Re: investigation into replacing ref. counts with leases

[Darin Adler <darin bentspoon com>]

On Wednesday, July 18, 2001, at 04:33  PM, Havoc Pennington wrote:

> Keeping in mind my earlier statement about in-process
> vs. out-of-process - the timer in an event loop is probably fine for
> renewing out-of-process leases, but I can't imagine any timer that
> would be remotely a good idea for in-process objects.

A timer in an event loop is not fine for out of process leases, is it? You 
don't want to do some long operation where the program doesn't respond to 
the user and then have all your components self-destruct, right?

The timer is irrelevant for in-process objects. Although the lease 
machinery is used in-process, there's really no chance of any lease 
expiring since the renewing of the leases and the aging of the leases 
would use the same timer.

> Thinking of this as a garbage collection algorithm, its properties
> are either:
>
>  - uncollected garbage can grow without bound as you are doing the
>    work of your program, if your GC happens only in an event loop or
>    the like

There's no garbage in normal operation with the scheme I was implementing.
 This is not like GC in that respect. The "garbage" only occurs when a 
process dies without cleaning up after itself. In normal operation, leases 
are destroyed, which is done immediately. Old leases expire, and that's 
the part that's done with a timer. But a local reference would never have 
an old lease unless there was a programming mistake.

> So leases are wholly inappropriate for small, pervasive objects.  Of
> course at the moment Bonobo objects are really too heavy for this use
> anyhow; even GObject is nearly too heavy for that, and the current
> Bonobo implementation is at least as heavy as GObject for obvious
> reasons.
>
> So you really want to stick to refcounting (or real GC) for
> in-process objects...

I think I have not been clear enough about this aspect of the thing. The 
interface for these leases is newLease, extendLease, and destroyLease. 
When the last owner lets go of an object, it's destroyed immediately as 
the result of a destroyLease that destroys the last lease, whether 
cross-process or in-process.

    -- Darin