The Inbox: System-cwp.662.mcz

On Sun, Jan 12, 2014 at 10:50:03AM -0500, Colin Putney wrote:
> That diff is a bit hard to read, so here's the implementation:
>
>
> allObjectsDo: aBlock
> "Evaluate the argument, aBlock, for each object in the system
> excluding SmallIntegers. New objects created by aBlock will
> be included in the enumeration, and it is up to the caller to avoid
> creating new objects faster than they are enumerated. It's
> important that the next object is fetched before the block is
> evaluated, because the block may use #become: to change the
> identity of object."
>
> | object nextObject |
> object := self someObject.
> [nextObject := object nextObject.
> aBlock value: object.
> object := nextObject.
> 0 == nextObject]
> whileFalse.
>
>
> This version has the following properties:
>
> - objects created inside the block will be enumerated

> - callers may use become inside the block
> - callers must avoid infinite loop
>
> I think these are the most desirable semantics for #allObjectsDo:. However,
> the risk with this implementation is that it will execute an infinite loop
> on VMs that create a new context object for each block activation. Cog and
> the StackVM don't, but John's Mac VM 5.7.4.1 does, according to a quick
> test I just did. I don't know what the behaviour of older Window and Linux
> VMs are.
>
> So  the question is, what VMs are we planning to support for in 4.5, and do
> they recycle activation contexts?
>

Certainly the up to date interpreter VM and Cog/StackInterpreter will be
used, but I think that there are some folks who still need to use John's
Mac VM, so it would be good if we could find a solution that works there
also.

But I also like your implementation here, it's simple and easy to understand.

Dave

Me too. We want to enumerate all objects *created until* sending #allObjectsDo.

Say you want to wrap every object in the system in a proxy.
You would need to use #allObjectsDo:. but when we enumerate all
object, the new ones too, we end up having proxies for proxies for
proxies ad infinitum (endless non-loop…) and we can’t know it…

On 12.01.2014, at 20:42, David T. Lewis <[hidden email]> wrote:

> On Sun, Jan 12, 2014 at 02:02:08PM -0500, Colin Putney wrote:
>> On Sun, Jan 12, 2014 at 1:45 PM, Tobias Pape <[hidden email]> wrote:
>>
>>
>>> Say you want to wrap every object in the system in a proxy.
>>> You would need to use #allObjectsDo:. but when we enumerate all
>>> object, the new ones too, we end up having proxies for proxies for
>>> proxies ad infinitum (endless non-loop?) and we can?t know it?
>>>
>>
>> Yeah, that's an example of creating objects faster than we can enumerate
>> them. (Or, exactly as fast as we can enumerate them, I suppose). It
>> wouldn't have worked pre-closures, either.
>>
>> The consensus so far is that we like the safety of a sentinel object, as it
>> lets us do whatever we want within the loop, and potentially not seeing all
>> the objects in the image is an acceptable trade-off for that safety. Fair
>> enough.
>>
>
> (Changing subject line because original discussion spans commit notices)
>
> The most recent inbox entry System-cwp.663 seems to work fine on an
> interpreter VM.
>
> For my understanding, can you clarify, with respect to the original concern
> that involves MC proxies - does the problem occur with the interpreter VM
> as well as Cog, or was it specific to one kind of VM?
>
> It is worth noting that allObjectsDo: relies on assumptions about how
> the objects memory works internally. It requires that #someObject will
> always answer the object at the lowest address in the object memory, and
> also that a newly allocated object will always be placed at a higher
> address location than all other objects. Either of these assumptions is
> likely to be a problem as new and better object memories and garbage
> collectors are implemented.
>
> Dave

Right (as Eliot's vm-dev post shows).

So IMHO the only sensible semantics of allObjectsDo: is as in "allObjects do:" - which might be implemented as a primitive in some VMs soonish. It *should not* enumerate objects created after calling the method.

- Bert -

On Sun, Jan 12, 2014 at 09:53:27PM +0100, Bert Freudenberg wrote:
>
> On 12.01.2014, at 20:42, David T. Lewis <[hidden email]> wrote:
>

> > It is worth noting that allObjectsDo: relies on assumptions about how
> > the objects memory works internally. It requires that #someObject will
> > always answer the object at the lowest address in the object memory, and
> > also that a newly allocated object will always be placed at a higher
> > address location than all other objects. Either of these assumptions is
> > likely to be a problem as new and better object memories and garbage
> > collectors are implemented.
> >
> > Dave
>
> Right (as Eliot's vm-dev post shows).
>
> So IMHO the only sensible semantics of allObjectsDo: is as in "allObjects do:" -
> which might be implemented as a primitive in some VMs soonish. It *should not*
> enumerate objects created after calling the method.
>

On Sun, Jan 12, 2014 at 12:01:00PM -0800, Eliot Miranda wrote:
>
> The bug is in implementing allObjects in terms of someObject and nextObject
> in the first place.  It's cheap and cheerful but horribly error-prone and
> restrictive.  It's cheap because the collection of objects doesn't have to
> be created, and on the original 16-bit Smalltalk machines that was really
> important.  It's horribly restrictive because it assumes much about the
> implementation.
>
> Before closures a sentinel wasn't even needed because enumerating the block
> didn't create a new object (the block context was reused).  So the code had
> to be rewritten just to support closures.
>
> Spur has a generation scavenger operating in a distinct new space and that
> doesn't jive well with a consistent ordering at all.  So far the system is
> limping along by tenuring all objects on someObject and someInstance (so
> that newSpace is either empty, or doesn't contain any instances of a
> specific class) and having nextObject enumerate only objects in oldSpace.
>
> But I think now we can afford a primitive that answers all the objects
> (remember that average object size means that such a collection will be ~
> 10% of the heap, average object size in Squeak V3 is about 10.6 words).  At
> least that's what Spur will do, along with an allInstancesOf: primitive.
>  And then the become example won't cause any problems at all.  Far more
> reliable.  I suppose there are circumstances when enumerating without a
> container is the only feasible approach, but VisualWorks has got along with
> only an allObjects primitive for a long time now.  I suspect we can too.
>

Implementation attached. Works on interpreter VM, not yet tested on Cog but
it should be ok there also. If no objections or better suggestions I will
commit it to VMMaker tomorrow.

InterpreterPrimitives>>primitiveAllObjects
        "Answer an array of all objects that exist when the primitive is called, excluding those
        that may be garbage collected as a side effect of allocating the result array. Multiple
        references to nil in the last slots of the array are an indication that garbage collection
        occured, such that some of the unreferenced objects that existed at the time of calling
        the primitive no longer exist. Sender is responsible for handling multiple references to
        nil in the result array."

Hi David,

On Sun, Jan 12, 2014 at 5:13 PM, David T. Lewis <[hidden email]> wrote:

On Sun, Jan 12, 2014 at 09:53:27PM +0100, Bert Freudenberg wrote:
>
> On 12.01.2014, at 20:42, David T. Lewis <[hidden email]> wrote:
>

> > It is worth noting that allObjectsDo: relies on assumptions about how
> > the objects memory works internally. It requires that #someObject will
> > always answer the object at the lowest address in the object memory, and
> > also that a newly allocated object will always be placed at a higher
> > address location than all other objects. Either of these assumptions is
> > likely to be a problem as new and better object memories and garbage
> > collectors are implemented.
> >
> > Dave
>
> Right (as Eliot's vm-dev post shows).
>
> So IMHO the only sensible semantics of allObjectsDo: is as in "allObjects do:" -
> which might be implemented as a primitive in some VMs soonish. It *should not*
> enumerate objects created after calling the method.
>

On Sun, Jan 12, 2014 at 12:01:00PM -0800, Eliot Miranda wrote:
>
> The bug is in implementing allObjects in terms of someObject and nextObject
> in the first place.  It's cheap and cheerful but horribly error-prone and
> restrictive.  It's cheap because the collection of objects doesn't have to
> be created, and on the original 16-bit Smalltalk machines that was really
> important.  It's horribly restrictive because it assumes much about the
> implementation.
>
> Before closures a sentinel wasn't even needed because enumerating the block
> didn't create a new object (the block context was reused).  So the code had
> to be rewritten just to support closures.
>
> Spur has a generation scavenger operating in a distinct new space and that
> doesn't jive well with a consistent ordering at all.  So far the system is
> limping along by tenuring all objects on someObject and someInstance (so
> that newSpace is either empty, or doesn't contain any instances of a
> specific class) and having nextObject enumerate only objects in oldSpace.
>
> But I think now we can afford a primitive that answers all the objects
> (remember that average object size means that such a collection will be ~
> 10% of the heap, average object size in Squeak V3 is about 10.6 words).  At
> least that's what Spur will do, along with an allInstancesOf: primitive.
>  And then the become example won't cause any problems at all.  Far more
> reliable.  I suppose there are circumstances when enumerating without a
> container is the only feasible approach, but VisualWorks has got along with
> only an allObjects primitive for a long time now.  I suspect we can too.
>

Implementation attached. Works on interpreter VM, not yet tested on Cog but
it should be ok there also. If no objections or better suggestions I will
commit it to VMMaker tomorrow.

InterpreterPrimitives>>primitiveAllObjects
        "Answer an array of all objects that exist when the primitive is called, excluding those
        that may be garbage collected as a side effect of allocating the result array. Multiple
        references to nil in the last slots of the array are an indication that garbage collection
        occured, such that some of the unreferenced objects that existed at the time of calling
        the primitive no longer exist. Sender is responsible for handling multiple references to
        nil in the result array."

Instead of filling the unused slots with nil or 0, I think you should shorten the object so that it contains each object only once, and contains only the objects.  Cog contains some code for shortening.  See [New]ObjectMemory>>shorten:toIndexableSize:. 

HTH

--
best,

Eliot

2014/1/13 Eliot Miranda <[hidden email]>

Hi David,

On Sun, Jan 12, 2014 at 5:13 PM, David T. Lewis <[hidden email]> wrote:

On Sun, Jan 12, 2014 at 09:53:27PM +0100, Bert Freudenberg wrote:
>
> On 12.01.2014, at 20:42, David T. Lewis <[hidden email]> wrote:
>

> > It is worth noting that allObjectsDo: relies on assumptions about how
> > the objects memory works internally. It requires that #someObject will
> > always answer the object at the lowest address in the object memory, and
> > also that a newly allocated object will always be placed at a higher
> > address location than all other objects. Either of these assumptions is
> > likely to be a problem as new and better object memories and garbage
> > collectors are implemented.
> >
> > Dave
>
> Right (as Eliot's vm-dev post shows).
>
> So IMHO the only sensible semantics of allObjectsDo: is as in "allObjects do:" -
> which might be implemented as a primitive in some VMs soonish. It *should not*
> enumerate objects created after calling the method.
>

On Sun, Jan 12, 2014 at 12:01:00PM -0800, Eliot Miranda wrote:
>
> The bug is in implementing allObjects in terms of someObject and nextObject
> in the first place.  It's cheap and cheerful but horribly error-prone and
> restrictive.  It's cheap because the collection of objects doesn't have to
> be created, and on the original 16-bit Smalltalk machines that was really
> important.  It's horribly restrictive because it assumes much about the
> implementation.
>
> Before closures a sentinel wasn't even needed because enumerating the block
> didn't create a new object (the block context was reused).  So the code had
> to be rewritten just to support closures.
>
> Spur has a generation scavenger operating in a distinct new space and that
> doesn't jive well with a consistent ordering at all.  So far the system is
> limping along by tenuring all objects on someObject and someInstance (so
> that newSpace is either empty, or doesn't contain any instances of a
> specific class) and having nextObject enumerate only objects in oldSpace.
>
> But I think now we can afford a primitive that answers all the objects
> (remember that average object size means that such a collection will be ~
> 10% of the heap, average object size in Squeak V3 is about 10.6 words).  At
> least that's what Spur will do, along with an allInstancesOf: primitive.
>  And then the become example won't cause any problems at all.  Far more
> reliable.  I suppose there are circumstances when enumerating without a
> container is the only feasible approach, but VisualWorks has got along with
> only an allObjects primitive for a long time now.  I suspect we can too.
>

Implementation attached. Works on interpreter VM, not yet tested on Cog but
it should be ok there also. If no objections or better suggestions I will
commit it to VMMaker tomorrow.

InterpreterPrimitives>>primitiveAllObjects
        "Answer an array of all objects that exist when the primitive is called, excluding those
        that may be garbage collected as a side effect of allocating the result array. Multiple
        references to nil in the last slots of the array are an indication that garbage collection
        occured, such that some of the unreferenced objects that existed at the time of calling
        the primitive no longer exist. Sender is responsible for handling multiple references to
        nil in the result array."

Instead of filling the unused slots with nil or 0, I think you should shorten the object so that it contains each object only once, and contains only the objects.  Cog contains some code for shortening.  See [New]ObjectMemory>>shorten:toIndexableSize:. 

HTH

--
best,

Eliot

Oh, I missed this one. I can't check now, could it cleanly address my hugly trick of modifying the header like explained at
http://smallissimo.blogspot.fr/2013/04/still-hacking-largeintegerplugins-in.html ?