Piattaforme Software per la Rete Titolo presentazione ... · Titolo presentazione sottotitolo Milano, XX mese 20XX Piattaforme Software per la Rete System Administration A.A. 2016/17

Titolo presentazionesottotitolo

Milano, XX mese 20XX

Piattaforme Software per la Rete

System Administration

A.A. 2016/17Federico Reghenzani, Alessandro Barenghi

Federico Reghenzani 2/24

Dipartimento di Elettronica, Informazione e Bioingegneria

Outline

1) Linux boot sequence and BASH forking

2) Process Management

3) Scheduling

4) Process Inspection

Linux boot process and BASH forking



The machine boot

Power on

BIOS

The system is powered on, the firmware (BIOS/EFI) is loaded into main memory

The BIOS or EFI firmware performs all the necessary checks and launch the bootloader

Bootloader

Linuxkernel

The kernel starts running

The bootloader mounts the bootloader filesystem and loads the correct kernel image (usually /boot/vmlinuz-*)



Tyipical bootloaders

● GRUB: GRand Unified Bootloader

– A complete bootloader

● LILO: LInux Loader

– It does not support GPT partitions

– Unmaintained since Jan. 2016

● Syslinux

– Minimalistic

– Great for removable drives (e.g. USB sticks)



The Linux boot

Low-levelinitialization

Kernel imagedecompression

The first code initializes the CPU, programs the MMU,transistion to 64-bit mode, etc.

The kernel image is decompressed

start_kernel() Several non-architecture specific initializations: setup interrupts, memory configuration, scheduler, load initrd, ...

Start the initprocess

The init process is started, usually /sbin/init



The `init` process

● The init process is the first user-space process

– It is the ancestor of all user-space processes in the system

– It starts all enabled services during startup (e.g. DHCP, web server, graphic server, ntp, ...)

● Most common implementations:

– SysVinit (legacy systems)● Load services from /etc/init.d/, /etc/rc.d/

– systemd● Load services from /etc/systemd/



The process tree structure

● In Linux every process has exactly one parent and may have

● An arbitrary number of children

● init process is the only exception: it has no parent

● Every process has:

– PID: Process IDentifier

– PPID: Parent Process IDentifier

● For init PID=1, PPID=0

● PID is of pid_t type, implemented as 32-bit integer, but limited by /proc/sys/kernel/pid_max special file



The login phase

● At the end of the startup, the init daemon executes the getty command

● getty with the help of login command is in charge of authenticating the user

● After a successfull authentication the default shell for the user is started (see /etc/passwd)



BASH forking

bash

bash bash

bash: wait() ./my_prog

bash

fork()

exec()

user@machine:/path/to/dir$ ./myprog

user@machine:/path/to/dir$

Process Management



Seeing processes

● : show the current processes

– Common attributes: : show every process : show process running by a certain user : show the number of threads : show details of every process … tons of other attributes, check the man page

– Ideal for scripting

● : show processes in a tree format

● : return all the PIDs of <command>

ps

-e

-u <user>

-Lf

pstree

u

pidof <command>



Seeing processes (interactively)

● System administrators use the command to check interactively the running processes and their attributes

● Often, an enhanced version is used:

top

htop



Seeing processes – under the hood

● The information read by , , comes from the proc filesystem, mounted under /proc directory

● It’s a virtual filesystem: nothing is present on the disk

● When a process try to read/write files in the proc filesystem, the kernel runs specific routines to reply to these I/O requests

● Instead of exposing tons of system calls, the Linux kernel provide several information and knobs via file-based interfaces

ps pstree [h]top



Running processes in background

● Running a command from the shell results in the shell waiting for its completion: this is known as running in foreground

● Appending a & at the end of a command starts the execution in background

● CTRL+Z sends a SIGSTOP signal to the process, freezing the execution

– and commands resume the execution of the frozen process, respectively in foreground and in backgroundfg bg

Scheduling



Schedulers

● Normal scheduler

– CFS: Completely Fair Scheduler

– The key idea is to maximize the CPU allocation without sacrifice the interactive applications

● Real-Time schedulers

– Priority-based Round-Robin

– Priority-based FIFO

– EDF: Earliest Deadline First



Priority and Nice value

● Linux implements two separate concepts of priority:

– The nice value Range [-20;19], higher values mean lower priority

(default 0) It represents the proportion of system’s processor

assigned to the process

– The real-time priority Range [0;99], higher values mean higher priority

(default 0) 0 → non real-time process > 0 → real-time process



Changing priority

● allows the user to start a program with a different nice value:

● To change an already running process can be used:

● is used to change the real-time priority:

–

–

–

nice

nice -n <nice> command

renice

renice -n <nice> -p <pid>

chrt

chrt -f -p <priority> <pid>

chrt -r -p <priority> <pid>

chrt -o -p 0 <pid>

FIFO

RR

Return to non real-time priority

Process Inspection



Process Inspection

● How to analyze a live process execution?

– Debugging (via gdb)

– Tracing (via strace or ltrace)

– Monitoring the interacting files



Debugging

● The standard Linux debugger is the GNU Debugger (gdb)

● It provides several functions to inspect the inner working of a process

● The debugging is performed via the ptrace system call

● Syntax:

–

–

gdb <progam>

gdb -p <pid>

Start a program with the debugger

Attach the debugger to a running process



Tracing

● A process doing something meaningful needs to interact with the operating system and libraries

● The command intercepts and records system calls and signals.

– Common usage:

– Groups can be used to log only certain calls:● ● <group> = process, network, file or signal

● The command intercepts and records dynamic library calls. It shares most of the options with

strace

strace -o <filename> <command>

-e=<group>

ltrace

strace



Monitoring the interacting files

● The command lists all the open files on a system, including special files (e.g. sockets, libraries, etc.)

● Common options:

– prints all the files opened by any command starting with <string>

– prints all open files in a directory

– prints all open files of a certain user

– prints all open files by a certain PID

lsof

-c <string>

+D <dir>

-u <user>

-p <pid>

Documents

Piattaforme Software per la Rete Titolo presentazione ... · Titolo presentazione sottotitolo Milano, XX mese 20XX Piattaforme Software per la Rete System Administration A.A. 2016/17