[Melbourne-pm] Perl garbage collection behaviour

Scott Penrose scottp at dd.com.au
Wed Aug 24 16:41:52 PDT 2005

Hmm... weird. I posted this yesterday morning, wonder why it only  
just came through... oh well I will have to ask the list manager - oh  
damn, that's me!


On 24/08/2005, at 16:43, Scott Penrose wrote:

> On 24/08/2005, at 16:12, Harley Mackenzie wrote:
>> I have an application that I have developed that regularly polls a  
>> database table for new records, sleeps for most of the time and  
>> otherwise periodically generates some reports and faxes. The  
>> application works fine except for the memory behaviour, wheret 8KB  
>> of memory is used up for each database poll (running on Windows  
>> Server 2003) as reported by Windows task manager, with the memory  
>> requirements starting out at about 5 KB and over a couple of days  
>> eventually exhausting all of the available virtual memory and  
>> causing mayhem on the server.
>> I initially suspected a circular reference in my Perl objects,  
>> except for the fact that if I click on the application that is  
>> usually running in a minimised state at the bottom of the screen,  
>> and bring it to the foreground, and then minimise again, something  
>> kicks off the garbage collection and it then reduces to about 5KB  
>> and starts the memory accumulation thing all over again.
>> As this is a remote server at a client location this is pretty  
>> inconvenient and has resulted in some nasty out of hours phone calls.
>> Am I correct in assuming that there cant be circular references  
>> preventing the reclamation of memory as it reclaims the memroy  
>> when the application comes to the foreground?
>> Why is this bizarre behaviour occuring at all with the garbage  
>> collection? Is it because it is in a sleep state for most of the  
>> time?
> It would seem unlikely. My guess is that you are not releasing data  
> - or more accurately, some code somewhere in a module somewhere is  
> not releasing data.
> For example, you often find modules keeping references to  
> themselves or other places - which means that reference count is up  
> for that variable. Next time, when you release the previous state,  
> you may not have released the reference to itself.
>> Can I initate a garbage collection in the program itself?
> No, it is out of your hands.
> You could use Devel::Size to look at what is going on.
> http://search.cpan.org/~dsugal/Devel-Size-0.63/Size.pm
> Hope you get rid of your niggle - hopefully it is not in something  
> you have no control over.
> Of course it could always be a bug in Perl - but I would assume  
> first that it is your own code - and then a module - and only then  
> perl :-)
> Scott
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Scott Penrose
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scott at cpan.org

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