[aarch64-port-dev ] RFR: 8216557 Aarch64: Add support for Concurrent Class Unloading
zgu at redhat.com
Fri Mar 27 14:01:15 UTC 2020
On 3/26/20 6:42 PM, Stuart Monteith wrote:
> Please review this change to implement nmethod entry barriers on
> aarch64, and hence concurrent class unloading with ZGC. Shenandoah will
> need to be separately tested and enabled - there are problems with this
> on Shenandoah.
I identified a problem that failed TestStringDedupStress.java, I have
fix for it.
Would you mind to share what else failed with Shenandoah?
> It has been tested with JTreg, runs with SPECjbb, gcbench, and Lucene as
> well as Netbeans.
> In terms of interesting features:
> With nmethod entry barriers, immediate oops are removed by:
> LIR_Assembler::jobject2reg and MacroAssembler::movoop
> This is to ensure consistency with the entry barrier, as otherwise with
> an immediate we'd otherwise need an ISB.
> I've added "-XX:DeoptNMethodBarrierALot". I found this functionality
> useful in testing as deoptimisation is very infrequent. I've written it
> as an atomic to avoid it happening too frequently. As it is a new
> option, I'm not sure whether any more is needed than this review. A new
> test has been added
> "test/hotspot/jtreg/gc/stress/gcbasher/TestGCBasherDeoptWithZ.java" to
> test GC with that option enabled.
> This method emits the barrier code. In internal review it was suggested
> the "dmb( ISHLD )" should be replaced by "membar(LoadLoad)". I've not
> done this as the BarrierSetNMethod code checks the exact instruction
> sequence, and I prefer to be explicit.
> Benchmarking method entry shows an increase of around 6ns with the
> nmethod entry barrier.
> The deoptimisation code was contributed by Andrew Haley.
> The bug:
> The webrev:
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