# [concurrency-interest] Relativity of guarantees provided byvolatile

David Holmes davidcholmes at aapt.net.au
Fri Aug 17 21:55:03 EDT 2012

```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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