Optimizing accesses to read-mostly volatile variables

Abstract

A computer-implemented method, computer program product, and computer processing system are provided for eliminating a memory fence for reading a read-mostly volatile variable of a computer system. The read-mostly variable is read from more than written to. The method includes writing data to the read-mostly volatile variable only during a Stop-The-World (STW) state of the computer system. The method further includes executing the memory fence in any mutator threads and thereafter exiting the STW state. The method also includes reading the read-mostly volatile variable by the mutator threads without executing the memory fence after the STW state.

Description

BACKGROUNDTechnical Field[0001]The present invention generally relates to memory devices, and more particularly to optimizing accesses to read-mostly volatile variables.Description of the Related Art[0002]A memory barrier, also known as a memory fence, is a type of barrier instruction that causes a processor or compiler to enforce an ordering constraint on memory operations issued before and after the barrier instruction. Essentially this means that operations issued prior to the memory barrier are guaranteed to be performed before operations issued after the barrier.[0003]In the Java® memory model, accesses to volatile variables are used as a kind of memory barrier that controls the visibility of data in memory. For example, if a thread-A writes data to a volatile variable v and another thread-B reads the latest value of the volatile variable (=1), a Java® semantic guarantees that thread-B reads the latest values of all data written to memory by thread-A before it writes the data t...

AI Technical Summary

Benefits of technology

[0007]According to another aspect of the present invention, a computer program product is provided for eliminating a memory fence for reading a read-mostly volatile variable of a computer system. The read-mostly variable is read from more than written to. The computer program product includes a non-transitory computer readable storage medium having program instructions embodied therewith. The program instructions are executable by a computer to cause the computer to perform a method. The method includes writing data to the read-mostly volatile variable only during a Stop-The-World (STW) state of the computer system. The method further includes executing the memory fence in any mutator threads and thereafter exiting the STW state. The method also includes reading the read-mostly volatile variable by the mutator threads without executing the memory fence after the STW state.

Problems solved by technology

The preceding semantic causes a large overhead on a POWER platform because reading volatile variables requires a memory fence.

However, there is an Application Programming Interface (API) to change it, so the variable needs to be volatile.

Since the API to get the current security manager (System.getSecurityManager( )) is frequently called, computer performance is detrimentally affected by the overhead of the memory fence.

However, accessing such variables still need a memory barrier because they are volatile, even though their values are almost runtime constants.

Images