Concurrency control

Concurrency control

1. Mutual Excusion

   • Race condition
     • Multiple processes or threads read and write data items
     • The final result of a thread/process depend on the order of the processes.
   • Ensure that simultaneous updates to the same part of the memory cannot occur
   • Critical sections
     • Only one process at the time is allowed to access data
     • Implementations?
       • Software
         • Algotithms
           • Dekker's
           • Peterson's
           • Lamport's
           • Szymanski's
           • Taubenfeld's black-white bakery
       • Hardware
         • Uniprocessor
           • Disable interrupt
             • Making it non-preemptive
             • Cons?
               • it's non preemptive and cannot respond to interrupts
               • Slow processes can lock up the processor for a long time
         • Multi/uni-processor
           • Busy-waiting
             • Shared memory
             • Atomic test and set
               • Gets a value that indicates if the process is occupied
                 • if it isnt
                   • Set the value idecating that it's occupied
           • Compare and swap (CAS)
   • Critical sections
     • Definitions
       • Only one program at a time can riside reside inside a critical section
       • A section of code within a process that requires access to shared resources and that must not be executed while another process is in a corresponding section of code.

2. Deadlock

   • A situation in which two or more processes are unable to proceed because each is waiting for one of the others to do something
   • example?
     • 2 processes waiting for a message from each other

3. Starvation

   • A situation in which a runnable process is overlooked indefinetly by the scheduler; although it is able to proceed, it is never chosen.
   • Example?
     • A low priority process resides in a queue which is promoting altering higher priority processes rendering no process time for the lower priority process