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Diffstat (limited to 'Doc/whatsnew')
| -rw-r--r-- | Doc/whatsnew/whatsnew20.tex | 139 |
1 files changed, 72 insertions, 67 deletions
diff --git a/Doc/whatsnew/whatsnew20.tex b/Doc/whatsnew/whatsnew20.tex index 9a3b9ce..8b96e15 100644 --- a/Doc/whatsnew/whatsnew20.tex +++ b/Doc/whatsnew/whatsnew20.tex @@ -289,6 +289,75 @@ words, \code{s.join(seq)} is equivalent to the old \code{string.join(seq, s)}. % ====================================================================== +\section{Optional Collection of Cycles} + +The C implementation of Python uses reference counting to implement +garbage collection. Every Python object maintains a count of the +number of references pointing to itself, and adjusts the count as +references are created or destroyed. Once the reference count reaches +zero, the object is no longer accessible, since you need to have a +reference to an object to access it, and if the count is zero, no +references exist any longer. + +Reference counting has some pleasant properties: it's easy to +understand and implement, and the resulting implementation is +portable, fairly fast, and reacts well with other libraries that +implement their own memory handling schemes. The major problem with +reference counting is that it sometimes doesn't realise that objects +are no longer accessible, resulting in a memory leak. This happens +when there are cycles of references. + +Consider the simplest possible cycle, +a class instance which has a reference to itself: + +\begin{verbatim} +instance = SomeClass() +instance.myself = instance +\end{verbatim} + +After the above two lines of code have been executed, the reference +count of \code{instance} is 2; one reference is from the variable +named \samp{'instance'}, and the other is from the \samp{myself} +attribute of the instance. + +If the next line of code is \code{del instance}, what happens? The +reference count of \code{instance} is decreased by 1, so it has a +reference count of 1; the reference in the \samp{myself} attribute +still exists. Yet the instance is no longer accessible through Python +code, and it could be deleted. Several objects can participate in a +cycle if they have references to each other, causing all of the +objects to be leaked. + +An experimental step has been made toward fixing this problem. When +compiling Python, the \verb|--with-cycle-gc| option can be specified. +This causes a cycle detection algorithm to be periodically executed, +which looks for inaccessible cycles and deletes the objects involved. +A new \module{gc} module provides functions to perform a garbage +collection, obtain debugging statistics, and tuning the collector's parameters. + +Why isn't cycle detection enabled by default? Running the cycle detection +algorithm takes some time, and some tuning will be required to +minimize the overhead cost. It's not yet obvious how much performance +is lost, because benchmarking this is tricky and depends crucially +on how often the program creates and destroys objects. + +Several people tackled this problem and contributed to a solution. An +early implementation of the cycle detection approach was written by +Toby Kelsey. The current algorithm was suggested by Eric Tiedemann +during a visit to CNRI, and Guido van Rossum and Neil Schemenauer +wrote two different implementations, which were later integrated by +Neil. Lots of other people offered suggestions along the way; the +March 2000 archives of the python-dev mailing list contain most of the +relevant discussion, especially in the threads titled ``Reference +cycle collection for Python'' and ``Finalization again''. + + +% ====================================================================== +\section{New XML Code} + +XXX write this section... + +% ====================================================================== \section{Porting to 2.0} New Python releases try hard to be compatible with previous releases, @@ -378,70 +447,6 @@ and Fredrik Lundh. %place. % ====================================================================== -\section{Optional Collection of Cycles} - -The C implementation of Python uses reference counting to implement -garbage collection. Every Python object maintains a count of the -number of references pointing to itself, and adjusts the count as -references are created or destroyed. Once the reference count reaches -zero, the object is no longer accessible, since you need to have a -reference to an object to access it, and if the count is zero, no -references exist any longer. - -Reference counting has some pleasant properties: it's easy to -understand and implement, and the resulting implementation is -portable, fairly fast, and reacts well with other libraries that -implement their own memory handling schemes. The major problem with -reference counting is that it sometimes doesn't realise that objects -are no longer accessible, resulting in a memory leak. This happens -when there are cycles of references. - -Consider the simplest possible cycle, -a class instance which has a reference to itself: - -\begin{verbatim} -instance = SomeClass() -instance.myself = instance -\end{verbatim} - -After the above two lines of code have been executed, the reference -count of \code{instance} is 2; one reference is from the variable -named \samp{'instance'}, and the other is from the \samp{myself} -attribute of the instance. - -If the next line of code is \code{del instance}, what happens? The -reference count of \code{instance} is decreased by 1, so it has a -reference count of 1; the reference in the \samp{myself} attribute -still exists. Yet the instance is no longer accessible through Python -code, and it could be deleted. Several objects can participate in a -cycle if they have references to each other, causing all of the -objects to be leaked. - -An experimental step has been made toward fixing this problem. When -compiling Python, the \verb|--with-cycle-gc| option can be specified. -This causes a cycle detection algorithm to be periodically executed, -which looks for inaccessible cycles and deletes the objects involved. -A new \module{gc} module provides functions to perform a garbage -collection, obtain debugging statistics, and tuning the collector's parameters. - -Why isn't cycle detection enabled by default? Running the cycle detection -algorithm takes some time, and some tuning will be required to -minimize the overhead cost. It's not yet obvious how much performance -is lost, because benchmarking this is tricky and depends crucially -on how often the program creates and destroys objects. - -Several people tackled this problem and contributed to a solution. An -early implementation of the cycle detection approach was written by -Toby Kelsey. The current algorithm was suggested by Eric Tiedemann -during a visit to CNRI, and Guido van Rossum and Neil Schemenauer -wrote two different implementations, which were later integrated by -Neil. Lots of other people offered suggestions along the way; the -March 2000 archives of the python-dev mailing list contain most of the -relevant discussion, especially in the threads titled ``Reference -cycle collection for Python'' and ``Finalization again''. - - -% ====================================================================== \section{Core Changes} Various minor changes have been made to Python's syntax and built-in @@ -672,8 +677,8 @@ the function to be called on exit. \module{dircmp} modules, which have now become deprecated. (Contributed by Gordon MacMillan and Moshe Zadka.) -\item{\module{linuxaudio}:} Support for the \file{/dev/audio} device on Linux, -a twin to the existing \module{sunaudiodev} module. +\item{\module{linuxaudiodev}:} Support for the \file{/dev/audio} +device on Linux, a twin to the existing \module{sunaudiodev} module. (Contributed by Peter Bosch.) \item{\module{mmap}:} An interface to memory-mapped files on both @@ -684,7 +689,7 @@ functions that expect ordinary strings, such as the \module{re} module. (Contributed by Sam Rushing, with some extensions by A.M. Kuchling.) -\item{\module{PyExpat}:} An interface to the Expat XML parser. +\item{\module{pyexpat}:} An interface to the Expat XML parser. (Contributed by Paul Prescod.) \item{\module{robotparser}:} Parse a \file{robots.txt} file, which is |