On Wed, Apr 12, 2017 at 11:35 AM, Denis Kudriashov <dionisi...@gmail.com>
wrote:
>
> 2017-04-12 10:55 GMT+02:00 Guillermo Polito <guillermopol...@gmail.com>:
>
>> PharoClassInstaller>>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
>>>
>>> Commenting garbage collection here increases performance 10 times.
>>> Then commenting class update loop increases performance 3 times more.
>>> But this loop is required. It adopts all instances of old class to new one.
>>> And time here spent in #allInstances method.
>>>
>>> Can we remove manual garbage collection here? Why it is needed?
>>>
>>
>> Well, there is the comment that explains it and makes pretty good sense.
>>
>
> But is does not explain why these bad zombies exist. We investigates
> possible reasons and could not reproduce them. We will try remove garbage
> collection here in Pharo 7
>
No, this will break stuff! I'll try to explain what does it mean by zombie
instances to make some sense:
- Imagine that you have class A + 10 instances of A.
- We add an instance variable to A.
- this means the class builder will generate class A' that is the new
version of A.
- then, it migrates all instances of A to class A'.
This migration is not magic:
- 10 new instances of A' are created
- the state is migrated from the instances of A to A'
- each instance of A is becomed into its corresponding instance of
A'
- finally we become class A into A'
This step will make that old instances of A now have:
- the old format
- but point to the new class A
If we do not garbage collect, this means that doing
A allInstances
will return not only the new 10 instances of A, but the old instances of A'.
And that will break LOOOTS of stuff.
