Talk:Cheney's algorithm
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Untitled[edit]
What is the difference between Cheney's garbage collector and the one proposed a year earlier by Fenichel? Why do we have an article on Cheney and not on Fenichel?
Fenichel, R.R. and Yochelson, J.C., A LISP garbage-collector for virtual-memory computer systems, Communications of the ACM, vol. 12, no. 11, pp. 611-612, 1969 --Philippe 09:05, 3 May 2007 (UTC)
- It seems to me that this article confounds Fenichel's work with Cheney's. Cheney's work is the application of breadth-first traversal to Fenichel's subspace solution which originally used a recursive algorithm to traverse live objects. The article could be modified to clearly delineate the work of Fenichel from that of Cheney. —Preceding unsigned comment added by 128.173.236.232 (talk) 22:06, 3 November 2009 (UTC)
Recursion[edit]
- Is this algorithm recursive? Will it copy objects referred to by something referred to by something referenced from the stack? If so, it is not described that way. -- Beland 17:19, 24 May 2007 (UTC)
- It isn't strictly recursive, but it does reach all the objects referenced even indirectly from the stack. Essentially, the stack and the heap both act as components of a queue, with you adding to the end of the heap any copied items, then forwarding the pointer in the queue. This results in a breadth-first approach to reaching all the items, since you won't reach the 'end' of the heap/queue until no more objects require copying and no more pointers require forwarding..
- The algorithm as shown is totally recursive (see the recursive call to copy(r)), and performs a depth first search. I believe this is an error in the article. The pseudocode should be updated to match your description. — Preceding unsigned comment added by 86.66.40.164 (talk) 13:50, 20 April 2016 (UTC)
- It isn't strictly recursive, but it does reach all the objects referenced even indirectly from the stack. Essentially, the stack and the heap both act as components of a queue, with you adding to the end of the heap any copied items, then forwarding the pointer in the queue. This results in a breadth-first approach to reaching all the items, since you won't reach the 'end' of the heap/queue until no more objects require copying and no more pointers require forwarding..
Regarding Efficiency[edit]
Cheney's algorithm seems to produce among the worst possible structures if locality-of-reference is desired - objects referenced by other objects are neatly splattered across the entire heap due to the breadth-first-search implicit to the algorithm. And, yet, the lack of any requirement for a stack (which is necessary for a depth-first search) is part of what keeps Cheney's algorithm itself quite simple. If you need a KISS garbage collector and wish to avoid other structures and bit manipulations, Cheney's is about as simple as it gets. If you need a more optimal one, you'll need to look elsewhere.
It seems to me that thi algorithm is also not thread-save - the whole system has to stop for the gc. A big concern in today (e.g. in smartphones with 8 cores)
212.95.7.81 (talk) 01:26, 15 March 2013 (UTC)
Breadth first search gives good locality of reference for most paths for accesses close to the root. Depth first search gives great locality of reference for a single path from the root, and awful locality for everything else. In most cases, I think BFS will actually perform better, not worse. In this case there is no such thing as *worst*: performance will vary between use cases. — Preceding unsigned comment added by 82.132.224.134 (talk) 10:50, 4 July 2015 (UTC)
Algorithm Example[edit]
The algorithm example is recursive, and has no indentation. The whole idea behind Cheney is to avoid recursion. It needs to be replaced. SamRushing (talk) —Preceding undated comment added 08:51, 30 January 2016 (UTC)