Question

I’m creating a concurrent memory reclamation algorithm in C++. Periodically, the stacks of executing mutator threads need to be inspected, so that I can see what references the threads are currently holding. In the process of doing this, I need to also check the registers of the mutator thread to check any references that might be in there.

Clearly many JVM’s and C# vm’s have no problem doing this as part of their garbage collection cycles. However, I haven’t been able to find a definitive solution to this issue.

I can’t quite tease apart what is going on in the Bohem garbage collector in order to inspect the root set, if you can (or know how its done), I’d really like to know.

Ideally I would be able to cause the mutator thread to be interrupted, and execute a piece of handler code which would report it’s PC and also flush any register-based references into the stack, and then perhaps help finish the collection cycle. I believe that most compilers in most systems will automatically flush the registers when interrupt or signal handlers are called, but I’m not clear on the specifics, or how to access that data. It seems that separate stacks might be used for interrupt and signal handlers. Additionally, I can’t find any information about how to target a particular thread, or how to send a signal. Windows does not seem to support this form of signaling anyway, and I would like my system to run on both Linux and Windows on x86-64 processors.

Edit: SuspendThread() is used in some situations, although safepoints seem to be preferred. Any ideas on why? Is there any way to deal with long-lasting I/O waits or other waits for kernel code to return?

Answer 1

I thought this was a very interesting question, so I dug into it a bit. It turns out that the Hotspot JVM uses a mechanism called “safepoints” which cause the threads of the JVM to cooperatively all stop themselves so that the GC can begin. In other words, the thread initiating GC doesn’t forcibly stop the other threads, the other threads voluntarily suspend themselves by various clever mechanisms.

I don’t believe the JVM scans registers, because a safepoint is defined such that all roots are known (I presume this means in memory).

For more information see:

• HotSpot Glossary — which defines safepoints
• safepoint.cpp — the source in HotSpot that implements safepoints
• A slide deck that describes safepoints in some detail (look 10 slides or so in)

In regards to your desire to “interrupt” all threads, according to the slide deck I referenced above, thread suspension is “unreliable on Solaris and Linux, e.g., spurious signals.” I’m not sure what mechanism even exists for thread suspension that the slides would be referring to.