I'm trying to understand the native threading model and it's
consequences for @Uninterruptible code
As I understood it - previously under Green Threads on a n core machine
invoking the rvm with -X:vm:processors=1, code in a @Uninterruptible
method would have been executed atomically as there was no possible way
for another thread to be scheduled.
Yes, that is true for processors=1, but generally not true for processors>1, so relying on Uninterruptible to enforce atomicity was never very good idea. All uninterruptible really meant for processors>1 was that the code would not hit a yieldpoint within the uninterruptible reagion. Another thread could still execute in parallel with that uninterruptible region on another processor.
With native threading each Java thread is now it's own pthread...
In user threading, every running Java thread was also identified with a pthread (=processor).
On a 1 logical processor machine am I right in thinking that an
@Uniterruptible method is no longer guaranteed to execute atomically and
that another mutator thread could be scheduled to run part way through
the @Uniterruptible method - the @Uninterruptible annotation merely
disables yieldpoints which are responsible for handshakes with the GC
Correct. Since system threads are scheduled by the system, they can yield to the system at any point in their execution, regardless of whether they are at a "yieldpoint" or not.
What is the most efficient method to prevent other threads from running
whilst my @Uninterruptible method is running on a 1 core machine?
That depends on what you are trying to achieve. There is no straightforwardly "efficient" way to do so now since we are prepared for multiprocessor execution. Thus, you must use standard concurrent programming techniques: take a lock that protects the resource you are trying to prevent others from manipulating, or use other non-blocking synchronization constructed from synchronization primitives like CAS, etc. There are plenty of places with the RVM that need to do this sort of thing, and you can browse for how they achieve it in their particular situation. There is a heavyweight alternative, which is to stop all the other threads from running (just like stop-the-world GC needs to do), but I suspect in your situation that is not a price you want to pay.
Indeed, these will need updating for the expected next release of Jikes RVM.
Antony Hosking | Associate Professor | Computer Science | Purdue University
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