Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Fri, 08 Jul 2005 12:14:02 -0500

Hi Ben,

I think it's true to say that the "simple" cords approach to strings is

only possible if you have a Garbage Collecting memory manager. For

proof of that statement, consider what happens if I extract a short

string from the middle of another huge string; with a cord

implementation this extraction does no copying, it just records a

pointer and the length. If the huge string then gets unreferenced by

its original owner, the memory holding the huge string must not be freed

since another cord is still pointing into the middle of it. A GC memory

manager may be able to work out that most of the string is unused and

hence release it, but using the regular malloc/free model can't do that.

 A reference-counted implementation would be possible, but now you're

complicating the string implementation and the major advantages of cords

start slipping away.

One of the tenets of Garbage Collection is that it is pretty much an

all-or-nothing kind of thing; either you use GC for everything, or you

can't use it at all, since references to GC memory may only exist in

non-GC memory such as in OS structures (think of the string that holds

the name of one of my tasks for instance). Somewhere in the

documentation you pointed to on the Boehm GC memory manager it was

stated that it has only been implemented on systems that have virtual

memory.

I don't know whether anyone has ever tried to use GC under RTEMS, but

I'm pretty confident that it would not be easy to implement in vxWorks.

 Writing and tuning a GC is not an easy thing to do, and certainly not

something the EPICS community has any experience with. We do however

understand the issues associated with the use of malloc/free in the

real-time systems that have to be implemented at many EPICS sites.

I think we have to be conservative in our design choices for EPICS 4.0,

and I would certainly not favour the idea of including a full-blown GC

memory manager in EPICS. I agree that we should discuss the topic of

memory management next week, and I've added it to my list of topics, but

I don't think you're going to win us around to adopting GC.

- Andrew
--
Podiabombastic: The tendency to shoot oneself in the foot.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System