[Vm-dev] Garbage collector issue

[Pavel Krivanek]

Hi,

in Pharo and in Squeak, if you try to print:

Point allInstances size.

several times, you will get fastly growing number of Point instances in
most cases. But when you do:

Smalltalk garbageCollect. Point allInstances size.

The number of instances is quite stable.

I realized that about every 5 milliseconds two new Point instances are
created and not collected in reasonable time. I tried to find what points
to them and I realized that nothing. That two instances contain display
size so it was easy to find the place where these points are created. In
Pharo it is in

MorphicUIManager>>#checkForNewDisplaySize

"Check whether the screen size has changed and if so take appropriate
actions"

Display extent = DisplayScreen actualScreenSize ifTrue: [^ Display].
DisplayScreen startUp.
World restoreMorphicDisplay.

As you can see in the code, the two point instances are only very temporary
objects. And with some experiments I realized that that behavior is not
Point specific (eg. some problem with the @ primitive). Because if you
replace the code of the method with only Object basicNew, the growth of
Object instances is similar. See the attached script.

pharo-ui Pharo st script2.st

Cheers,
-- Pavel
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