Friday, January 18, 2008

Garbage collection: Follow up

Given some of the feedback on my previous post, I thought a follow up would be in order

One of the most contentious points was my comment that "A well written and tuned garbage collector can be faster than manual allocation". I will cover this in a separate post as it needs more than a couple of lines.

Why would you want to use a GC in delphi
I will cover this in a separate post as well. There is probably little gain in just adding a gc to an existing delphi app (unless it's leaky, but we don't write apps like that). If you are writing a new app based around having a gc in place, then you can do things differently.

Once of the original quotes referred to objects referencing each other not being released. I read this as talking about cyclic references. That is a problem with simple reference counting, but not with a tracing (ie mark and sweep) gc such as used by Boehm, .net and java (1).

A gc is not a silver bullet, nor will it catch all memory leaks. I am not suggesting that it will.

One point I forgot to mention. Some gc algorithms will allocate extra memory for flags, counts etc (1). This can push up the memory use compared to manual allocation. However Fast mm 4 (2) also allocations a 32 bit flag ahead of every memory block so it is probably a wash.

Fast mm
Fast mm will not catch all memory leaks. It will catch memory that hasn't been freed when the application exits (2) which is not the same thing.

If you have poor testing coverage, then the untested code can have memory leaks.
Fast mm will not catch objects freed on application shutdown (ie forms owned by the application). A gc won't catch this either.

Fast mm will help with double frees, but it won't help with a/v errors (unless I am missing something, it certainly hasn't helped me). A gc will help with both of those (1).

1 The wikipedia article on Garbage collection provides a lot of the background.
2 Fast mm details are taken from

"However most Delphi memory managers request large chucks of memory from windows and then parcel it out to the app on request," See (2) and Nexus MM

The quotes in the original article are taken from the newsgroup thread "Garbage collection"

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