EMBEDDED KERNEL and its COMPONENTS.pptx

Dr.M.Lordwin Cecil Prabhaker
EMBEDDED KERNEL &
COMPONENTS
UNIT - 4

Contents
• Embedded File System: RAMDisk – RAMFS –
CRAMFS
• Journaling Flash File Systems: JFFS and JFSS2
• NFS and PROC File system
• Optimizing storage Space: Kernel space
optimization - Application Space Optimization
• Applications for Embedded Linux
• Tuning kernel memory.

Embedded File System
• Types:
– Flash based (Widely Used)
– Memory based
• used at boot-up time to hold a root file system
• for storing volatile data that need not be saved across
reboots.

RAM Disk
• It is a way by which Linux emulates a hard disk using memory.
• For storing a root file system when there is no traditional
storage device such as a hard disk or a flash.
• .initrd that holds the ramdisk image
– It is used to load the kernel image along with a root file system into
memory.
• Creating the initrd image and packing it with the kernel image.
• It gives a mechanism by which you can switch to a new root
file system at a later point of time during system execution
• Uses :
– system recovery and upgrade procedures

Process
The script creates an initrd image “/tmp/ramdisk.img”
mounted as an “ext2” file system on the target
It uses a loopback device “/dev/loop0” to copy files
from the root file system folder “<rfs-folder> “ to the
target image “/tmp/ramdisk.img”

A generic
INTRD
script

• Its Usage is,
mkinitrd <rfs-folder> <ramdisk-size>
where,
• <rfs-folder> is the absolute path of the parent directory
containing the root file system.
• <ramdisk-size> is the size of initrd.

RAMFS (RAM File System)
• To store files that are not useful across reboots at /tmp directory
– It was observed that initializations such as,
• finding the root device,
• mounting the root file system either locally or over NFS, and so on that were part of
the kernel boot-up sequence can easily be handled in user space.
• Initramfs was introduced in the 2.6 kernel to provide early user space.
The “initramfs” image
Its usage is:

RAM DISK vs RAMFS

CRAMFS (Compressed RAM File
System)
• This is a very useful file system for flash storage and was
introduced in the 2.4 kernel.
• It is a high-compression “read-only” file system.
• CRAMFS is a regular file system; that is, it goes via the buffer
cache to talk to a block device holding the actual data.
• Hence you need to enable the MTD block device driver mode
to use CRAMFS.
• CRAMFS uses the zlib routines for compression and it does
compression for every 4-KB (page size) block.
• The CRAMFS image that needs to be burned in flash can be
created using a program called mkcramfs.

Journaling Flash File Systems — JFFS
and JFFS2
• What is in JFFS?
– The Journaling Flash File System
(or JFFS) is a log-structured file system for
use on NOR flash memory devices on the
Linux operating system.
– No data corruption on sudden power outage
– Directly use the MTD-level APIs instead of
going through the flash translation layers
– Linux 2.0 kernel and 2.2 & 2.3
– A journaling flash file system is
specifically designed to minimize frequent
overwrites.
– Files which are read many times but
written infrequently work well on main
flash.

• JFFS2
– Added with compression technique
– 2.4 kernel
• Both JFFS and JFFS2 are log-structured file systems; any
change to a file gets recorded as a log, which gets directly stored
on the flash (logs are also called nodes).
• The contents of this log will be:
– Identification of the file to which the log belongs
– Version that is unique per log belonging to a particular file
– Metadata such as timestamp
– Data and data size
– Offset of the data in the file

Parameters used in JFFS
• Folders and files written to a JFFS2 flash drive are logged in sections called
nodes.
– There are two types of nodes:
• inode – metadata with optional data
• Directory Entry (dirent) node – directory name with inode number.
– When the file is the same but in two different locations, the inode number is the same (hard link).
• When a single file is created, the node is considered valid.
• When a file is modified and created in another location, the node is obsolete.
• Note:
– When a file is renamed,
• a new dirent node is created with the new file name and the inode number. This is similar to a
hard link being created.
• Then, the inode number for the original dirent entry is changed to zero. Changing the number
to a zero unlinked the older entry.

The main features of the JFFS2 file
system are as follows.
• Management of erase blocks:
– In JFFS2, the erase blocks are placed in three lists:
• clean, dirty, and free.
– The clean list contains only valid logs (i.e., logs not
invalidated by newer ones).
– The dirty list contains one or many logs that are
obsolete and hence can be purged when garbage
collection is called.
– The free list contains no logs and hence can be used
for storing new logs.

• Garbage collection:
– JFFS2 garbage collection happens in the context of a separate
thread, which gets started when you mount a JFFS2 file
system.
• For every 99 out of 100 times, this thread will pick up a block from
the dirty list and for the other one out of 100 will pick up a block
from the clean list to ensure wear leveling.
• JFFS2 reserves five blocks for doing garbage collection (this number
seems to have been decided by heuristics).
• Compression:
– The distinguishing factor between JFFS and JFFS2 is that
JFFS2 gives a host of compression including zlib and rubin.

NFS — Network File System
• The network file system can be used to mount a file system over the
network.
• The Linux kernel provides a mechanism for automatically mounting a file
system using NFS at boot time using following steps,
– The config options CONFIG_NFS_FS, which allows to mount a file
system from a remote server
– CONFIG_ROOT_NFS, which allows to use NFS as root file system
needs to be turned on at build time.
– Because an IP address becomes necessary to connect to a NFS server at
boot time, you might want to turn on the network auto configuration
using BOOTP, RARP, or DHCP.
– Kernel command-line parameters to specify the NFS server have to
be specified

PROC File System
• The proc file system is a logical file system used by the kernel to export its
information to the external world.
• The proc file system can be used for,
– system monitoring, debugging, and tuning.
• The proc files are created on the fly when they get opened.
• The files can be read-only or read-write depending on the information
exported by the files.
• The read-only proc files can be used to get some kernel status that cannot
be modified at runtime.
• Examples of read-only proc files are the process status information.
• The read-write proc files contain information that can be changed at
runtime; the kernel uses the changed value on the fly.
• The proc file system gets mounted by the startup scripts at the standard
mount point /proc.

Configuration

Optimizing Storage Space
• To effectively use the storage space.
• Techniques
– Kernel Space Optimization
• To remove the unwanted code in kernel space
• Making smaller kernel images (“Linux tiny kernel project”)
– Option to remove printk and all the strings passed to printk
– Avoid too much usage of inline function.
• CONFIG_EMBEDDED option is used to build a sleeker kernel
• SWAP subsystem can be disabled at build time
– Application Space Optimization
• Pruning individual applications with respect to unwanted code
• Using tools such as the library optimizer
• Using smaller and sleeker programs/distributions aimed at embedded systems
• Using a smaller C library such as uClibc
– Using compressed file systems
• file systems such as CRAMFS and JFFS2

Kernel space optimization techniques
• Unrolling Loops.
• Pipelining Loops.
• Pipelining Work Items.
• Enabling Concurrent Processing with DATAFLOW.
• Reducing Kernel to Kernel Communication Latency
with OpenCL Pipes.
• Improving Kernel Frequency.
• Exploring Kernel Optimizations Using Vivado HLS.

Applications for Embedded Linux
• Some popular distributions and applications used for embedded linux systems are,
– Busybox
• Multicall program (single small executable implements some commonly used programs)
• The required programs for the system can be chosen at build time
– Tinylogin
• Similar to Busybox and is used for implementing UNIX log-in and access applications.
• list of functionalities implemented by Tinylogin
– Adding and deleting users
– login and getty applications
– Changing password passwd application
– Ftp Server
• To copy files to and from
• Two Servers : 1. wu-ftpd server and 2. proftpd server,
– Web Server
• Needed for remote management of an embedded device
• For Example: BOA, mini_httpd and GoAhead

Busybox contains the following main programs
known as applets in Busybox terminology:
• Shells such as the
– ash, lash, hush, and so on
• Core utilities such as
– cat, chmod, cp, dd, mv, ls, pwd, rm, and so on
• Process control and monitoring utilities such as
– ps, kill, and so on
• Module-loading utilities such as
– lsmod, rmmod, modprobe, insmod, and depmod
• System tools such
– reboot, init, and so on
• Networking utilities such as
– ifconfig, route, ping, tftp, httpd, telnet, wget, udhcpc (dhcp client), and so on
• Log-in and password management utilities such as
– login, passwd, adduser, deluser, and so on
• Archival utilities such as
– ar, cpio, gzip, tar, and so on
• System logging utilities such as
– Syslogd

Tuning Kernel Memory (to reduce memory usage by the kernel)
• Linux kernel does not participate in paging and hence the entire
kernel (code, data, and stack) always resides in main memory.
• How to estimate the memory utilized by the kernel?
– by using the size command
– The /proc/meminfo records the runtime memory on the system.

Tuning Kernel Memory - Techniques
• Cutting down on statically allocated data structures
– Statically allocated data structures reside either in the .data or in
the .bss section
• System.map file
– This file contains the symbol addresses of each symbol
• Cutting down unused code within the kernel
• Improper usage of kmalloc ()
– By avoiding waste memory allocation
• Using the .init directive
• Cutting down on holes in physical memory
– To prevent from huge hole “CONFIG_DISCONTIGMEM”
• XIP or “eXecute In Place”
– It is a technology by which a program executes directly from flash

Conclusion
• File System
– Embedded File system
• RAM Disk – RAMFS – CRAMFS
– Journaling Flash File Systems
• JFFS and JFSS2,
– NFS and PROC File system
• Optimizing storage Space:
– Kernel space optimization
– Application Space Optimization
– Compression
• Applications for Embedded Linux
• Tuning kernel memory

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EMBEDDED KERNEL and its COMPONENTS.pptx

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