[concurrency-interest] Relativity of guarantees provided by volatile

David Holmes davidcholmes at aapt.net.au
Fri Aug 17 18:58:56 EDT 2012


Hi Marko,

I think the "surprise" is only in the way you formulated this. Said another
way a write takes a finite amount of time from when the instruction starts
to execute to when the store is actually available for a read to see.
(Similarly a read takes a finite amount of time.) So depending on those two
times a read and write that happen "around the same time" may appear to have
occurred in either order. But when you program with threads you never know
the relative interleavings (or should never assume) so it makes no
difference how the program perceives the order compared to how some external
observer might perceive it.

As for your optimization to "chunk" volatile writes, I don't see a problem
here if you are basically asking if given:

w = 1;  // w is volatile
w = 2;
w = 3;

that this could be reduced to the last write alone? I see no reason why not.
Without some additional coordination between a reader thread and the writer
thread, reading w==3 is a legitimate outcome. If you are thinking about how
the hardware might chunk things then that is a different matter. We have to
use the hardware in a way that complies with the memory model - if the
hardware can't comply then you can't run Java on it.

David Holmes
------------

> -----Original Message-----
> From: concurrency-interest-bounces at cs.oswego.edu
> [mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of Marko
> Topolnik
> Sent: Saturday, 18 August 2012 7:24 AM
> To: concurrency-interest at cs.oswego.edu
> Subject: [concurrency-interest] Relativity of guarantees provided by
> volatile
>
>
> Consider the following synchronization order of a program
> execution involving a total of two threads, R and W:
>
> - thread R begins;
>
> - thread R reads a volatile int sharedVar several times. Each
> time it reads the value 0;
>
> - thread R completes;
>
> - thread W begins;
>
> - thread W writes the sharedVar several times. Each time it
> writes the value 1;
>
> - thread W completes.
>
> Now consider the wall-clock timing of the events:
>
> - thread R reads 0 at t = {1, 4, 7, 10};
> - thread W writes 1 at t = {0, 3, 6, 9}.
>
> As far as the Java Memory Model is concerned, there is no
> contradiction between the synchronization order and the
> wall-clock times, as the JMM is wall-clock agnostic. However, I
> have yet to meet a single Java professional who wouldn't at least
> be very surprised to hear that the specification allows this.
>
> I understand that the SMP architecture that dominates the world
> of computing today practically never takes these liberties and
> makes the volatile writes visible almost instantaneously. This
> may change at any time, however, especially with the advent of
> massively parrallel architectures that seem to be the future. For
> example, an optimization technique may choose to chunk many
> volatile writes together and make them visible in a single bulk
> operation. This can be safely done as long as there are no
> intervening read-y actions (targets of the synchronizes-with
> edges as defined by JLS/JSE7 17.4.4).
>
> Now, my questions are:
>
> 1. Is there a loophole in my reasoning?
>
> 2. If there is no loophole, is there anything to worry about,
> given that practically 100% developers out there consider as
> guaranteed something that isn't?
>
>
> -Marko
>
>
>
>
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