[Ironpython-users] LightweightScopes and scopes in general

[Igor Brejc]

On Sun, Feb 12, 2012 at 6:44 PM, Keith Rome <rome at wintellect.com> wrote:

> I am fairly sure it is no different than when using disposable objects in
> any .NET application. If you don’t call IDisposable.Dispose() directly (or
> via the “using()” statement), then those objects will be disposed via their
> finalizer when the GC collects them. This happens automatically and fairly
> frequently in a .NET application. Those objects will be cleaned up for you
> – assuming you really aren’t holding references to them. If you find they
> never seem to go away, then I highly recommend attaching a memory profiler
> (SciTech’s MemProfiler is superb) and exploring the heap graph for the root
> paths that are responsible for pinning them.
>

I'm not a GC expert, but if I understand correctly, GC doesn't really call
Dispose(), you have to implement the finalizer explicitly to do this (
http://stackoverflow.com/questions/45036/will-the-garbage-collector-call-idisposable-dispose-for-me).
Since (from my experience) most user-written disposable classes don't
implement finalizers, this means that if you don't call Dispose()
explicitly or using "using", the resource won't be released until the
process stops. Even FxCop doesn't suggest writing finalizers, it just
recommends using the proper Disposable pattern (
http://geekswithblogs.net/sdorman/archive/2010/05/25/the-dispose-pattern-and-fxcop-warnings.aspx
).

On the other hand, I notice .NET Framework classes do seem to implement
finalizers (FileStream, Image...):
http://msdn.microsoft.com/en-us/library/system.io.filestream.finalize.aspx


> ****
>
> ** **
>
> Take note that your ScriptScope itself is holding references to all of the
> objects contained within. You will need to discard all references to that
> ScriptScope if you want it (and its contents) to be collected by the
> runtime.
>

Good suggestion, thanks.


> ****
>
> ** **
>
> If you conclude that they truly are eligible for collection, and you just
> can’t wait for the system to clean up on its own, then you can always call
> GC.Collect() directly. I really don’t recommend that kind of brute force
> approach though. There are very few situations where calling GC.Collect()
> is really the right thing to do.****
>
> **
>

I agree.


>  **
>
> ** **
>
> *Keith Rome*
>
> *Senior Consultant and Architect*
>
> MCPD-EAD, MCSD, MCDBA, MCTS-WPF, MCTS-TFS, MCTS-WSS****
>
> Wintellect | 770.617.4016 | krome at wintellect.com <rome at wintellect.com>****
>
> www.wintellect.com****
>
> ** **
>
> *From:* ironpython-users-bounces+rome=wintellect.com at python.org [mailto:
> ironpython-users-bounces+rome=wintellect.com at python.org] *On Behalf Of *Igor
> Brejc
> *Sent:* Saturday, February 11, 2012 10:11 AM
> *To:* Dino Viehland
> *Cc:* Ironpython-users at python.org
>
> *Subject:* Re: [Ironpython-users] LightweightScopes and scopes in general*
> ***
>
> ** **
>
> I'm thinking of this scenario: my host application allows users to run
> their own Py scripts. OK, in the ideal case, their scripts should take care
> of the disposal of the reserved resources, but a lot of times user will
> write trail-and-error scripts which will often fail. Is there a mechanism
> for the host application to somehow track (and possibly dispose) of the
> stuff that has been allocated in a failed user script? Using SetTrace() or
> by simply examining the ScriptScope after the script has finished? Or
> should I simply ignore this?****
>
> ** **
>
> I'm wondering how other people who use IronPython as an embedded scripting
> engine deal with these issues (if at all).****
>
> ** **
>
> Thanks,****
>
> Igor****
>
> ** **
>
> On Fri, Feb 10, 2012 at 10:40 PM, Dino Viehland <dinov at microsoft.com>
> wrote:****
>
> We won’t call IDispose on them.  Usually they’ll follow the pattern of
> implementing IDisposable + having a finalizer so they will eventually
> release their resources.  Also they can be used with the “with” statement
> so that their Dispose methods are eagerly called.****
>
>  ****
>
> *From:* Igor Brejc [mailto:igor.brejc at gmail.com]
> *Sent:* Friday, February 10, 2012 10:51 AM
> *To:* Dino Viehland
> *Cc:* Jeff Hardy; Ironpython-users at python.org
> *Subject:* Re: [Ironpython-users] LightweightScopes and scopes in general*
> ***
>
>  ****
>
>  ****
>
> On Fri, Feb 10, 2012 at 7:46 PM, Dino Viehland <dinov at microsoft.com>
> wrote:****
>
>
>
> Jeff wrote:
> > I'm taking a stab off the top of my head based on my limited knowledge
> of the
> > internals, but I do know that debug code is not collectable.
> > It's possible that the code itself may be holding references that keep
> objects
> > alive longer than expected.
> >
> > I believe if you run the code in a separate AppDomain you can unload the
> > AppDomain and the code will be collected, which should take everything
> else
> > with it.
> >
> > >
> > > I've also tried running in the release mode and turning on the
> > > options["LightweightScopes"] = true mode, which seems to help. But I
> > > cannot find any information about what this option actually does and
> > > what happens with the scope variables in general. Any info would be
> > appreciated.
> >
> > That one I'll have to leave to Dino.****
>
> The difference between the different types of scopes just comes down to how
> we hold onto global variable and call site objects.
>
> The most efficient way to hold onto these is to use CLR static fields to
> store the
> global variables.  That allows the CLR to generate a direct access to the
> field and
> we can quickly access the global variables.  But it comes at the cost of
> not being
> able to recover the static fields and leaking the values that are stored
> in there.
>
> The light weight scopes store the variables in a field or an array instead
> of storing
> them in static fields.  To do that we need to pass in the field or close
> over it and then
> on each access we need to do the field or array access (and if it's an
> array access, it
> needs to be bounds checked).  But the end result is that the only thing
> which is keeping
> the global variable / call site objects alive are the delegate which
> implements the
> ScriptCode.  Once the ScriptCode goes away all of those call sites and
> PythonGlobal
> objects can be collected.
>
> So lightweight scopes come at a performance cost, but they are more
> applicable
> Where you're actually re-compiling code regularly.****
>
>  ****
>
> Thanks for that explanation, Dino.****
>
>  ****
>
> Another question that pops up: what happens with Disposable objects (like
> files, GDI+ bitmaps etc.) created within the Py scope (if they are not
> referenced from the outside)?   ****
>
>  ****
>
> Igor****
>
> ** **
>
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