deleting objects present in a list

[Sandy]

On Apr 21, 10:48 am, Dave Angel <da... at ieee.org> wrote:
> (For some reason you posted your response before the message you were
> replying to.  That's called Top-posting, and is bad form on these
> mailing lists)
>
>
>
> Sandy wrote:
> > Thanks for the replies.
>
> > Terry,
> > What does 'immediately' mean? I did a small test and here are the
> > results.
>
> > import psutil
>
> > def testing():
> >     class Object():
> >         pass
>
> >     l =}
> >     apm =sutil.avail_phymem()/(1024*1024)
> >     print 'Before creating objs: ' + repr(apm)
>
> >     for i in xrange(500000):
> >         l.update({Object():1})
>
> >     apm =sutil.avail_phymem()/(1024*1024)
> >     print 'After creating objs: ' + repr(apm)
> >     return l
>
> > def hello():
> >     myl =esting()
>
> >     apm =sutil.avail_phymem()/(1024*1024)
> >     print 'Before deleting: ' + repr(apm)
>
> >     del myl
>
> >     # Here I want to delete the objects in the list
> >     # deleting myl doesn't seem to change the memory
>
> >     apm =sutil.avail_phymem()/(1024*1024)
> >     print 'After deleting: ' + repr(apm)
>
> > if __name__ ='__main__':
> >     hello()
>
> > OUTPUT:
> > Before creating objs: 2516L
> > After creating objs: 2418L
> > Before deleting: 2418L
> > After deleting: 2430L
>
> > In my original case the memory is not getting released even after long
> > time.
>
> > - dksr
>
> > On Apr 20, 8:44 pm, Terry Reedy <tjre... at udel.edu> wrote:
>
> >> On 4/20/2010 3:21 PM, Sandy wrote:
>
> >>> Hi all,
> >>> I have large number of objects created and to handle them properly, I
> >>> store them in a list. How can I delete all of these objects (delete I
> >>> mean here is to remove the object from memory not just from list)?
> >>> I cannot use the list to iterate through the objects to delete them.
> >>> Because 'del' only reduces the reference count and as it is present in
> >>> the list it is not deleted. I cannot delete the list because I loose
> >>> control over the objects.
>
> >> Deleting the list is the best you can do. If that deletes the last
> >> reference, then the interpreter will delete the object when it feels
> >> like it. For *current* CPython, this will be immediately. For other
> >> implementations, whenever.
>
> First, you're using some 3rd party library for measuring some kind of
> memory usage.  I'd guess you're probably usinghttp://code.google.com/p/psutil/         Since I'm not familiar with
> how they derive these numbers, I can only make a good guess as to how
> valid they are.   And if avail_phymem refers to what its name implies,
> it has little to do with Python.  Python doesn't control physical
> memory, only virtual.
>
> So let's skip those numbers and talk about what CPython actually does.  
> As others have pointed out, other implementations will be different.
>
> When you delete a large number of objects (which you can do with
> myl=None), CPython may keep some of the memory under its own control for
> reuse.  A future object of the same size will fit nicely in the hole,
> and that may be faster than calling free() and malloc() again.
>
> When CPython calls free(), the C runtime library almost certainly keeps
> the memory for reuse by subsequent calls to malloc(). Operating system
> calls for allocating and freeing memory are (usually) done in larger,
> fixed-size blocks.  In Windows for example, the granularity is 4k or 64k
> for the more efficient methods of memory allocation.  Swapfile
> allocation, for example, is always in 4k multiples.  See function call
> VirtualAlloc().  Anyway, if there's even a single allocated byte in a
> block, it can't release the block.  And searching for such blocks is slow.
>
> When the operating system is told to free something, it usually does not
> "free" physical memory immediately.  In the case of Windows, it marks
> the block as available, and eventually a daemon task will zero it.  But
> it could very well be "charged to" the current process until some other
> process needs the physical memory.  What it does do is free it from
> virtual memory.  But notice that Virtual memory is represented by a
> swapfile on disk, and Windows doesn't support a file with gaps in it
> (sparse allocation).  So unless this particular allocation is at the end
> of the file, the size isn't likely to go down.
>
> If you really want to "observe" that the memory has been "released,"  
> I'd suggest calling a similar testing() function a second time, with
> objects of the same size, and see whether the numbers get any worse.  
> I'd say they won't, at least not by much, assuming there's any validity
> to this avail_phymem() function.
>
> I also have to point out that using "del myl" is not necessary for
> freeing up the memory.  All that's necessary is for the myl name to stop
> referring to the list, and the list will go away.  So myl=42 will work
> just as well.
>
> HTH
> DaveA

Hi,
Thanks for detailed reply. I tried running testing() function a second
time and as expected the
total memory available is not decreased.

Cheers,
dksr