I started reading Tanenbaum's Distributed Systems book a while ago. I read about two phase locking and timestamp reordering in transactions chapter. While having a deeper look from google I heard of lightweight transactions/lightweight transactional memory. But I couldn't find any good explanation and implementation. So what is lightweight memory? What are the benefits of lightweight locks? And how can I implement them?
1 Answer
The term "lightweight," when it is applied to locking and transactions, is generally used to indicate a locking or transaction scheme that has low overhead. Low overhead means that your computer is chewing up less CPU time on housekeeping tasks, and spending more time doing actual work.
The term "lightweight," when applied to memory locking and transactions, is probably referring to Software Transactional Memory (STM). Some languages, like Clojure, have STM built in.
Unlike the locking techniques used in most modern multithreaded applications, STM is very optimistic: a thread completes modifications to shared memory without regard for what other threads might be doing, recording every read and write that it is performing in a log. Instead of placing the onus on the writer to make sure it does not adversely affect other operations in progress, it is placed on the reader, who after completing an entire transaction verifies that other threads have not concurrently made changes to memory that it accessed in the past. This final operation, in which the changes of a transaction are validated and, if validation is successful, made permanent, is called a commit. A transaction may also abort at any time, causing all of its prior changes to be rolled back or undone.
The benefit of this optimistic approach is increased concurrency: no thread needs to wait for access to a resource, and different threads can safely and simultaneously modify disjoint parts of a data structure that would normally be protected under the same lock.
answered Oct 24, 2013 at 17:39
