CSC/ECE 506 Spring 2012/ch9a cm
Introduction
Different Mutex solutions and Overheads
Software Overhead
Reducing the overhead - solutions
Thick locks
Thin locks
Language supported synchronization leads to inefficiency due to the useless synchronization requirements due to the thread safe nature of the Java Libraries. The necessity of locking introduces the use of heavy weight objects. In Java, synchronization is introduced even when there is no requirementfor synchronization. This causes slowdowns of upto a factor of 2.In Sun JDK implementation, the monitors are placed outside the objects and are looked up from monitor caches.This is highly inefficient and requires locking of the monitor caches to avoid race conditions. It also leads to considerable increase in space overhead due to the size of the monitor structures. This problem is solved by using a new algorithm that requires fewer machine instructions and lesser number of bits per lock.
Thin Locks are an implementation of monitors in IBM's version of the JDK 1.1.2 for AIX. It has the following desirable characteristics over thick locks.
- In the absence of contention, initial and nested locking performance is much better. There is a significant improvement in performance even in th presence of contention, as compared to the JDK implementation.
- Only 24 bits in an object are used for locking.These 24 bits are obtained using encoding techniques on the other values stored in the header.`
- The structure of the 24 bit lock has been engineered to make the most common locking and unlocking functionalities to be executed with minimal machine instructions.
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