On 9/3/22 9:35 AM, frame wrote:
I'm not sure I fully understand how it works. I know that the OS creates
read only memory pages for both and if a memory section is about to be
written, the OS will issue a copy of the pages so any write operation
will be done in it's own copy and cannot mess up things.
But then is the question, how can memory be marked as free? The forked
process cannot since it writes into a copy - how it is synchronized then?
Is the GC address root somehow shared between the processes? Or does the
forked process communicate the memory addresses back to the parent?
It definitely communicates back to the parent. I'm not sure the
mechanism, either shared memory or a pipe.
The information communicated back is which blocks can be marked as
unreferenced, then the sweep is done in the original process.
Are freeing GC operations just locked while the forked process is running?
I'm not sure, but I would think it's possible not to. Only during the
freeing of the blocks does it need to lock the GC.
What happens if a manually `GC.free()` is called while the forked
process marks the memory as free too but the GC immediately uses the
memory again and then gets the notification to free it from the forked
child? Can this happen?
No, because if you can free it, you should have had a reference to it
when you forked, which should mean it's not garbage.
There's a talk on it from the 2013 dconf by the inventor:
https://dconf.org/2013/talks/lucarella.html
-Steve
