[concurrency-interest] On A Formal Definition of 'Data-Race'

Zhong Yu zhong.j.yu at gmail.com
Wed Apr 17 11:29:19 EDT 2013


On Wed, Apr 17, 2013 at 1:38 AM, Nathan Reynolds
<nathan.reynolds at oracle.com> wrote:
> Couldn't JIT hoist the non-volatile writes out of the loop?

Certainly, sorry if my statement sounds too absolute.

> For example, the following code...

But, is this a valid example? Can JMM really reorder around
System.out.println()?

> for (i = 0; i < 1_000_000_000; i++)
> {
>     System.out.println(i);
>     shared = 2 * i;
> }
>
> ... could be transformed into ...
>
> for (i = 0; i < 1_000_000_000; i++)
> {
>     System.out.println(i);
> }
>
> shared = 2 * 1_000_000_000;
>
> ... If so, then the non-volatile write may not happen for a very long time.
>
> Nathan Reynolds | Architect | 602.333.9091
> Oracle PSR Engineering | Server Technology
> On 4/16/2013 10:27 PM, Zhong Yu wrote:
>
> On Tue, Apr 16, 2013 at 8:51 PM, thurstonn <thurston at nomagicsoftware.com>
> wrote:
>
> Vitaly Davidovich wrote
>
> The code works as-is.
>
> Absolutely.  volatile is not needed for correctness
>
> Vitaly Davidovich wrote
>
> Why?
>
> Well, for performance reasons given the 'undefined/indefinite' visibility of
> #hash to other threads.
> At least according to the JMM (which has nothing to say about CPU cache
> coherency), it is *possible* that each distinct thread that invoked
> #hashCode() *could* result in a recalculation of the hash.
>
> In practice though, application threads contain very frequent
> synchronization actions, or other operations that force VM to
> flush/reload. So it won't take very long for any non-volatile write in
> one thread to become visible to other threads.
>
> Imagine a long-lived Map<String, ?>; and many threads accessing the map's
> keyset and for some unknown reason invoking #hashCode() on each key.
> If #hash was declared volatile, although there is no guarantee that #hash
> would only be calculated once, it is guaranteed that once a write to main
> memory was completed, every *subsequent* (here meaning after the write to
>
> In JMM though, we cannot even express this guarantee. Say we have
> threads T1...Tn, each thread Ti burns `i` seconds CPU time first, then
> volatile-reads #hash, and if it's 0, calculates and volatile-writes
> #hash which takes 100 ns. We can find no guarantee from JMM that
> there's only one write; it's legal that every thread sees 0 from the
> volatile read.
>
> Zhong Yu
>
> main memory) read no matter from which thread would see #hash != 0 and
> therefore skip the calculation.
>
>
>
> Vitaly Davidovich wrote
>
> String is too high profile (especially
> hashing it) to do the "naive" thing.
>
> Nothing wrong with being naive; naive can be charming.
>
>
> Vitaly Davidovich wrote
>
> Also, some architectures pay a
> penalty for volatile loads and you'd incur that each time.
>
> Fair point; the JDK authors only get one shot and they can't assume that
> volatile reads are cheap
>
>
>
>
>
> --
> View this message in context:
> http://jsr166-concurrency.10961.n7.nabble.com/On-A-Formal-Definition-of-Data-Race-tp9408p9466.html
> Sent from the JSR166 Concurrency mailing list archive at Nabble.com.
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