1. Exercise 6 from Gary Nutt’s Kernel Projects for Linux
Joe Malta
Kris Wright
John Bird
Jason Gallick
Tom Fortmuller

2. Overview of Presentation
Review shared memory
Explain the steps we took to complete our project
Discuss the problems we encountered

3. Shared Memory Basics
Part of InterProcess Communication (IPC) mechanism
Developed so that processes could easily communicate with each other and work together
Each process is allocated a certain amount of virtual memory when created, but only a small part of this is initially used. The rest can be used to hold data.

4. Shared Memory Basics (con’t)
A block of main memory may be defined as a block of shared memory
Any process may map the block of shared memory to its own virtual address space
Processes communicate by reading from and writing to the shared memory block. When one process changes information in the shared block of memory, all processes have access to the changed data.
Segments of program code that change data in shared memory is called the critical section

5. Shared Memory Basics (con’t)

6. Project Implementation
Understanding the Project requirements

7. Project Implementation (con’t)
Understanding the Project requirements
Learning methods of shm.c

8. Functions of Shm.c Manages the shared memory of Linux
Important functions: find_key, newseg, shm_get, shm_init
Array based

9. Shm.c Variables and more about Shm_get
SHMNI and SHMALL are constants, IPC_UNUSED and IPC_NOID are flags
Shm_get could be the most important function in shm.c
Heavily dependant on newseg and find_key

10. Project Implementation (con’t)
Understanding the Project requirements
Learning methods of shm.c
Developing a program to test shared memory

11. Importance of Test Program
Showed how shared memory works on a Linux machine
Made sure that shared memory was being recycled
Proved that linked list was functioning properly
Compared results from array based shm.c file and linked list shm.c file

12. Project Implementation (con’t)
Understanding the Project requirements
Learning methods of shm.c
Developing a program to test shared memory
Changing shm.c from array to linked list

13. Modifying the Shm.c File
Additions required
struct shm_list
Pointer to next element in list
Pointer to shared memory data structure
struct shmid_kernel *findseg(int id)
Returns idth shared memory struct in list
int newshm(void)
Appends a new node to the end of list
void setseg(int id,struct shmid_kernel *value)
Sets idth shared memory struct in list to value

14. Modifying the Shm.c File (con’t)
List Initialization
Create head of list
Original code replacement
Replace variable = shm_segs[x] statements with variable = findseg(x)
Replace shm_segs[x] = value with setseg(x,value)

15. Project Implementation (con’t)
Understanding the Project requirements
Learning methods of shm.c
Developing a program to test shared memory
Changing shm.c from array to linked list
Compiling shm.c

16. Problems - Compiling
To properly compile the kernel the following environment variables must be passed to gcc
<gcc –D__KERNEL__ -I/user/src/linux/include –Wall
–Wstrict-prototypes –02 –fomit-frame -pointer –fno-strict -aliasing –pipe –03 –fomit-frame-pointer –fno-exceptions –fno -rtti –pipe –s –mpentium –mcpu=pentium –march=pentium –ffast-math –fexpensive optimizations -m486 –malign-loops=2 –malign-jumps=2
–malign-functions=2 –DCPU=586 -c –o shm.o shm.c>
These variables were found by redirecting outputting from make to a text file, and GREPing the text file for shm.c

17. Problems (con’t) - Kernel
We were working on a Linux Red Hat 9 machine, kernel version Gary Nutt’s text refers to Linux Mandrake 7, kernel version The two versions are considerably different.
We installed Mandrake 7, kernel version 2.2, which is current with Gary Nutt’s Kernel Projects for Linux

18. End