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From "Weldon Washburn" <weldon...@gmail.com>
Subject Re: [drlvm][sablevm] Desing of Class Unloading Support
Date Mon, 30 Oct 2006 15:24:25 GMT
I like it.  I don't fully understand the fine details yet.  But overall it
seems to be a clean design.  Maybe it makes sense for someone to prototype
this in drlvm.

On 10/30/06, Etienne Gagnon <egagnon@sablevm.org> wrote:
>
> Hi all,
>
> Here's a more structured proposal for a simple and effective
> implementation of class unloading support.
>
> In accordance with Section 2.17.8 of the JVM spec, class unloading (and
> its related native resource cleanup) can only happen when the class
> loader instance becomes unreachable.  For this to happen, we put in
> place the following things:
>
> 1- Each class loader is represented by some VM internal structure.
> [We'll call it the "class loader structure"].
>
> 2- Each class loader internal structure, except (optionally) the
> bootstrap class loader, maintains a weak reference to an object
> instance of class ClassLoader (or some subclass).  The Java instance
> has some opaque pointer back to the internal VM structure.   The Java
> instance is usually created before the internal VM structure.  The
> instance constructor is usually in charge of creating the internal VM
> structure.  [We'll call it the "class loader instance"]
>
> 3- Each class loader instance maintains a collection of loaded classes.
> A class/interface is never removed from this collection.  This
> collection maintains "hard" (i.e. "not weak") references to
> classes/interfaces.
>
> 4- [Informative] A class loader instance is also most likely to maintain
> a collection of classes for which it has "initiated" class loading.
> This collection should use hard references (as weak references won't
> lead to earlier class loading).
>
> 5- Each class loader instance maintains a hard reference to its parent
> class loader.  This reference is (optionally) null if the parent is the
> bootstrap class loader.
>
> 6- Each j.l.Class instance maintains a hard reference to the class
> loader instance of the class loader that has loaded it.  [This is not
> the "initiating" loaders, but really the "loading" loader].
>
> 7- Each class loader structure maintains a set of boolean flags, one
> flag per "non-nursery" garbage collected area (even when thread-local
> heaps are used).  The flag is set when an instance of a class loaded by
> this class leader is moved into the related GC-area.  The flag is unset
> when the GC-area is emptied, or (optionally) when it can be determined
> that no instance of a class loaded by this class loader remains in the
> GC-area.  This is best implemented as follows: a) use an unconditional
> write of "true" in the flag every time an object is moved into the
> GC-area by the garbage collector, b) unset the related flag in "all"
> class loader structures just before collecting a GC-area, then setting
> the flag back when an object survives in the area.
>
> 8- Each method invocation frame maintains a hard reference to either its
> surrounding instance (in case of instance methods, i.e. (invokevirtual,
> invokeinterface, and invokespecial) or its surrounding class
> (invokestatic).  This is already required for synchronized methods
> (it's not a good idea to allow the instance to be collected before the
> end of a synchronized instance method call; yep, learned the hard way
> in SableVM...)  So, the "overhead" is quite minimal.  The importance of
> this is in the correctness of not letting a class loader to die while a
> static/instance method of a class loaded by it is still active, leading
> to premature release of native resources (such as jitted code, etc.).
>
> 9- A little magic is required to prevent premature collection of a class
> loader instance and its loaded j.l.Class instances (see [3-] above), as
> object instances do not maintain a hard reference to their j.l.Class
> instance, yet we want to preserve the correctness of Object.getClass().
>
> So, the simplest approach is to maintain a hard reference in a class
> loader structure to its class loader instance (in addition to the weak
> reference in [2-] above).  This reference is kept always set (thus
> preventing collection of the class loader instance), except when *all*
> the following conditions are met:
> a) All nurseries are empty.
> b) All GC-area flags are unset.
>
> Actually, for making this practical and preserving correctness, it's a
> little trickier.  It requires a 2-step process, much like the
> object-finalization dance.  Here's a typical example:
>
> On a major collection, where all nurseries are collected, and some (but
> not necessary all) other GC-areas are collected, we do the following
> sequence of actions:
> a) All class loader structures are visited.  All flags related to
>   non-nursery GC-areas that we intend to collect are unset.  If this
>   leads to *all* flags to be unset, the hard reference to the class
>   loader instance is set to NULL (thus enabling, possibly, the
>   collection of the class loader instance).
>
> b) The garbage collection cycle is started and proceeds as usual.
>   Note that the work mandated in [7-] above is also done, which might
>   lead to setting back some flags in class loader structures that had
>   all their flags unset in [a)].
>
> c) After the initial garbage collection is applied, and just before
>   the usual treatment of weak references (where they are set to NULL
>   when pointing to a collected object), all class loader structures
>   are visited again.  The hard pointer of every class loader structure
>   that has any flag set is set back to point to the class loader
>   instance if it was NULL (same as how object instances are preserved
>   for finalization).
>
> d) If [c)] has triggered any change (i.e. it mandates the survival of
>   additional class loader instances that were due to die), the garbage
>   collection cycle is "extended" to rescue the additional class loader
>   instances and all objects they can reach.
>
> e) Any additional work of the garbage collection cycle is done (e.g.
>   soft, weak, and phantom references, finalization handling).
>
> f) All class loader structures are visited again.  Every structure for
>   which the weak reference has NOT been set to NULL has its hard
>   reference set to the weak reference target.  Every structure for
>   which the weak reference has been set to NULL is now ready to be
>   unloaded (i.e. release all of its native resources, including jitted
>   code, class information, method information, vtables, and so on).
>
>
> In addition,I highly recommend using the approach proposed in Chapter 3
> of http://sablevm.org/people/egagnon/gagnon-phd.pdf for managing
> class-loader related memory.  It has many advantages:
>
> 1- No "header space" overhead for very small allocations.  [This is a
> typical "hidden" space overhead of malloc() implementations to allow
> for later free() calls].
> 2- Minimal memory fragmentation.  [Allocation only happens in large
>   blocks].
> 3- Simple and very efficient allocation.  [No overhead for complex
>   management of freeing small areas later].
> 4- Efficient freeing of large memory blocks on class unloading.
> 5- Possibility of clever usage of this memory; see Chapter 4 of the same
>   document for the implementation of sparse interface virtual tables
>   enabling invokeinterface at the simple cost of invokevirtual.  :-)
>
>
> I hope this is useful to both projects [drlvm][sablevm]  :-)
>
> Etienne
>
> (C) 2006 by Etienne M. Gagnon <egagnon@sablebm.org>
> This text is licensed under the Apache License, Version 2.0.
>
> [You may add this document in svn;  I am willing to sign the required
> Apache agreement to make it so, if you intend to use it in drlvm's
> implementation].
>
> --
> Etienne M. Gagnon, Ph.D.            http://www.info2.uqam.ca/~egagnon/
> SableVM:                                       http://www.sablevm.org/
> SableCC:                                       http://www.sablecc.org/
>
>
>


-- 
Weldon Washburn
Intel Enterprise Solutions Software Division

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