Re: Timing of garbage collection

[Marshall]

Perl doesn't have "garbage collection" in the sense that it never gives memory back to the OS.

Once Perl has memory from the OS, it has it forever.
A long-lived Perl app will reach a maximum memory usage and just stay at that number forever (provided no memory leaks).

If you un-define an object ref: $obj_a = undef;
Or say $obj_a = Method_X->new(...), reuse/reassign an object ref that has the same effect as long as no reference to an in internal structure within obj_a is in existence.

Perl has its own memory management and reuses memory when it can.

[eyepopslikeamosquito]

Perl doesn't have "garbage collection" in the sense that it never gives memory back to the OS

Sorry, but that statement is garbage.

First, the statement is wrong, as already demonstrated by BrowserUk. Whether perl releases memory back to the OS or not depends simply on the implementation of malloc/realloc, as described at Re: Not able to release memory (malloc implementation).

Second, whether it returns memory to OS is not related to garbage collection! As already noted by dave_the_m, perl uses a reference-counted garbage collector, so you get "deterministic destructors" for free (i.e. in perl, you are guaranteed that an object is destroyed (and destructor called) immediately its reference count goes to zero). BTW, deterministic destructors are a feature of the C++ RAII idiom yet are problematic when using a mark-and-sweep garbage collector, such as that used by Java, which is why Java has a "finally" clause (see also Dispose pattern).

[BrowserUk]

Perl doesn't have "garbage collection" in the sense that it never gives memory back to the OS.

That's demonstrably not exactly true:

C:\test>perl -E"say `tasklist|find \"$$\"`; $x=chr(0); $x x= 2e6; say 
+`tasklist|find \"$$\"`; undef $x; say `tasklist|find \"$$\"`"
perl.exe                    139252 Console                    1      4
+,660 K
perl.exe                    139252 Console                    1      6
+,680 K
perl.exe                    139252 Console                    1      4
+,724 K

That shows that perl allocating a 2MB scalar and then returning that 2MB back to the OS.

On my Perl/system, the break point for the size of allocations that are released back to the system is 1040351 bytes. anything more and it is; less and it is not:

C:\test>perl -E"say `tasklist|find \"$$\"`; $x=chr(0); $x x= 1040352; 
+say `tasklist|find \"$$\"`; undef $x; say `tasklist|find \"$$\"`"
perl.exe                    129340 Console                    1      4
+,688 K
perl.exe                    129340 Console                    1      5
+,788 K
perl.exe                    129340 Console                    1      4
+,780 K

C:\test>perl -E"say `tasklist|find \"$$\"`; $x=chr(0); $x x= 1040351; 
+say `tasklist|find \"$$\"`; undef $x; say `tasklist|find \"$$\"`"
perl.exe                    241476 Console                    1      4
+,704 K
perl.exe                    241476 Console                    1      5
+,788 K
perl.exe                    241476 Console                    1      5
+,792 K

That number is a around 8k less than 1MB, so presumably it is 1MB internally but then some is used for internal management.

[Marshall]

What version of Perl are you running? I am still on Ver 5.10.1 with Active State.

[zwon]

It doesn't depend on Perl version, it depends on malloc implementation.

[BrowserUk]

Those tests were run using that exact same combination:

C:\test>perl -v

This is perl, v5.10.1 built for MSWin32-x64-multi-thread
(with 2 registered patches, see perl -V for more detail)

Copyright 1987-2009, Larry Wall

Binary build 1007 [291969] provided by ActiveState http://www.ActiveSt
+ate.com
Built Jan 27 2010 14:12:21

But the same has been true for every build since at least 5.8.1 and probably earlier, and continues through 5.16.1:

C:\test>perl -v

This is perl 5, version 16, subversion 1 (v5.16.1) built for MSWin32-x
+64-multi-thread
(with 1 registered patch, see perl -V for more detail)

...

C:\test>perl -E"say `tasklist|find \"$$\"`; $x=chr(0); $x x= 1040351; 
+say `tasklist|find \"$$\"`; undef $x; say `tasklist|find \"$$\"`"
perl.exe                     17672 Console                    1      4
+,992 K
perl.exe                     17672 Console                    1      6
+,080 K
perl.exe                     17672 Console                    1      6
+,084 K


C:\test>perl -E"say `tasklist|find \"$$\"`; $x=chr(0); $x x= 1040352; 
+say `tasklist|find \"$$\"`; undef $x; say `tasklist|find \"$$\"`"
perl.exe                    270080 Console                    1      4
+,912 K
perl.exe                    270080 Console                    1      6
+,016 K
perl.exe                    270080 Console                    1      5
+,012 K