[Vm-dev] Instance mutation [Was [Pharo-dev] threading in Pharo]

Eliot Miranda eliot.miranda at gmail.com
Tue Mar 25 16:46:30 UTC 2014

Hi Phil, Hi ClassBuilder people,

On Mar 25, 2014, at 5:16 AM, "phil at highoctane.be" <phil at highoctane.be>

On Tue, Mar 25, 2014 at 1:05 PM, Igor Stasenko <siguctua at gmail.com> wrote:

> On 24 March 2014 22:54, phil at highoctane.be <phil at highoctane.be> wrote:
>> On Mon, Mar 24, 2014 at 8:23 PM, Alexandre Bergel <
>> alexandre.bergel at me.com> wrote:
>>> >> I am working on a memory model for expandable collection in Pharo.
>>> Currently, OrderedCollection, Dictionary and other expandable collections
>>> use a internal array to store their data. My new collection library recycle
>>> these array instead of letting the garbage collector dispose them. I simply
>>> insert the arrays in an ordered collection when an array is not necessary
>>> anymore. And I remove one when I need one.
>>> >
>>> > Hm, is that really going to be worth the trouble?
>>> This technique reduces the consumption of about 15% of memory.
>>> >> At the end, #add:  and #remove: are performed on these polls of
>>> arrays. I haven't been able to spot any problem regarding concurrency and I
>>> made no effort in preventing them. I have a simple global collection and
>>> each call site of "OrderedCollection new" can pick an element of my global
>>> collection.
>>> >>
>>> >> I have the impression that I simply need to guard the access to the
>>> global poll, which is basically guarding #add:  #remove: and #includes:
>>> >
>>> > One of the AtomicCollections might be the right things for you?
>>> I will have a look at it.
>>> >> What is funny, is that I did not care at all about multi-threading
>>> and concurrency, and I have not spotted any problem so far.
>>> >
>>> > There isn't any 'multi-threading' like in Java, you got a much more
>>> control version: cooperative on the same priority, preemptive between
>>> priorities.
>>> > So, I am not surprised. And well, these operations are likely not to
>>> be problematic when they are racy, except when the underling data structure
>>> could get into an inconsistent state itself. The overall operations
>>> (adding/removing/searing) are racy on the application level anyway.
>>> >
>>> > However, much more interesting would be to know what kind of benefit
>>> do you see for such reuse?
>>> > And especially, with Spur around the corner, will it still pay off
>>> then? Or is it an application-specific optimization?
>>> I am exploring a new design of the collection library of Pharo. Not all
>>> the (academic) ideas will be worth porting into the mainstream of Pharo.
>>> But some of them yes.
>>> Thanks for all your help guys! You're great!
>>> Cheers,
>>> Alexandre
>>> --
>>> _,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:
>>> Alexandre Bergel  http://www.bergel.eu
>>> ^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;.
>> An interesting method I stumbled upon which may help in understanding how
>> these things do work.
>> BlockClosure>>valueUnpreemptively
>>  "Evaluate the receiver (block), without the possibility of preemption
>> by higher priority processes. Use this facility VERY sparingly!"
>> "Think about using Block>>valueUninterruptably first, and think about
>> using Semaphore>>critical: before that, and think about redesigning your
>> application even before that!
>>  After you've done all that thinking, go right ahead and use it..."
>> | activeProcess oldPriority result semaphore |
>>  activeProcess := Processor activeProcess.
>> oldPriority := activeProcess priority.
>> activeProcess priority: Processor highestPriority.
>>  result := self ensure: [activeProcess priority: oldPriority].
> I would not recommend you to use this method for anything.
> This method heavily relies on how process scheduler works, and in case of
> any changes, it may break everything.
> For the sake of programming, one shall never assume there is a way to
> "stop the world while i busy doing something".

If you reshape the world, it makes sense. I was looking at how classes were
migrated, that's why I found it.
And all of the new Pharo way of doing these things.

Hey, it is becoming really cool down there. Martin and Camille have been
hard at work. Kudos!

migrateClasses: old to: new using: anInstanceModification
 instanceModification := anInstanceModification.
old ifEmpty:  [ ^ self ].
 1 to: old size do: [ :index |
self updateClass: (old at: index) to: (new at: index)].
old elementsForwardIdentityTo: new.
 " Garbage collect away the zombie instances left behind in garbage memory
in #updateInstancesFrom: "
" If we don't clean up this garbage, a second update would revive them with
a wrong layout! "
 " (newClass rather than oldClass, since they are now both newClass) "
Smalltalk garbageCollect.
 ] valueUnpreemptively

The global GC here is pretty unfortunate.  It is there because the VM used
to leave old instances lying around.  It works like this:

we want to reshape instances of class C, e.g. by adding an inst var, and so

1. create C', which is C plus an inst var
2. create a parallel set of instances of class C', one for each instance of
class C
3. for each corresponding pair of instances copy state from the instance of
C to the instance of C'
4. forward-become the instances of C to the instances of C' (now no
references to the instances of C remain)
5. become C to C' (now C' is the new C)

The bug is that the old instances of C are still in the heap.  Because of
the become in 5. they look like instances of the new C, but are the wrong
size; they lack space for the new inst var.  They're not reachable (4.
replaced all references to them with references to the instances of C') but
they can be resurrected through allInstances (someInstance,nextInstance)
which works not by following references from the roots (Smalltalk and the
activeProcess) but by scanning objects in the heap.

However, this was "fixed" in

Name: VMMaker.oscog-eem.254
Author: eem
Time: 11 January 2013, 7:05:37.389 pm
UUID: 74e6a299-691e-4f7d-986c-1a7d3d0ec02c
Ancestors: VMMaker.oscog-eem.253

Fix becomeForward: so that objects whose references are deleted are
freed and can no longer be resurrected via allObjects or allInstances.

The change is to free the objects replaced in a forwardBecome so they are
no longer objects (effectively their class is null (not nil, but 0)).  So
they can't be resurrected and hence the global GC is un necessary.  The
Newspeak folks, in particular Ryan Macnak, spotted this and encouraged me
to make the change.  It of course speeds up instance mutation considerably.

I say fixed because there was a bug tail:

Name: VMMaker.oscog-eem.258
Author: eem
Time: 18 January 2013, 11:01:23.072 am
UUID: da1433f1-de50-475f-be33-f462b300a2ea
Ancestors: VMMaker.oscog-eem.257

Fix becomeForward: when the rootTable overflows.  There were two
bugs here.  One is that initializeMemoryFirstFree: used to clear the
needGCFlag so if the rootTable overflowed noteAsRoot:headerLoc:'s setting
of the needGCFlag would be undone after the sweep.
The other is that rootTable overflow was indicated by
rootTableCount >= RootTableSize which could be undone by
becomeForward: freeing roots which need to be removed from
the rootTable.  At some point in becomeForward the rootTable would
fill but at a later point a root would be freed, causing the table to
become not full.

The fix is two fold.  1. Add an explicit rootTableOverflowed flag
instead of relying on rootTableCount >= RootTableSize.
2. move the clearing of the needGCFlag to the GC routines.
Remove unnecessary senders of needGCFlag: false, and remove
the accessor.

Name: VMMaker.oscog-eem.255
Author: eem
Time: 12 January 2013, 6:28:41.398 pm
UUID: 51e53ec1-8caf-41f6-9293-1088ef4b82d8
Ancestors: VMMaker.oscog-eem.254

Fix freeing of objects for becomeForward:.  Remove freed young
roots from the rootsTable.  Filter freed objects pointet to from the
extraRootsTable (because these locations can change it is wrong
to remove entries from the extraRootsTable).

But the bottom line is that, at least on the current Cog VM, that global GC
is unnecessary.  David, Tim, this still needs to be folded into
ObjectMemory in the standard interpreter. But doing so is very worth-while.
 Monticello loads are noticeably faster.


> --
> Best regards,
> Igor Stasenko.

Eliot (phone)
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