[erlang-questions] [Q] Copying memory between processes

[David Hopwood]

Bjorn Gustavsson wrote:
> Ladislav Lenart <lenartlad@REDACTED> writes:
> 
>>But as soon as this structure is sent to one other process
>>the system is suddenly out of resources. The only reason
>>this could happen (that I could think of) is: When the
>>structure is copied to the heap of the other process, the
>>identity is not preserved, that is each item references its
>>own list of previous items (though equal). This would explain
>>expontential growth of the memory.

Quadratic in the number of items, actually. But that's bad enough.

> Yes.

I'd say this was a bug. Even if this degree of sharing in a data structure
is unusual, it is not pathological enough that this increase in memory
usage should be considered acceptable.

>>Am I right? And if it so, what is the reason (performance)?
> 
> Yes. In typical applications, shared data structures are uncommon.

A deep copy that preserves sharing can be implemented efficiently.
The easiest approach is to use forwarding pointers: as you copy an
object, set a pointer in its header to point to the copy.

-- 
David Hopwood <david.hopwood@REDACTED>