RFR: fix loop unswitching with -XX:-ShenandoahWriteBarrierMemBar

[Roman Kennke]

Am 1. Februar 2018 11:04:17 MEZ schrieb Roland Westrelin <rwestrel at redhat.com>:
>
>> With the patch, -XX:-ShenandoahWriteBarrierMemBar does not crash 
>> anymore, but it's significantly slower than with membar... like 75% 
>> slower. Which seems illogical.
>>
>> tried with:
>> -XX:+UseShenandoahGC -XX:+UnlockDiagnosticVMOptions 
>> -XX:+UnlockExperimentalVMOptions -Xms4g -Xmx4g 
>> -XX:ShenandoahGCHeuristics=traversal -XX:ShenandoahFreeThreshold=17 
>> -XX:-ShenandoahWriteBarrierMemBar
>>
>> on specjvm compiler.
>
>I don't see that:
>
>-ShenandoahWriteBarrierMemBar
>Compiler.compiler  thrpt   30  378.604 ± 18.569  ops/min
>
>+ShenandoahWriteBarrierMemBar
>Compiler.compiler  thrpt   30  398.564 ± 23.585  ops/min
>
>current repo with a product build.

Yes. But without membars, I would expect it to be faster or equal compared to with membars. But not slower... Or am I missing something?

Roman

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