[Python-Dev] Linus on garbage collection (original) (raw)

Mark Shannon marks at dcs.gla.ac.uk
Fri May 6 19:46:37 CEST 2011

Michael Foord wrote:

On 06/05/2011 18:26, Mark Shannon wrote:

Michael Foord wrote:

On 06/05/2011 17:51, Stefan Behnel wrote:

Mark Shannon, 06.05.2011 18:33:

skip at pobox.com wrote:

Antoine> Since we're sharing links, here's Matt Mackall's take: Antoine> http://www.selenic.com/pipermail/mercurial-devel/2011-May/031055.html

From that note: 1: You can't have meaningful destructors, because when destruction happens is undefined. And going-out-of-scope destructors are extremely useful. Python is already a rather broken in this regard, so feel free to ignore this point. Given the presence of cyclic data I don't see how reference counting or garbage collection win. Ignoring the fact that in a pure reference counted system you won't even consider cycles for reclmation, would both RC and GC have to punt because they can't tell which object's destructor to call first? It doesn't matter which is called first. May I quote you on that one the next time my software crashes? Arbitrarily breaking cycles could cause a problem if a destructor attempts to access an already collected object. Not breaking cycles definitely leaks memory and definitely doesn't call finalizers. You don't need to break the cycles to call the finalizers. Just call them, then collect the whole cycle (assuming it is still unreachable). The GC will never reclaim a reachable object. Objects awaiting finalization are reachable, by definition. Well it was sloppily worded, so replace it with: if a finalizer attempts to access an already finalized object.

A finalized object will still be a valid object. Python code cannot make an object unsafe. Obviously C code can make it unsafe, but that's true of C code anywhere.

For example, a file object will close itself during finalization, but its still a valid object, just a closed file rather than an open one.

Michael

Michael

It may not make a difference for the runtime, but the difference for user software may be "dead" or "alive".

Stefan

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