UNIX Concurrency Mechanisms

Signals

💡 유저 레벨의 핸들링 메카니즘이라고 쉽게 말할 수 있다.
A software mechanism that informs a process of the occurrence of asynchronous events (similar to a hardware interrupt)

✔️ Sending a signal

• Kernel sends (delivers) a signal to a destination process by updating some state in the context of the destination process
• Kernel sends a signal for one of the following reasons:
  • Kernel has detected a system event such as divide-by-zero (SIGFPE) or the termination of a child process (SIGCHLD)
  • Another process has invoked the kill system call to explicitly request the kernel to send a signal to the destination provess

✔️ Receiving a signal

• A destination process receives a signal when it is forced by the kernel to react in some way to the delivery of the signal
• Two possible ways to react
  • Default action (ignore, terminate the process, terminate & dump)
  • Catch the signal by executing a user-level function called a signal handler
• Akin to a hardware exception handler being called in response to an asynchronous interrupt

Signal Concepts

✔️ A Signal is pending if it has been sent but not yet received

• There can be at most one pending signal of any particular type
• Signals are not queued
  • If a process has a pending signal of type k, then subsequent signals of type k that are sent to that process are discarded

✔️ A process can block the receipt of certain signals

• Blocked signals can be delivered, but will not be received until the signal is unblocked

✔️ Kernel maintains pending and blocked bit vectors in the context of each process

Memory Management

Terminology

💡 Frame
A fixed-length block (unit) of main memory

💡 Page
파일의 고정적인 크기의 물리적 구성 단위
A fixed-length block of a program. It is a unit of transfer between secondary storage and main memory

💡 Segment
파일의 가변적인 크기의 논리적 구성 단위
A variable-length block of a program. An entire segment may be copied into main memory (segmentation) or the segment may be divided into pages which can be individually copied into main memory (segmented paging)

Memory Organization

✔️ Logical Organization

• Users view programs as a collection of modules
  • modules can be written and compiled independently
  • different protection (read-only, execute-only) are given to different modules
  • module level sharing corresponds to the user's way of viewing the problem
• Segmentation is the tool that most readily satisfies requirement

✔️ Physical Organization

• Computer memory is organized as main memory and secondary memory
• Main memory is organized as a linear array of bytes
  • main memory available for a program may not be sufficient
  • programmer does not know how much space will be available

✔️ Memory management technique involve

• Paging, segmentation, overlaying, and virtual memory, etc
• Overlaying allows various modules to be assigned the same region of memory with a main program responsible for siwtching the modules in and out

Memory partitioning

가상메모리 이전에 메모리 관리 기법 Memory partitioning 에 대해 살펴본다.

✔️ Fixed partitioning

• Physical Memory를 고정적인 크기의 파티션으로 시스템이 생성될 때 쪼개는 방법이다. 한 프로세스는 자신의 이미지보다 작거나 큰 파티션에 로딩되는데 이 방법은 구현이 쉽지만 꽤 비효율적이다.
• Problem
  • Internal Fragmentation : wasted space inside a fixed partition (낭비되는 공간)
  • 파티션 수에 따라 실행가능한 프로세스의 수 (degree) 가 정해져있다.

✔️ Buddy System

• Use both fixed and dynamic partitioning (절충안)

✔️ Dynamic partitioning

• 다양한 크기를 가질 수 있는 프로세스에 맞출 수 있다.
• 프로세스 이미지에 따라서 프로세스가 요청한 만큼 할당한다. 파티션 수에 따라 실행가능한 프로세스의 수가 유동적으로 변한다.
• Problem
  • External Fragmentation (hole) : Memory becomes more and more fragmented
• Compaction
  • Technique to overcome External Frangmentation
  • It is a overhead time consuming process, wasting CPU time

💡 Process Image
프로그램 코드, 데이터와 순간적 process 상태(레지스터 내용, 프로세서 상태 워드, 기타) 의 집합이며, 일종의 스냅샷이라고 생각하면 될 듯하다. PCB 에는 이미지와 process context가 저장된다
(아래 그림 참고)

Paging

💡 Paging

• Partition memory into equal fixed-size chunks (page frames)
• Process image is divided into the same fixed-size chunks (pages)

💡 Page Table
어떤 페이지가 어떤 페이지 프레임에 로드되어있는지?

• Contains the mapping between pages and frames
  • For each page in the process, PTE (page table entry) contains the frame number
• Maintained by operating system for each process
• CPU must access the page table to generate a physical address for the current process
• TLB(MMU) 의 사용 이유?

Segmentation

💡 Segmentation

• A program is divided into variable-length sgments
• The address consists of segment number + offset
• No internal fragmentation
• But, external fragmentation
  • Similar to dynamic partitioning

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🔗 Reference

프로세스 이미지에 관해서 참고한 블로그
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