Fwd: Tracking object relocations in Java 8

[Thomas Schatzl]

Hi,

On 19.11.20 10:47, David Griffiths wrote:
> BTW doesn't have to be a call to a "copy(from, to)" style function - if
> there is some convenient place where a list of pointers is traversed to do
> the update to the new location I'm quite happy to duplicate that code to do
> the same traversal. I'm not yet familiar with the code so just a nudge in
> the general direction would be great.

In Java 8, parallel gc is default. The relevant method is 
PSPromotionManager::copy_to_survivor_space() there for young gc.

There is no general task list that is guaranteed to contain all objects 
that are copied in any collector I'm aware of although there is some 
"task queue". Objects are copied without going to that queue in some 
cases though (e.g. if they are directly referenced by some VM internal 
pointer, i.e. a "root").

So hooking into these "copy(from, to)" methods is your best bet.

Apart from that, traversal of objects occurs in several different places 
("OopClosures") dependent on where that object is processed from. E.g. 
the do_oop() methods in PSRootsClosure, PSScavengeFromCLDClosure, 
PSKeepAliveClosure and others, so that's fairly inconvenient to hook into.

> 
> Another alternative - don't suppose there is any option to force hashcodes
> to always be stored in the mark word (i.e. even if the hash code is not
> requested)? Couldn't see any sign of one in the code but just thought I'd
> ask. I guess -XX:hashCode=4 is just going to use the current address and
> only if hash code requested so of no use to me.
> 

I am not aware of such an option but maybe others can help.

Hth,
   Thomas