Re: Peterson's Algorithm in java, sequencial instruction execution
?
Daniel Pitts wrote:
....
I think the reordering of instructions you are talking about is at the
machine code level. I don't know if Java's JVM will do this, but here
is what I would suspect.
....
There can be reordering at a level below machine code, as part of the
processor implementation. The JVM does not have direct control, but
processors have instructions for forcing extra ordering, and the JVM is
required to use them to achieve what the JLS specifies e.g. for volatile
data.
The mental model of a processor doing one instruction at a time in
program order, and doing all work for each instruction before going on
to the next one, used to be reasonably accurate, except for some
supercomputers. Now, it is a gross simplification for most processors.
Instead, think of the processor as a sort of factory, with a main
production line several instructions wide, but many specialized stations
where different processing gets done and instructions can wait if
something they need is not available. Some operations are shunted off
e.g. to a specialized production line for doing floating point. The work
of making a store globally visible, not just visible to this processor,
may go into another production line.
A processor can have many dozens of instructions at some stage in the
processing.
It's all designed to make it look, to code executing on the processor,
as though the processor is doing one instruction at a time in program
order. However, a thread on another processor monitoring the behavior of
shared data can see the order in which some things really happened.
Patricia