RFR: 8266821: G1: Prefetch cards during merge heap roots phase [v2]

[Thomas Schatzl]

> Hi all,
> 
>   can I have reviews for this change that improves performance of the merge heap roots phase in g1?
> 
> Merging heap roots (remembered sets and log buffers) into the card table for later scanning to a large degree constitutes random accesses to the card table.
> 
> Putting a small prefetch queue between calculating the card table address and inspecting the given card table value decreases merge remembered set time by 20-30% and merge log buffers time by 40-50% (on x64. AArch64 shows similar if not better improvements).
> 
> Applications not having a significant amount of either remembered sets or log buffers do not show significant difference. 
> 
> The cache size constants for arm64 and x86 were derived from testing with a benchmark creating lots of references and log buffers (i.e. BigRamTester). Could not do any corresponding tests on other architectures due to lack of machines.
> 
> Testing: tier1-3, lots of tier1-tier8 runs with [JDK-8017163](https://bugs.openjdk.java.net/browse/JDK-8017163).

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  kbarrett review

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3975/files
  - new: https://git.openjdk.java.net/jdk/pull/3975/files/9a1d8ef9..288ba6dc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3975&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3975&range=00-01

  Stats: 25 lines in 7 files changed: 2 ins; 0 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3975.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3975/head:pull/3975

PR: https://git.openjdk.java.net/jdk/pull/3975