GetPreemptionLatency in Sun's RTJ

Question

Hello,
I've tried the following piece of code to obtain information about the interference introduced by the garbage collector:
System.out.println( RealtimeSystem.currentGC().getPreemptionLatency());in the following system:
- Ubuntu (9.1)
- Real-time Linux Patch (linux-rt package)
- Sun's VM
- 1,6 Ghz Laptop
My results showed that the preemption time for JRTS 2.1 is 1 ms. After testing this value empirically, running one low-priority thread which is executing System.gc() and measuring the interference on a high-priority NoHeapRealTimeThread, some questions come to my mind.
- Is this the maximum interference any NoHeapRealTimeThreadshould expected from the garbage collector?
- How is it possible that a garbage collector obtain this value?
- Is there any strategy to reduce the preemption time?
Thanks,
Pablo
Edited by: pbasanta@ on Dec 3, 2009 10:41 AM

Answer 1

Hi Bertrand,
Bertrand.Delsart wrote:
Hi Pablo,
By construction, NHRTs do not have any reference to the Heap on their stack. Hence the RTGC need not scan their >stack.
So, if you have a NHRT that has a reference to an object in immortal memory that has a connection to an object allocated in the heap.
(NHRT->[scopedmemory or immortal object]->Heap-object)
Then, according to your comment, el heap object is not destroyed if:
1) There is another object in immortal memory that has a reference to the object (even if the object is not reachable)
2) The whole scope (*LTMemory*) is not reclaimed (even if the object is not reachable).
>
The interference of the RTGC logic on the NHRT is negligible (mainly activating and deactivating barriers). We can still reach latencies in the tens of microseconds with NHRT when the RTGC is running.
Of course, at this level of determinism, you have to be careful with hardware issues (cache misses, bus saturation, ...) and use for instance processor sets and interrupt shielding.In this case, the garbage collector is similar to another thread. Nice solution .;)
Thanks,
Pablo
Edited by: pbasanta@ on Dec 4, 2009 4:16 AM