Lecture #8-9: Linux advanced

7/27/2019 Lecture #8-9: Linux advanced

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High performance computing

for non-programmers

Lecture #8:Linux internals

Glib Ivashkevych

junior researcher, NSC KIPT



Linux boot

bird’s-eye view1 → BIOS (UEFI)

2 → GRUB1 & GRUB23 → Kernel image

4 →

init/systemd

5 → Functional system



Linux boot

BIOS stage

1→ power up, CPU resetone CPU becomes bootstrap, other - “application” processors

2→ CPU in real mode, POSTCPU behaves like Intel 8086, HW is tested and initialized (kinda)

3→ Search for boot device, MBRMBR - first sector (512 bytes) of boot drive, stage-1 BL + partition table.

To get MBR: # dd if=/dev/sda of=/tmp/mbr.bkp bs=512 count=1

bootstrap code

446 bytes

partition table

72 bytes 5

5 A A



Linux boot

GRUB1→ GRUB stage 1 BL from MBR

can do little (since it’s about 446 bytes)

2→ GRUB stage 1.5knows filesystems, can access by normal path

3→ GRUB stage 2presents menu, loads kernel image, maneuvers in unreal mode,since kernel image can be larger than 640K

Config files:/boot - they are here. /boot/grub2/grub.cfg is a main config file.



Linux boot

Kernel1→ Kernel image is loaded

from /boot/vmlinuz-*. bootstrap kernel code is <1Mb, CPU in real mode

2→ Kernel bootstrap codeinitializations and preparations to transition to protected mode

GDT & IDT are set here, A20 line is opened by kernel

image is decompressed, paging is set up and

3→ Kernel kernel is now PID 0, setups remaining CPUs, hardware is fully functional

kernel launches init (tries /sbin/init, /etc/init, /bin/init, /bin/sh)kernel thread waits for requests



Linux boot

init (systemd)1→ systemd reads config

/usr/lib/systemd/system/default.target

2→ systemd starts launching unitsunit can be: target, service, socket, etc.

unit config file specifies requirements and dependenciesif no unit config file - use file in /etc/init.d instead

systemd becomes a parent of any processwhich lost its direct parent (and what the hell does that

mean?! believe it or not, any process in Linux has a parent. except PID 0 and

PID 1).



Linux boot

processInstance of running program

Has ID called PID and parent

Can have command line args and has

access to environment

Has current working directory



Linux

birth of a processfork/exec pair of system calls

forkis called from parent processreturns twice (0 in child, PID of created process in parent)

exec

loads new program to memory

typically, never returns



Linux

process: CL args and env

argv[0] is a name of a program by convention

Depending on actual form of exec environment can be passed from parent ornot. If so, it is passed as a copy

Process can change its environment variables



Linux

process: memory layoutProcess has its own address space, which islinear and flat

Memory is virtual. Logical addresses inprogram do not correspond directly tophysical addresses

Memory is allocated in pages (4KB)



Linux

process: memory layout

text

initialized data

uninitialized data

heap

stack

0x00000000

text: program code, literals, constants

initialized: explicitly initialized data

uninitialized (bss): obvious, mainly

stack: per function call, contains local

variables, return values and where toreturn. One stack per thread

heap: dynamically allocated memory

args, env

v i r

t u a l a d d r e s s



Linux

process: memory paging

page 0 v i r t

u a l a d d r e s s

page 1

page 2

page 3

page 4

page 5

page 6

page frame 3

page frame 1

page frame 2

page frame 0

page frame 6

page frame 4

page frame 5

frame 0

frame 1

frame 2

frame 3

frame 4

frame 5

frame 6

page table

R A M



Linux

process infoVirtual filesystem /proc

/proc/<PID>/ - directory per process

/proc/<PID>/status - various info about process/proc/<PID>/cmdline

/proc/<PID>/cwd

/proc/<PID>/environ

/proc/<PID>/maps, /proc/<PID>/smaps



Linux

process executionCooperative vs. preemptive

multitasking

schedulerinterrupts

system callssignals

mechanismsof control



Linux

process schedulingCPU-bound processes

spend most of the time executing code

high throughput

vs.

I/O bound processesspend most the time waiting for I/O

low latency

conflict



Linux

process schedulingLinux Completely Fair Scheduler

Tries to emulate ideal multiprocessing

CPU time is divided into epochs

Process has nice value, which weight theirtimeslice

Processes with lowest CPU time are

scheduled



Linux

interruptsInterrupt: signal, that breaks current

CPU activity

Async: generated by hardware

Sync: generated by CPU itself(exceptionally conditions, etc.)



Linux

interruptsKernel should handle interrupts

peripheral device

interrupt

controller

CPUstop executionload kernel routine called ISR

(interrupt service routine)

critical actions (otherinterrupts disabled)

non-critical

deferred

kernel decides what to do:

schedule process, return topreviously running



Linux

system callsA way to utilize kernel services from

user-space (say, open file or fork)

Used via C standard library

Kernel enters the scene and performstask in kernel space



Linux

process deathProcess died→ Zombie

Zombie = entry in process table with exit code

Zombies retain little memory, but consumePIDs

Parent should use wait system call, or exit, sothat zombie child can be inherited by init

process



Linux

processes: tools to managetop

useful keys: -p <PID> -n -b -u <UID>

shortcuts:P (sort by CPU%)M (sort by memory%)k (send signal to process)c, z

iotop to check disk usageuseful keys: -o -k -b -t

shortcuts:← → r (choose sorting, reverse)



Linux

processes: tools to managepstree

display processes tree

useful keys: -p -s

ps

takes info from /proc

many different options



Linux

virtual filesystemSkipped: learn it yourself

Consider this:

Ch. 13 in Linux Kernel Development by Robert Love

or

Ch. 2 in How Linux Works by Brian Ward



Linux

The test of the machine is the satisfaction it gives you.There isn't any other test. If the machine producestranquility it's right. If it disturbs you it's wrong until either

the machine or your mind is changed.

Robert Pirsig,

in Zen and the Art of Motorcycle Maintenance



Questions?

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Lecture #8-9: Linux advanced

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