RFR: Disable matrix for good when -UseShenandoahMatrix

[Roman Kennke]

Am 24.03.2017 um 18:25 schrieb Aleksey Shipilev:
> Hi,
>
> Turns out, clearing matrix is costly, and it sometimes happens during "prepare
> evac" at final mark pause:
>
> [56.054s][info][gc,stats]   Prepare Evacuation        =     0.22 s
>     (a =    16725 us) (n =    13)
>      (lvls, us =      137,    16992,    17383,    18555,    19583)
>
>
> This fix disables matrix operations for good when matrix is disabled:
>   http://cr.openjdk.java.net/~shade/shenandoah/matrix-disable-for-good/webrev.01/
>
> This returns final mark pauses to normal. We would need to look into fixing this
> performance pothole for the case the matrix is enabled and needed by GC.
>
> Testing: hotspot_gc_shenandoah, benchmarks
>
> Thanks,
> -Aleksey
>
Can we avoid clearing the bitmap if we don't need it and the
clear_all()/internal_clear_all() distinction?

Other than that, good!

Roman