UNIX

yozzum·2023년 9월 30일
ubuntuunix기술면접
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LINUX

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3/5

OS

  1. init == systemd(in modern)

    responsible for transitioning the system between runlevels.

  2. runlevel

    there are different run levels that define the state or mode in which the system operates; single-user mode, multi-user mode, graphical mode. The init process switches between these run levels as needed.

  3. Order of operating system boot process (after powering on)

    • Self Test(Power on self Test) > Loading OS > Loading System Utilities

  4. Safe Mode

    a diagnostic mode that starts OS with a limited set of files and drivers to debug system malfunctions.

  5. Fence Register(memory fence / memory barrier)

    a hardware mechanism that ensures that memory operations are performed in a specific order, especially in multi-threaded or multi-core environments. The primary purpose is to ensure proper synchronization and memory consistency for CPU operations. It primarily protects the CPU.

  6. Threads

    • the execution units within a process.
    • each thread has its own execution context, state, stack, program counter.
    • within the same process, they share the heap, stack, code section, and global/environment variables.

  7. Addressed generated by CPU

    • logical addresses used in virtual memory systems.

  8. Kernel

    • a core component of the OS responsible for managing HW resources, providing system services and mediating interactions between SW and HW.
    • remains in memeory throughout the entire computer session.
    • the first part of the OS to load into memory during booting.
    • Power-On-Self-Test and BIOS/UEFI Initialization
    • consists of various modules and not all of them are loaded into memery at once when the OS is running
    • critical code is loaded into a separate area of memory.

  9. Trouble shoot Too Many Open Files Error

    • sudo lsof : lists Open Files to identify the process that's generating "Too many open files" error
    • ulimit -n : Check the resource limits for the user and the system.
    • ulimit -n 9999 : Increate Resource Limits

  10. CPU

    • speed is measured in Gigaherts(GHz)

  11. Fully-distributed mode

    • all daemons execute the process in separate nodes.

Memory

  1. Code Section

    • a section where the executable code of the process resides

  2. Data Section

    • a section where global and static variables used by the process reside. These variables are shared within a process.

  3. Heap

    • a region of memory where dynamically allocated data is stored.
    • All threads within the same process share the same heap.
    • meaning that memory allocated on the head by one thread can be accessed and modified by other threads in the same process.

  4. Stack

    • a region of memory used for function call and local variable storage.

  5. Memory-Mapped Files

    • processes can use memory-mapped files to map files into their memory space. use the data as if it were in memory

  6. Shared Memory

    • processes can share data using shared memory, pipes, sockets, interprocess communition(IPC)

Cache memory

  1. Mapping methods

    • direct mapping
    • associative mapping
    • set-associative mapping

  2. When to use caching

    • Short running queries on databases : △
    • Computation intensive processing : O
    • Working with inconsistent data : X
    • high latency network requests : O
    • long running queries on databases : O
    • Frequently accessed data : O

Multi-tasking/processing

  1. Multi-tasking (Single-Threaded)

    • run tasks/processes concurrently on a single CPU core (rapidly switching between tasks > illusion!)
    • share the resources of a single CPU core. consumtion of a task can affect the other.
    • slower/unstable

  2. Multi-processing

    • run multiple processes on multiple CPU cores.
    • each CPU core can execute its own process independently
    • no need to share the resources of a single CPU core as each has its own CPU core.
    • faster/stable

CLI

  1. Chmod 755 file.txt

    • Change mode.
    • User Group Others
    • rwx → 421

  2. ps aux / top(real-time)

    • to view a list of running processes

  3. dmesg

    • provides system-level information related to kernel messages including HW, drivers and system events.
    • not detailed CPU info.

  4. lscpu

    • gets detailed CPU info.

  5. netstat

    • provides information about network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.

  6. ss(socket statistics)

    • display detailed socket statistics, including information about network connections and their state.

Network

  1. Checking port on a server

    • "Ping" directly checks whether the server itself is reachable and responsive using ICMP(Internet Control Message Protocol). NOT directly checking the status of a specific port on a server.
    • telnet/nc/nmap (commands) can be used to check a specific port on a server.
    • When using telnet/nc etc to check the status of a port on a remote server, TCP(Transmission Control Protocol) is commonly used.

  2. SNMP

    • a widely used protocol for monitoring and managing network devices such as routers, switches and servers.

  3. Get an IP from DHCP

    • Discover → Offer → Request → Acknowledge(Confirming IP Lease)
      ※ lease is a concept for the DHCP IP assignment.

  4. CIDR

    • private
    • 10.0.0.0 ~ 10.255.255.255
    • 172.16.0.0 ~ 172.31.255.255
    • 192.168.0.0 ~ 192.168.255.255
    • public
    • 0.0.0.0 ~ 127.255.255.255
    • 128.0.0.0 ~ 191.255.255.255
    • 192.0.0.0 ~ 233.255.255.255
    • 224.0.0.0 ~ 239.255.255.255
    • 240.0.0.0 ~ 255.255.255.255

  5. IPv4 VS IPv6

    • Anycast Address is specific to IPv6 and not present in IPv4

  6. A network with 5 subnets, each with at least 16 hosts. which subnet masks should be used ?

    • Subnet mask: 255.255.255.240 (or /28 in CIDR notation)

  7. Port numbers

    • SFTP(22; SSH File Transfer Protocol)
    • SFTP and SCP(Secure Copy Protocal) use SSH(Secure Shell) protocol to secure the traffic and data transfer.

  8. OSI Model

    • Application > Presentation > Session > Transport > Network > Data Link > Physical Layer

  9. TCP Sliding Window

    • a fundamental mechanism used to optimize and control the flow of data between two endpoints(devices.)
    • crucial for TCP's reliability and efficiency.

  10. DNS

    • RD(Recursion Desired) flag in the DNS message header is used to determine if a query is recursive. (client sends the message to DNS server)

  11. Trouble shoot Intermittent network

    • Run a Packet Capture to Analyze the Traffic : packet loss/excessive latency, unusual traffic patterns or malformed packets.
    • Review the system and network device logs
    • User Follow the Path : check others' connections.

  12. ping

    • 상대 서버 점검 : ping X.X.X.X
    • 내 랜카드 점검 : ping localhost / 127.0.0.1
    • 유무선 공유기 점검 : ipconfig > ping 192.168.0.1
    • 통신사(ISP) DNS 점검 : nslookup > exit > ping 168.126.63.1
    • 인터넷망 점검 : nslookup dns.google > ping 8.8.8.8

  13. when unable to communicate on the network.

    • check the host local logs.

  14. When an HTTP request to a host is failing

    • Review Logs on All devices
    • Perform tcpdump on the host(or a packet capture tool)

  15. When Ping is OK, SSH not working

    • SSH Client key is Invalid
    • Only ICMP Ports are Allowed(TCP Ports) : review the server's firewall

  16. CORS(Cross-Origin Resource Sharing)

    • related to missing or incorrectly configured headers in an HTTP response
    • there are specific HTTP headers that needdto be set in the response from the server to allow the origins to access resources from the server.
    • Access-Control-Allow-Origin/Methods/Headers/Credentials

  17. traceroute

    • traceroute/tracepath uses UDP(User Datafram Protocol) so it is one way.

  18. nslookup

    • used mainly to resolve domain → IP or vice versa

  19. Check for DB connection

    • telnet db.ip 3306
    • connect to db using psql/mysql
    • ping db.ip

  20. Purpose of ARP(Address Resolution Protocol)

    • maps an IP to a MAC(Media Access Control) address on a local network.

  21. TCP

    • SSH, FTP, SMTP uses TCP
    • DHCP, ICMP does not use TCP

  22. SFP

    • SFP(Sender Policy Framework) is a DNS-based email authentication protocol used to help prevent email spoofing and phshing attacks.

  23. MX(Mail Exchanger)

    • responsible for accepting incoming email for a domain.

  24. NTFS ;;;

  25. Layer responsible for encryption and decryption

    • Transport Layer(4), Network Layer(3), Application Layer(7)

Database

  1. DDL

    • CREATE, ALTER, DROP, TRUNCATE, COMMENT, RENAME, GRANT/REVOKE

  2. RDBMS

    • designed with transactional processing in mind
    • ACID (Atomicity, Consistency, Isolation, Durabiolity) transactions to ensure data integrity and reliability.

  3. NoSQL

    • typically use key-value stores
    • non-relational Database

  4. Secondary indexing

    • technique that creates an additional index structure for a non-primary key column
    • should be a 유일성 + 최소성 == candidate key field

  5. Querying 500 people from over 200 countries

    • partition with low volume of data and Bucket data

  6. Keys

    • super key : 유일성
    • candidate key : 유일성 + 최소성
    • primary key : 유일성 + 최소성
    • foreign key : there can be multiple FKs

  7. DB caching types

    • Intergrated caches
    • Distributed caches : on multiple nodes
    • Remote caches : to share cached data
    • Local caches : to reduce latency

  8. RDBMS the adequate normal form.

    • the most commonly aimed-for normal forms : 1NF, 2NF, 3NF

  9. Role of a parser in the query lifecycle

    • a crucial role in the initial stages of query processing
    • Syntax Analysis

  10. Both runtime skewness and ata skwness have impact on query performances.

  11. which table columns should be avoided when distributing table data

    • foreign key : often used as join keys.
    • join column : can lead to data skew(uneven data distributions)
    • columns with skewed data
      ※ PK is usually good.
      so having duplicates in the distributing column is not good.

  1. DW

    • maintaining de-normalized tables in a DW is recommended for improved query performance, aggregation efficiency, reduced query complexity.
    • Catesian product joins(cross joins) are not used in a DW.

  2. Processing order

    • Verify Syntax → Create Query Plan → Acquire Locks

  3. Buffer(DB Cache)

    • minimizes the number of block transfers between the disk and memory in a DB system.

Web

  1. A Three-Tier Architecture

    • Presentation Tier (Front-end)
    • Application Tier (Middle)
    • Data Tire (Back-End)

  2. HTTP Status codes

    • HTTP 3XX : indicated client request was redirected.
    • 503 : Server Overload/Server Failed
    • 443 :
    • 500 : Internal Server Error
    • 400 : Bad Request
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