM, etc). Or it is the markers put onto the disk media which the OS uses to decide where to write files to (inodes, blocks, superblocks etc). The actual meaning can almost always be inferred from context.
A filesystem is the methods and data structures that an operating system uses to keep track of files on a disk or partition; that is, the way the files are organized on the disk. The word is also used to refer to a partition or disk that is used to store the files or the type of the filesystem.
inode:
-------
A data structure holding information about files in a Unix file system. There is an inode for each file and a file is uniquely identified by the file system on which it resides and its inode number on that system. Each inode contains the following information: the device where the inode resides, locking information, mode and type of file, the number of links to the file, the owner's user and group ids, the number of bytes in the file, access and modification times, the time the inode itself was last modified and the addresses of the file's blocks on disk. A Unix directory is an association between file leafnames and inode numbers. A file's inode number can be found using the "-i" switch to ls.
master boot record:
--------------------
(MBR) The first logical sector on a disk, this is (usually) where the BIOS looks to load a small program that will boot the computer.
Superblock and inode:
----------------------
The superblock contains information about the filesystem as a whole, such as its size (the exact information here depends on the filesystem). An inode contains all information about a file, except its name. The name is stored in the directory, together with the number of the inode. A directory entry consists of a filename and the number of the inode which represents the file. The inode contains the numbers of several data blocks, which are used to store the data in the file.
Filesystem block size:
----------------------
The block size specifies size that the filesystem will use to read and write data. Larger block sizes will help improve disk I/O performance when using large files, such as databases. This happens because the disk can read or write data for a longer period of time before having to search for the next block.
On the downside, if you are going to have a lot of smaller files on that filesystem, like the /etc, there the potential for a lot of wasted disk space.
For example, if you set your block size to 4096, or 4K, and you create a file that is 256 bytes in size, it will still consume 4K of space on your harddrive. For one file that may seem trivial, but when your filesystem contains hundreds or thousands of files, this can add up.
Mounting:
---------
-attaching the new file system to existing file system.
-Since all files in UNIX are in a single directory tree, the mount operation will make it look like the contents of the new filesystem are the contents of an existing subdirectory in some already mounted filesystem.
NFS:
------
The Network File System (NFS) is probably the most prominent network service using RPC. It allows you to access files on remote hosts in exactly the same way you would access local files. A mixture of kernel support and user-space daemons on the client side, along with an NFS server on the server side, makes this possible. This file access is completely transparent to the client and works across a variety of server and host architectures.
NFS offers a number of useful features:
Data accessed by all users can be kept on a central host, with clients mounting this directory at boot time. For example, you can keep all user accounts on one host and have all hosts on your network mount /home from that host. If NFS is installed beside NIS, users can log into any system and still work on one set of files.
Data consuming large amounts of disk space can be kept on a single host. For example, all files and programs relating to LaTeX and METAFONT can be kept and maintained in one place.
Administrative data can be kept on a single host. There is no need to use rcp to install the same stupid file on 20 different machines.
NIS:
-----
This is why Sun developed the Network Information System (NIS). NIS provides generic database access facilities that can be used to distribute, for example, information contained in the passwd and groups files to all hosts on your network. This makes the network appear as a single system, with the same accounts on all hosts. Similarly, you can use NIS to distribute the hostname information from /etc/hosts to all machines on the network.
Add loop device to swap to expand the mamory:
----------------------------------------------
You can add a loop device to swap to expand your memory;
this will make compilation much slower, but at least it will work on such
devices that have insufficient memory:
dd if=/dev/zero of=/tmp/swapspace bs=1M count=128
losetup /dev/loop0 /tmp/swapspace
mkswap /dev/loop0
swapon /dev/loop0
will set up 128MB of swap space (or virtual memory). If it still fails,
increase the amount of disk space used (count=256; count=512; count=XXX). If
you are done compiling or want to increase the size, remove the added swapspace
with:
swapoff /dev/loop0
losetup -d /dev/loop0
rm /tmp/swapspace
Kernel:
------------
The kernel is the heart of the operating system. In fact, it is often mistakenly considered to be the operating system itself, but it is not. An operating system provides provides many more services than a plain kernel.
It keeps track of files on the disk, starts programs and runs them concurrently, assigns memory and other resources to various processes, receives packets from and sends packets to the network, and so on. The kernel does very little by itself, but it provides tools with which all services can be built. It also prevents anyone from accessing the hardware directly, forcing everyone to use the tools it provides. This way the kernel provides some protection for users from each other.
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