On Wed, Sep 9, 2020 at 5:49 AM St��phane Ducasse <stephane.ducasse@inria.fr> wrote:
>
> Hi
>
> I would to be able to see when an incremental GC is happening.

You can't.�� All GC activity happens while the mutator (the rest of the Smalltalk VM) has stopped executing. ��i.e. GC does not occur in parallel with the rest of the VM.
��
> is there a way to know when a GC is happening?

It is impossible,��because effectively GCs happen between sends,��and even between bytecodes.�� But the VM collects statistics that show how many GCs of the various kinds have occurred and how much time has been sent in them ��The statistics are made available through the vm parameterAt: primitives.�� They are typically displayed in a SystemReporter dialog.�� Here's an example from a Squeak image:

Virtual Machine Statistics

--------------------------

uptime 2d 5h 9m 56s (runtime 10h 27m 28s, idletime 1d 18h 42m 28s)

memory 238,690,304 bytes

old 229,738,272 bytes (96.2%)

young 7,190,528 bytes (3%)

used 198,213,464 bytes (83%)

free 32,454,736 bytes (13.6%)

GCs 110,155 (1737.5 ms between GCs 341.8 ms runtime between GCs)

full 115 totalling 17,644 ms (0.05% runtime), avg 153.4 ms

marking 6,600 ms (37.4%) avg 57.4 ms,

compacting 11,044 ms (62.6%) avg 96 ms

scavenges 110,040 totalling 68,213 ms (0.18% runtime), avg 0.6 ms

tenures 82,870,154 (avg 753 tenures per scavenge)

Code compactions

699 totalling 625 ms (0.002% runtime), avg 0.9 ms

These times were collected on a 2.9 GHz Intel Core i9 MacBook Pro.

A different question is can one be informed when a GC just happened?�� The current answer is no, but it would be very easy to add.�� In VisualWorks, for example, a WeakArray is primed with an Object instance, and this gets collected every scavenge.�� So the WeakArray is notified.�� From this VW builds a notification system.�� I would engineer it differently.�� If there is a Semaphore in either of two slots in the specialObjectsArray the VM could signal one Semaphore whenever a scavenge occurs and signal the other when a full GC occurs.

P.S> ��can we please stop using the term "incremental GC" for a young space collection?�� In Spur the young��space GC is a classic Ungar-style scavenger, so I like to call these GCs scavenges.�� Soon I hope that the Spur VM will have a proper incremental global GC. This will comprise a mark-sweep collector and a compactor fort old space, but both will operate incrementally.�� The mark phase will be done in increments, allowing very small pause times as marking proceeds.�� Likewise sweeping to free objects and especially compacting, to coalesce��storage, will be done in small increments.�� So there will be pause times of the order of 2 or 3 millisecondes due to the incremental collector and overall the system will avoid the relatively long pauses the global GC causes.

HTH
_,,,^..^,,,_
best, Eliot