Code • Systemy Operacyjne (Informatyka)

Semaphores in SHM#

make sem_shm
./sem_shm

Source: sem_shm.c

Named semaphores#

Example presents parallel data producers: sensor processes which periodically generate files picked up by a single aggregator process.

graph LR
    S1[Sensor 1]
    S2[Sensor 2]
    S3[Sensor 3]
    subgraph Filesystem
    F1[sensor_1.dat]
    F2[sensor_2.dat]
    F3[sensor_3.dat]
    end
    A[Aggregator]

    S1 == write() ==> F1
    S2 == write() ==> F2
    S3 == write() ==> F3
    
    F1 == read() ==> A
    F2 == read() ==> A
    F3 == read() ==> A
    
    A == write() ==> stdout

make sensor aggregator
./aggregator 3

Source: sensor.c aggregator.c

./sensor 0 &
./sensor 1 &
./sensor 2 &
wait

See into the Linux debug fs:

ls -l /dev/shm/sem.can_overwrite /dev/shm/sem.data_ready

xxd /dev/shm/sem.can_overwrite

Bounded Buffer Problem#

make bounded_buffer_sem
./bounded_buffer_sem

Source: bounded_buffer_sem.c

Shared mutexes#

make record_db_setup
./record_db_setup

make record_db_worker
./record_db_worker 0 10000

Simulate load:

time {
for i in {1..1000}
do ./record_db_worker $((i % 5)) 10000 & 
done
wait
}

Compare timing with global locking strategy:

make record_db_setup_global
./record_db_setup_global

make record_db_worker_global
time {
for i in {1..1000}
do ./record_db_worker_global $((i % 5)) 10000 & 
done
wait
}

Mutex attributes#

make mtx_attributes
./mtx_attributes

Try changing mutex type to PTHREAD_MUTEX_ERRORCHECK and then PTHREAD_MUTEX_RECURSIVE.

Robust mutexes#

Note that too many steps in the record worker cause stack overflow:

make record_db_setup record_db_worker
./record_db_setup
./record_db_worker 0 100000

This leaves the shared memory block corrupted:

./record_db_worker 0 1000

Other records are ok (at least for now):

./record_db_worker 1 10000

Use robust mutexes to detect owner dead condition:

make record_db_setup_robust record_db_worker_robust
./record_db_setup_robust
./record_db_worker_robust 0 100000
./record_db_worker_robust 0 1000

Proceed with the real test (chaos engineering!):

for i in {1..1000}
do ./record_db_worker_robust $((i % 5)) $(( RANDOM % 10 == 0 ? 100000 : 10000 )) 
done
wait

Conditional Variables#

make bounded_buffer_cv
./bounded_buffer_cv

make readers_writers_cv
./readers_writers_cv