Week 12

Lecture recording here.

Lab recording here.

Introduction

This week we are going to look at memory mapping. We will look at process memory some more, and we will look at virtual memory. We will look at the various memory mapping schemes across processes.

Videos

Quiz

• Quiz 10 will be on memory mapping. Study chapter 49 of The Linux Programming Interface

Lecture Material

• Chapters 8 and 9 of Operating System Concepts
• Chapters 6, 49 and 50 of The Linux Programming Interface

Labs

• Lab 9 - Hacking RSA.
• Lab 10 - Work Package.

Assignment

Assignment 2.

Additional Lecture Material

An overview of virtual memory can be seen at:

Locations of shared memory, memory mappings, and shared libraries can be seen below:

Two processes with a shared mapping of the same region of a file can be seen at:

Sample Code

Banking Application

• A banking application, several of which can be run in parallel, for updating a bank account can be found at:
  BankApplication.h, BankApplication.cpp and Makefile.
  The common file that is mapped to all banking application processes can be found at BankDatabase.txt. Note that this is a binary file.
• Code that demonstrates mprotect can be found at:
  BankDatabase.txt, BankApplication.h, BankApplication.cpp and Makefile.
  
• Code that demonstrates mlockall, mincore and madvise can be found at:
  BankDatabase.txt, BankApplication.h, BankApplication.cpp and Makefile.
  
• Code that demonstrates anonymous mapping via /dev/zero can be found at:
  BankApplication.h, BankApplication.cpp and Makefile.
  This type of mapping does not work for unrelated processes. For related processes, it works. See:
  BankApplication.h, BankApplication.cpp and Makefile.
• Code that demonstrates anonymous mapping without any files (no database file) whatsoever can be found at:
  BankApplication.h, BankApplication.cpp and Makefile.
  This type of mapping does not work for unrelated processes. For related processes, it works. See:
  BankApplication.h, BankApplication.cpp and Makefile.

Transaction Applications

• Transactions between parent and child using anonymous mapping without any files can be found at:
  Transaction.h, Transaction.cpp and Makefile.
• Failed transactions between a parent and an exec'ed process using anonymous mapping without any files can be found at:
  Transaction.h, Parent.cpp, Child.cpp and Makefile.

Memory Mapping vs Shared Memory

Underneath it all, memory mapping and shared memory are basically the same. But memory mapping gives you much more flexibility. In week 9 we looked at code that simulates 16 clients trying to write to a radio with four channels, with shared memory. To do the same with memory mapping, we would make use of its many features to ensure the performance and operation are the same. See:



We would use semaphores for synchronization. We would use msync to ensure our data writes are synchronized with the memory. We would use mlock to ensure our memory is not swapped to disk. We would use mprotect to indicate read/write. We would use madvise to advise the kernel that this memory region will be very busy.