HPC 中的计算机系统¶

Introduction¶

重要概念：抽象

• Problem
• Algorithm (Code)
• Program (Executable)
• Runtime System
• ISA (Instruction Set Architecture)
• Microarchitecture
• Logic
• Circuits

Turing Complete on Steam

Operating System Basics¶

Abstraction of CPU¶

• Program counter (PC)
• Register: stores data processed by the CPU（寄存器）

Memory Hierarchy¶

Not all CPU operations are created equal

CPU 执行的不同指令，耗时差异很大！

What is an OS?¶

Operating System is a resource abstractor and resource allocator.

• Event 事件
• Handler 处理器（根据事件的类型选择）

OS Events¶

Traps¶

• Interrupts（中断）: 外部异步事件
  • Hardware-generated
  • e.g. 鼠标点击、硬盘拔出
• Exceptions（异常）: unusual condition occurring at instruction run time
  • Software-generated
  • e.g. division by zero

Trapped Control Flow¶

表面上是执行了不同位置的程序，实际上改变了 program counter (PC) 的值

System Calls¶

When a user program needs to do something privileged, it calls a system call. A system call is a special kind of trap.

strace ./hello

Timers¶

一上来就一个 while True 的死循环，别的程序不就都没法运行了！？

• The OS must keep control of the CPU
  • Programs cannot get stuck in an infinite loop and lock up the computer
  • Programs cannot gain an unfair share of the computer
• One way in which the kernel retrieves control is when an interrupt occurs.
• To make sure that an interrupt will occur reasonably often, the OS uses a timer.

定期制造中断：定时器

Process¶

A process is a program in execution, a unit of resource allocation and protection.

一个进程不能越权访问其他进程的内存！

$ file hello
hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, ...

ELF: Executable and Linkable Format, 被分成一个个 section

Process Address Space¶

• In Memory (Unique Address Space)
  • Code: Text section, initially stored on disk
  • Data Section:
    • BSS: uninitialized data 全局变量，未初始化
    • Data: initialized data 全局变量，已初始化
  • Stack: Function frames, local variables, etc, grows downwards
  • Heap: Dynamically allocated memory, grows upwards
• Context
  • Program Counter: Points to the next instruction to execute (i.e. an address in the code)
  • Content of the process's registers

Stack Frame¶

存放函数调用的参数、局部变量、返回地址等

• sp (stack pointer) 指向栈顶
• fp (frame pointer) 指向当前函数的栈帧底部

在 HPC 的 profiling 中，perf 工具通过分析栈帧来提供函数调用的性能数据。

#include <stdio.h>
#include <stdlib.h>
int x;
int y = 15;
int main(int argc, char *argv[]) {
    int *values;
    int i;
    values = (int *)malloc(sizeof(int) * 5);
    for (i = 0; i < 5; i++) {
        values[i] = i;
    }
    return 0;
}

Process Schedule¶

• Non-preemptive: 非抢占式调度
• Preemptive: 抢占式调度，the OS can interrupt a running process to schedule another one.
  • Round Robin: 时间片轮转
  • Priority-based: 优先级调度
• Also decide which process to run on which core. (Control by Core Affinity, 绑核)

Process Switch¶

• Save the context of the current process 保存上下文

Inter-Process Communication (IPC)¶

• Message Passing: We use MPI to send / receive messages.
• Shared Memory, Signals, Pipes, Sockets, etc. 都有缺点！

Disadvantages of IPC: It's too heavy!

Thread¶

• A thread is a basic unit of execution within a process.
  • Thread ID
  • Program counter
  • Register set
  • Stack
• Shares the following with other threads within the same process:
  • Code section
  • Data section
  • Heap (dynamically allocated memory)
  • Open files and signals
• Concurrency: A multi-threaded process can do multiple things at once.

Virtual Memory¶

Virtual memory is another abstraction layer between process and physical memory.

Paging¶

• Page and Frame
  • Physical memory is split into fixed-size frames.
  • Virtual memory is split into fixed-size pages.
• Memory Mapping
  • Hardware support: MMU (Memory Management Unit)
  • Function: Translate virtual page to physical frame
  • Data structure: Page Table
• Page Fault
  • If a page is not in physical memory, a page fault occurs.
  • The OS will load the page from disk to physical memory, and update the page table.

cat /proc/self/maps