Udacity/ud923/Intro to Operating System

学习纪录

L20 / Datacenter Technologies

• multi-tier architectures for internet services
  
  • homogeneous architecture: each node can do any processing step, so that it keeps the front-end simple
    
    
  • heterogeneous architecture: different nodes/tasks/requests/data don't uniformly be accessed
    
    
    
  
  
• cloud computing benefits
  
  • capacity scales elastically with demand
    
    
  • scaling in instantaneous, both up and down
    
    
  • cost is proportional to demand to revenue opportunity
    
    
    
  
  
• separation of responsibilities
  
  • infrastructure as a service: Amazon EC2
    
    
  • platform as a service: Google App Engine
    
    
  • software as a service: gmail
    
    
    
  
  
  

L12 / Memory Management(这章有点复杂，尤其是在计算page table时)

* How does OS manage virtual/physical memory
    - Allocation
        - allocate/replace spaces between virtual and physical memory
        - For page based management, it means that allocate pages(virtual) to page frames(physical)
    - Arbitration
        - address translation and validate address
        - For page based management, it's implemented by page tables
        - For segment based management, it's implemented by segment registers
    - OS has hardwares support in CPU package called MMU
* Memory management mechanisms
    - Page based mechanism
        - physical/virtual pages have the same size, so that they only stores the beginning entry point and its offset
        - For the purpose of validation, it also stores the validation bits called flags, such as the right of writing.
        - Multi-level pages helps better utilize the space in physical memory since a process will not fully used the virtual memory it gets.
        - When pages should be swapped out
            - when memory usage is above a threshold
            - when cpu usage is below a threshold
        - Which pages should be swapped out
            - pages that won't be used by using policies such as LRU
            - pages that won't be written out
    - Segment based mechanism
        - It will have un-fixed memory size like page based does.
        - It's often used with pages.
* Memory Allocator
    - It's used to determine VA to PA mapping
    - It has different levels, kernel-level and user-level.
        - For kernel-level allocator, it allocates memory for kernel including kernel states and static process state
        - For user-level allocator, it allocates memory for user-level process including dynamic process state(heap), malloc/free
    - One memory allocation challenge is aggregation of free memory(简单来说，就是如何有效合并闲置的小记忆区块成一个大的区块)
        - buddy allocator: it divides memory into the power of 2 memory size
        - slab allocator: it uses internal fragmentation to create custom size for different purposes

L15 / IO Management

* OS support for I/O devices
    - Basic devices features
        - control registers: command, data transfers, and status
        - micro-controller: cpu
        - on device memory
        - other logic: A/D converters
    - devices are connected through peripheral component interconnect(PCI)
    - access device register is equal to memory load/store
    - There are two paths a device can use to reach CPU
        - Interrupt
            - pros: can be generated as soon as possible
            - cons: interrupt handling steps
        - Polling
            - pros: it's convenient for OS
            - cons: delay or CPU overhead
        - Typical device access: a user process -> kernel -> driver -> device
    - virtual file system can solve the following three challenges
        - files are on more than one device
        - devices work better with other file systems implementations
        - files are not on a local device
    - virtual file system abstractions on linux
        - file: an element on which the VFS operate
        - file descriptor: OS representation of a file
        - inode: persistent representation of a file, "index"
        - dentry: directory entry
        - superblock: file system specific information regarding the layout

这个是不是没有作业？
感觉咋样？
是好课吗？

基本要求知識點：
1. C programming
2. Command line usage
3. Using makefiles

wowmomsos 发表于 2019-1-8 21:21
这个是不是没有作业？
感觉咋样？
是好课吗？

似乎没有作业 只有一些内容的问答。
还没看完 但感觉蛮基础 楼主有推荐关于OS的进阶课程吗

L3: Introduction to OS
Take way notes:
* OS 可被定义为三个部分，abstraction(such as process management...), mechanism(such as allocate memory...), and policies(such as least recently accessed).
* OS 是介于application(such as browser...)和physical device(such as hard drive..)中间的介质

L4: Process and Process Management
Take way notes:
* What is a process: 它是一个程式执行中的状态，包含了变数，指令计数...
* How are processes represented by OS:
    - 它被表示成一个记忆区块(virtual address space)，包含 text, data, stack, and heap
    - OS使用process control block(PCB, 一种数据结构)去管理processess
    - process有两种开启的基本方式，fork(创建和parent process相同的context)和exec(创建全新的context)
* How are multiple concurrent processes managed by OS:
    - OS使用cpu scheduler来管理process的运作，包含：preempt(中断), schedule(排成), and dispatch(发包)
    - process之间的互动称为inter process communication(IRP)

L5: Threads and Concurrency(这章节内容有点多, 主要建立于[https://s3.amazonaws.com/content.udacity-data.com/courses/ud923/references/ud923-birrell-paper.pdf]"An Introduction to Programming with Threads" by Birrell[/url])
Take way notes:
* What are threads: 他是一个工作中的process，它帮助更有效率使用CPU的计算能力
* How threads different from processes:
    - thread可以分享virtual spaces
    - process花更多时间context switch
    - thread通常有hot cache
* What data structures are used to implement and manage threads:
    - thread data structure
    - mutex 避免资源冲突
    - condition variables 提供mutex可以在指定情况下产生或解除
    - critical section structure 使用资源mutex的proxy，避免过度保护资源
    - fork(proc, args)
    - join(threads)
* Common pitfalls:
    - spurious wakeup 提早唤醒其他threads
    - dead locks 资源冲突
* threads design patterns
   - boss/workers
   - pipeline
   - layered

L6: Threads case study(这章蛮棒的，简单易懂，并且可以让前一章的内容更清晰)
Take way notes:
* What is PThreads: 它是POSIX(portable operating system interface) Threads的简称
* 相关数据结构：
    - thread data structure: pthread_attr_t
    - mutex: pthread_mutex_t
    - condition variable: pthread_cond_t
    - wait mechanism: pthread_wait_cond
* mutex小技巧：
    - 分享的资料需要记得使用同一个mutex，避免dead lock
    - mutex的scope必须分享给所有的threads
    - 将所有mutex使用全域性排列
    - 永远记得解除mutex
* condition variable小技巧:
    - 当条件被更动时，记得通知所有在等候的threads
    - 当不清楚使用signal/broadcast时，可先使用brodcast处理，但要记得会有效能损失(spurious wakeup)
    - 在使用signal/broadcast时，不一定需要mutex

想问楼主为什么花时间在理论课程里面? intro to OS 应该都是理论，没有实际的project 和coding practice， 在时间紧的情况之下会效果不明显?

hengde12 发表于 2019-1-15 00:34
想问楼主为什么花时间在理论课程里面? intro to OS 应该都是理论，没有实际的project 和coding practice，  ...

这来自于K大最近这篇帖子，https://www.1point3acres.com/bbs ... p;extra=&page=1

因为想往Machine Learning Engineer的方向前进，根据K大目前帖子里提及的技能应该是要有Operating System的理论知识，所以开始修了这门课。另外一个原因是直接啃Modern Operating Systems这本，实在晦涩难懂

如果有更应用导向的OS课程，还烦请推荐，谢谢