F_____ 2: Systems Round!

Position	Intern - Production Engineer
Time	45 Minutes
Rounds	2
Offer	No
NDA	No

Preparation

2 Weeks of in-depth preparation of Linux + Operating Systems + Networks. Linux SysAdmin Github Academy was my primary guide.

• My solutions can be found in this fork

What happens when a linux system boots?

My Answer:

• BIOS chip checks hardware is ok
• BIOS Chip loads first sector (Bootloader) from primary HD
• Bootloader like grub knows locations of other operating system
• Bootloader starts kernel code
• Kernel Code checks hardware again and starts init
• init starts daemons
• 1 daemon is xserver responsible for GUI
• GUI shows login

What I missed:

• How File system is loaded?
• What about / directory?
• What about /boot?
• How EXT4 gets loaded? what if drivers not there?
• What about Swap partition and primary partition?

What happens when ls -l foo* is run?

My Answer:

• Terminal is a bash program
• Bash program is listening for inputs
• Every time I press a keyboard button, hardware interrupt to cpu
• CPU switches to kernel mode, reads keyboard input, serves process
• bash displays keyboard input
• when enter is pressed, bash starts something called as Fork-Execute-Wait Cycle
• bash forks and creates replica
• I talked about switching into kernel mode too.
• replica runs exec to switch into ls
• ls takes command line arguments
• ls runs and prints and exits
• returns and bash which was waiting now continues

What I missed:

• foo* is actually passed into shell globbing program
  • Example ./*cat*
  • it will linearly pass arguments such as cat netcate.sh tcat.sh
  • Follow Up: What if arguments are too long?
    • my answer xargs but not able to convince
    • second answer find -name 'foo*' -exec rm {} +

What is Fork-Execute-Wait Cycle?

My Answer:

• Pattern where Fork creates replica to the point PC is also same
  • Follow up How Parent, child process know which ones they are?
    • fork returns non zero PID for Parent
    • returns zero PID for child
  • Follow up Can Fork Fail?
    • Yes, PIDs are limited
    • If a parent process keeps creating zombie children PIDs will run out.
    • how to handle that?
      • returns negative
• Exec
  • replica runs exec to switch into ls
    • exec loads executable mostly in ELF format.
    • Loads into memory which has typical C program format of growing stack from above, growing heap from below, initialised and uninitialised variables etc.
• Wait
  • Waits for child process to finish
  • Successfully or unsuccessfully is determined by return value

What I missed:

• Not Actually missed, but told irrelevant information about throwing exceptions in C language (goto).

What are Zombie Processes?

My Answer:

• child processes which have finished executing
• Parents are still running
  • Follow up How to limit no of max processes that can be created?
  • Follow up Heard of ulimit?
    • yes, explained

What I missed:

• Didn’t know about limiting max no of processes.

Explain VMSTAT output

• showed me vmstat output and asked me whether I knew what the output was.
  • First I said no, but realized I knew about it.. so I interrupted and told
• Explained to me each column, but given my feeble memory I already forgot about them by the time he finished.
• I didn’t do so well here.
• What is interruptible vs uninterruptible sleep?
  • Answered (hardware interrupt for uninterruptible vs signal for interruptible)
• Two modes of CPU are IO mode and Computational mode.
• What is wait time? is it good?
  • CPU sleeping.
  • Not good, because precious CPU cycles are wasted.
• There were other questions but didn’t do so good here.

My Wifi Went Down during the interview

• Of all the possible times for the Wifi to not work…. it had to be now.
• Switched immediately to hotspot on my phone
• Lost 4 minutes
• Apologized frantically

My Summary:

• Did OK.
• May not be positive.
• But definitely not negative.
• PS. preparing for this interview was really useful
  • It was a good refresher for Operating Systems and networks.
  • Learned new things about linux, for instance nohup instead of tmux
  • Thanks Spandan Chowdhury (The networks guru) for explaining to me all the fundamentals in Networks.