Transactional Memory

 

Transactional Memory is the concept of using transactions rather than locks to synchronise processes that execute in parallel and share memory.

At a very simplified level, to synchronise with locks you identify sections of code (called critical sections) that must not be executed simultaneously by different threads and acquire and release locks around the critical sections. Since each lock can only be held by one thread at a time, this guarantees that once one thread enters a critical section, all of the section's operations will have been completed before another thread enters a critical section protected by the same lock(s).

Transactional memory instead lets you designate sections of code as transactions. The transactional memory system (which can be implemented in hardware, software, or both) then attempts to give you the guarantee that any run of a program in which multiple threads execute transactions in parallel will be equivalent to a different run of the program in which the transactions all executed one after another, never at the same time.

The transactional memory system does this by allowing transactions to execute in parallel and monitoring their access to transaction variables. If the system detects a conflict between two transactions' access to the same variable, it will cause one of them to abort and "rollback" to the beginning of the transaction it was running; it will then automatically restart the transaction, and the overall state of the system will be as if it had never started the earlier run.