RFR (S): 8155230: Tune thread usage for mark bitmap clear

[Thomas Schatzl]

Hi all,

  during testing of 8077144 I found that mark bitmap clearing can be
significantly sped up by not using that many threads.

A good conservative estimate (tends to use too many threads) that still
cuts mark bitmap clear time by >50% is to give each thread at least 1
MB of data to clear.

CR:
https://bugs.openjdk.java.net/browse/JDK-8155230
Webrev:
http://cr.openjdk.java.net/~tschatzl/8155230/webrev/
Testing:
jprt, webrev, various performance tests

Depends on 8155232.

Thanks,
  Thomas