1. Shared Memory David Ferry, Chris Gill
CSE 522S - Advanced Operating Systems
Washington University in St. Louis
St. Louis, MO 63130

2. CSE 522S – Advanced Operating Systems
The Common Theme So Far?
What we have seen…
Signals:
Kernel carries/delivers event notifications
Pipes/FIFOs:
Intra-machine communication
Many-to-many
Kernel reads/writes through the pipe file system
Sockets:
Inter/Intra-machine communication
Point-to-point
Kernel reads/writes through networking system
CSE 522S – Advanced Operating Systems

3. Kernel’s Involvement in IPC
Saves data into kernel address space
Delivers data from kernel address space
Sleeps and wakes processes that block on communications channels
Delivers asynchronous notifications
Useful, but incurs overhead!
Multiple copies of data
Event delivery timing at mercy of kernel
Access requires system calls
CSE 522S – Advanced Operating Systems

4. Shared Memory Concepts
Processes share a physical memory region:
Communication by modifying memory values
Kernel only invoked at creation & deletion
No read or write overhead vs. regular memory
0x1000
Process A
shm_ptr 1 2 3 5 8 13
Process B
shm_ptr
CSE 522S – Advanced Operating Systems

5. CSE 522S – Advanced Operating Systems
Recall - Paging
All physical memory is divided into pages
Pages get mapped to virtual memory
Virtual addresses require translation
Virtual Address Translation:
Process A
Process B
Virtual Address
Page #
Offset
4KB
4KB
4KB
4KB
4KB
4KB
Physical Page
Phys.
Addr
CSE 522S – Advanced Operating Systems

6. Efficient Shared memory
Same address translation as usual (little or no overhead)
Addresses map to same physical page
Page must be readable/ writable by both processes
Process A
Virtual Address
Page #
Offset
Process B
Virtual Address
Page #
Offset
Physical Page
CSE 522S – Advanced Operating Systems

7. CSE 522S – Advanced Operating Systems
Shared Memory in Linux
Three steps to use shared memory:
Create a shared file descriptor - shm_create()
Resize shared region - ftruncate()
Map region into process - mmap()
Shared regions identified by name
Creator can specify user-level permissions
CSE 522S – Advanced Operating Systems

8. CSE 522S – Advanced Operating Systems
Using Shared Memory
Basic interface: void* pointer
Programmer must impose organization on shared memory region:
Structs
Libraries
Provides multi-threaded-like environment
Concurrency libraries may or may not work when shared across processes
E.g. Pthreads and PTHREAD_PROCESS_SHARED
May need own concurrency code
CSE 522S – Advanced Operating Systems