[concurrency-interest] Relativity of guarantees provided byvolatile

David Holmes davidcholmes at aapt.net.au
Fri Aug 17 22:29:39 EDT 2012


Well that's your choice, but it makes more sense temporally to consider
completetion when the actual result of the action is available, in my
opinion.

David

> -----Original Message-----
> From: Zhong Yu [mailto:zhong.j.yu at gmail.com]
> Sent: Saturday, 18 August 2012 12:21 PM
> To: dholmes at ieee.org
> Cc: concurrency-interest at cs.oswego.edu
> Subject: Re: [concurrency-interest] Relativity of guarantees provided
> byvolatile
>
>
> In the model, a write completes as soon as the mail is sent, the same
> thread can immediately make another write. A read completes when the
> return mail is received; the reading thread is suspended during the
> wait.
>
> The example is designed so that reads are short too, so we can treat
> actions as points in time, to order them easily.
>
> On Fri, Aug 17, 2012 at 8:55 PM, David Holmes
> <davidcholmes at aapt.net.au> wrote:
> > As I keep saying, for this to "make sense" you have to make temporal
> > measurements when an action completes.
> >
> > David
> >
> >> -----Original Message-----
> >> From: concurrency-interest-bounces at cs.oswego.edu
> >> [mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf
> Of Zhong Yu
> >> Sent: Saturday, 18 August 2012 11:34 AM
> >> To: Yuval Shavit
> >> Cc: concurrency-interest at cs.oswego.edu; dholmes at ieee.org
> >> Subject: Re: [concurrency-interest] Relativity of guarantees provided
> >> byvolatile
> >>
> >>
> >> Consider this physical model:
> >>
> >> Each thread is a person Tx.
> >>
> >> There's a person V managing all variables.
> >>
> >> To make a write, Tx sends a paper mail to V. No return mail is
> >> required, therefore a write is very short.
> >>
> >> To make a read, Tx sends a paper mail to V, and waits for return mail.
> >>
> >> The synchronization order is the order the mails received by V.
> >>
> >> This seems to be a valid JMM model.
> >>
> >> --
> >>
> >> Now suppose thread R is very close to V, therefor reads are also very
> >> short. (it's easier to talk about temporal order between short
> >> actions) Suppose thread W is very far away from V.
> >>
> >> To realize OP's example, consider the numbers are in hours, and route
> >> W -> V takes 48 hours.
> >>
> >> On Monday, W writes v=1, it reaches V on Wednesday. On Tuesday R reads
> >> v=0. So the write is after read in sync order, yet write is before
> >> read in temporal order(even considering relativity - all persons are
> >> on Earth)
> >>
> >> Therefore sync order doesn't have to be consistent with temporal order.
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> >>
> >
>



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