RFR: JDK-8326586: Improve Speed of System.map [v3]

[Thomas Stuefe]

> A small optimization that speeds up `jcmd pid System.map` by orders of magnitude by optimizing NMT cache lookup for sequential accesses. This cache is used to map NMT against System information - and the latter usually comes in sequential form, e.g. scanned from `/proc/<pid>/maps` or via VirtualQuery.
> 
> On Linux x64, `System.map`for a JVM with an artificially bloated process space (1 million mappings) takes:
> 
> before this patch: 6.3 minutes
> with this patch: 4.2 seconds
> 
> which is a 87x speed improvement.
> 
> Ping @jdksjolen and @gerard-ziemski?

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

  Typos and stuff

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17984/files
  - new: https://git.openjdk.org/jdk/pull/17984/files/91ea5966..6efe8829

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17984&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17984&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17984.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17984/head:pull/17984

PR: https://git.openjdk.org/jdk/pull/17984