1. Semaphore and Multithreading
-Sagar Panchariya

2. Semaphore A hardware or software flag.
A Semaphore is a variable with a value that indicates the status of a common resource.
It's used to lock the resource that is being used. A process needing the resource checks the semaphore to determine the resource's status and then decides how to proceed

3. Resource Semaphore Load data(signal); Wait(signal); Process(data);)
Buffer
I/O Device
Start processing
Load data

4. Multithreading
The ability to execute different parts of a program, called threads, simultaneously.
Different parts of a program may be:
I/O operations
Data Processing
Advantages:-
Multithreading, when done correctly, offers better utilization of processors and other system resources.

5. Multithreading Problems to be taken care of: Race conditions:
Two or more threads try and update the same resource <variable> without proper synchronization.
Solution:
Use of semaphores, to bring in synchronization among threads.

6. C” Programming example
Mono Memory
Mono Memory
Input Data
Read Sem
async_memcpym2p
Read Sem
async_memcpyp2m
Output Data
Write Sem
SRC_addr
DST_addr
96 PE Array Memory
Background buffer
Elements[0]
Active buffer
Elements[1]

7. 1. Read into the active buffer.
2. Wait for the background buffer to be empty then read the next chunk into it.
3. Wait for the active buffer to be full then process the contents.
4. Write out the active buffer.
5. Swap the active and background buffers.
6. Repeat from 2 until all chunks are processed Wait for the write of the last active buffer to finish then print the results to the console.

8. Two semaphores are used:
one to synchronize reading a buffer and processing that data;
the second to synchronize writing the data with refilling the buffer
Code snapshot:--
async_memcpym2p(READ_SEMAPHORE_a, &element[active], src_addr, sizeof(float));
sem_wait(READ_SEMAPHORE_a);
//do the processing on all data in parallel element[active] *= scale;
async_memcpyp2m(WRITE_SEMAPHORE, dst_addr, &element[active], sizeof(float));

9. Semaphore debug commands
sem help
To give description of all semaphore commands
sem display all
Displays all semaphore
sem display allval
Display all semaphores with value
Sem display <semaphore no>

10. Thanks