RFR: 8253478: (se) epoll Selector should use eventfd for wakeup instead of pipe [v6]

[Brian Burkhalter]

> Please review this change which modifies the Linux `epoll(7)`-based `Selector` to use `eventfd(2)` instead of `pipe(2)` in its wakeup mechanism. The change passes all tier 1-tier 3 tests on Linux. Based on rudimentary testing, there does not appear to be any appreciable change in performance. One improvement however is that only one file descriptor instead of two is used for the wakeup. No test is included as the code is covered well by existing tests.

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8253478: clean up comment in epoll selector

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2082/files
  - new: https://git.openjdk.java.net/jdk/pull/2082/files/839873bd..b231c536

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2082&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2082&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2082.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2082/head:pull/2082

PR: https://git.openjdk.java.net/jdk/pull/2082