RFR: 8303215: Make thread stacks not use huge pages

[Poonam Bajaj]

When a system has Transparent Huge Pages (THP) enabled (/sys/kernel/mm/transparent_hugepage/enabled is set to 'always'), thread stacks can have significantly more resident set size (RSS) than they actually require. This occurs when the stack size is 2MB or larger, which makes the memory range of the stack more likely to be aligned on a 2MB boundary. This in turn makes the stack eligible to be backed by transparent huge pages resulting in more memory consumption than it would otherwise when standard small pages are used. This issue is more apparent on AArch64 platforms where the default stack size is 2MB. 


Example mapping from smaps illustrating this issue:
fffced200000-fffced204000 ---p 00000000 00:00 0 
Size:                16 kB     # guard pages
KernelPageSize:       4 kB
MMUPageSize:          4 kB
...
fffced204000-fffced400000 rw-p 00000000 00:00 0 
Size:              2032 kB    # stack space
KernelPageSize:       4 kB
MMUPageSize:          4 kB
Rss:               2032 kB    # entire stack resident in memory


This fix addresses this issue with the following two main changes:

1. Change the default stack size to 2040KB, which is 2 pages less than 2MB. This ensures that stacks don't get 2MB aligned. And why 2 pages less than 2MB, because for non-JavaThreads, glibc adds an additional guard page to the total stack size. To keep it simple and to keep the default stack size value for  all options - ThreadStackSize, CompilerThreadStackSize, and VMThreadStackSize same, we use the default value as 2040K.

Example mapping for a JavaThread:

ffff6e913000-ffff6e917000 ---p 00000000 00:00 0 
Size:                 16 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
...
ffff6e917000-ffff6eb11000 rw-p 00000000 00:00 0 
Size:               2024 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                  92 kB

Example Mapping for a non-JavaThread (WatcherThread):

ffff6eb11000-ffff6eb12000 ---p 00000000 00:00 0 
Size:                  4 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
...
ffff6eb12000-ffff6ed10000 rw-p 00000000 00:00 0 
Size:               2040 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                  12 kB


2. If the requested stack size is greater than 2MB and can be 2MB aligned, then add an additional page to the stack size. This reduces the chances of stacks getting large page aligned.

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

Commit messages:
 - 8303215: Make thread stacks not use huge pages

Changes: https://git.openjdk.org/jdk/pull/14105/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14105&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303215
  Stats: 13 lines in 2 files changed: 8 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/14105.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14105/head:pull/14105

PR: https://git.openjdk.org/jdk/pull/14105