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Re: NSAutoreleasePool does not free all memory

From: Richard Frith-Macdonald
Subject: Re: NSAutoreleasePool does not free all memory
Date: Mon, 30 Apr 2001 05:49:52 +0100

On Sunday, April 29, 2001, at 09:39 PM, Gerald Siebke wrote:


I have installed the latest cvs-version on NetBSD 1.5_ALPHA
with the default egcs-1.1.2.

I have the following problem with an NSAutoreleasePool:

The allocated memory inside the NSAutoreleasePool is not completely
freed outside.

line '16' contains : autorelease

before allocation                                       : 4.9 MB
after allocation                                        : 7.8 MB
after destroy  NSAutoreleasePool        : 7.0 MB        <-- why ???

we changed in line '16' : autorelease --> release

before allocation                                       : 4.9 MB
after allocation                                        : 7.5 MB
after destroy  NSAutoreleasePool        : 4.9 MB        <-- this is okay !

The free() call can only give memory back to the operating system if it has
a nice big simple block of unused memory extending to the end of the
allocated memory range.  If that's not the case, the memory will remain
'owned' by the program and will be re-used in later calls to malloc().

If you use autorelease, as well as allocating memory for the strings and array, yopu allocate some memory for the autorelease pool itsself, and as the pool grows, this memory is reallocated to make the pool bigger. This means that
somewhere near the end of the allocated memory will be memory used by an
autorelease pool.

Autorelease pool memory is *not* freed (unless the thread they belong to
is terminated) ... the autorelease pool objects are
cached for later re-use ... so this memory will remain in use, and will
stop the whole block of memory being returned to the operqating system.

This is of course harmless, since the memory will be re-used the next time any object is allocated, (so memory usage will not grow indefinately) and if nothing
ever is allocated again, it will be paged out to disk.

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