Moving objects between safepoints in hotspot?

[Simone Bordet]

Erik,

On Wed, Mar 5, 2014 at 7:39 PM, Erik Österlund <erik.osterlund at lnu.se> wrote:
> Hi guys,
>
> My name is Erik Österlund and I'm new to this mailing list. So first of all, hi everyone! Here's a short introduction of what I'm up to:
> I intend to make a real-time GC for OpenJDK. My main focus is concurrent wait-free compaction. I made a wait-free compaction algorithm which is wait-free both for mutators (memory accesses, no safepoints) but also for the GC (no need for handshakes either and hence alloc() can be made wait-free if root sampling, tracing and their transitions can be made wait-free which I can but that's a different story). This was all implemented in my own prototype VM and now my intention is to give my GC a new home: hotspot.
>
> This brings me to my first question in this mailing list: In order for an object comparison (o1 == o2) to work correctly, it is currently assumed in hotspot that the location of objects do not change between any two GC checkpoints.
>
> Assuming o1 and o2 are pointing to the same object, I need to avoid the following race:
>
> 1. mutator thread: r1 = load o1 (resulting in from-space address of the object)
> 2. collector thread: move o1 to to-space, installing to-space object and copying it completely
> 3. mutator thread: r2 = load o2 (resulting in to-space address of the object)
> 4. mutator thread: compare (resulting in false negative since fromspace and tospace addresses are not equal even though it is the same object)
>
> For this reason, I would need to either 1) conservatively cancel any object movement for objects touched by the mutator between any two GC checkpoints (which is a bit sad) or 2) emit a custom object equals function that deals with this properly and hence allows objects to move between checkpoints.
>
> Therefore I'm wondering, how deep does this assumption go and how modular is the current implementation of object equals in hotspot? Is there just a few entry points used everywhere or quite many all over the place?

A similar topic has been asked for shenandoah in this mailing list,
see http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2014-February/009485.html

-- 
Simone Bordet
http://bordet.blogspot.com
---
Finally, no matter how good the architecture and design are,
to deliver bug-free software with optimal performance and reliability,
the implementation technique must be flawless.   Victoria Livschitz