Shared memory #

Which variables are shared? #

• Programmer decides
• All memory potentially shareable (in process)
• Stack locals: private
• Globals: shared
• Pointers stored in globals: shared
• Pointers passed as arguments when instantiating threads: shared

void my_func(int x) {
    int y; // private
    Pool p; // private
}

Pool p2; // public

Thread safety #

• Refers to a class designed for shared use
• std::vector is not thread-safe
• For thread-unsafe classes: must manually lock externally

Condition variables #

std::condition_variable

Wait for particular state:

wait(lock): atomically releases lock, blocks thread (reacquire lock before returning). On return condition may not exist

notify_one(): if any waiting threads, wake up the first one

notify_all(): wake all waiting threads

Using locks #

How many locks? #

• As few as possible
• One lock for program doesn’t allow for concurrency
• Kernel-call lock causes contention
• Lock per variable is complicated, deadlock-prone, and inefficient

Monitor: #

• Shared data structure (class instance)
• One lock
• More than 1 condition variable
• Every method of class:
  • Lock at beginning
  • Unlock at end