From: Robert Klemme <shortcutter@...>
Date: 2007-10-11T22:38:49+09:00
Subject: Re: Should *most* memory be release back to the system?

2007/10/11, Blackie <mudsweatandears@gmail.com>:
> I do appreciate your help (and no that link is lost to the mists of
> time as far as I can google.) :)
>
> I do understand that they are not useful for leak detection, but this
> is just observing the peak during the life of the script. During
> "other_task" I can see the usage rise *and fall* so I know the OS is
> reclaiming *some* memory in places...but the part I don't understand
> is why it isn't returning to "tasks" original base of 50M when only
> "task" is running.
>
> I hate to sound dense, but I need to convince some fairly hard-headed
> sysadmins.

:-)

There are a few things to say to this.

First, it seems reasonable to hold on to memory that has once been
grabbed because you can expect that your process needs that memory
again.  There is no point in allocating and deallocation memory from
the OS all the time.

Then, even though memory is allocated does not mean that physical
memory is actually used.  IIRC pure allocation just reserves memory,
only when you try to access it the first time the OS generates a page
fault and you get physical memory.  If memory is unused for a while
chances are that it's paged out to disk *if* other processes need
resources.  If not, there is no problem anyway.  Granted, if a machine
does not have enough virtual memory configured this can lead to
problems for long running programs but then I'd say the machine is
probably misconfigured anyway.

Now, what was the third point I had in mind?  Ah yes: I believe
typically JVM's behave the same. However, Sun's JVM also does heave
copying around which might help the OS because objects will be packed
on fewer memory pages so that more pages are idle and can be swapped
out.  I believe the current Ruby interpreter does not copy objects but
you can verify this by looking at the source code.

All in all, this is a strange reason to ban a programming language
from a machine IMHO.  Other reasons seem more reasonable to me
(management overhead for keeping the installation up to date etc.).

Kind regards

robert