Re: What does GC.GetTotalMemory really tell us?

From: Jon Skeet [C# MVP] (skeet_at_pobox.com)
Date: 04/20/04 

Date: Tue, 20 Apr 2004 15:06:23 +0100

Daniel Billingsley <dbillingsley@NO.durcon.SPAAMM.com> wrote:
> I did try using GC.Collect() and it didn't make any difference. I didn't
> add the second step.
>
> I also had noted that the documentation is conspicuously incomplete in
> describing the exact effect of the true parameter.
>
> I'm still curious why the memory is apparently never deallocated even when
> the variable is set to null. Or, in the case of the actual application
> where I was using it, when the form's Close() is called. I mean in that
> case even a minute later. I thought the whole point of being a good little
> Disposing boy was to free up memory resources, but if GetTotalMemory is
> telling us anything at all it's not nearly so simple.
>
> Would the following be correct?
>
> Calling Dispose() does not guarantee that a resource will be freed, it just
> guarantees that the GC will be able to free it if necessary. The GC may
> very well only free it if in fact it does believe it necessary. Maybe it
> makes sense that the memory would remain allocated as long as it wasn't
> needed as it would help performance if the item was needed again later.

Dispose doesn't really say anything about GC - it isn't usually used
for managed resources. It's more usually used for unmanaged resources
or references to unmanaged resources.

I suspect the more likely culprit for the "strange" results is that
you're actually allowing the garbage collector to collect before you
think anyway. For instance:

MemoryHog hog = new MemoryHog();
howMuchDuring = GC.GetTotalMemory(true);
Console.WriteLine("Memory with hog:\t{0}", howMuchDuring);
hog = null;

There's nothing to stop the GC from collecting the new object in the
second line - the local variable isn't read at any time after its
initial assignment. 

-- 
Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet
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