Integrated: 8324817: Parallel GC does not pre-touch all heap pages when AlwaysPreTouch enabled and large page disabled

[Wang Zhuo]

On Mon, 29 Jan 2024 08:34:37 GMT, Wang Zhuo <wzhuo at openjdk.org> wrote:

> AlwaysPreTouch requires all freshly committed pages to be pre-touched. While currently PS GC does not pre-touch heap pages with -XX:+AlwaysPreTouch.
> It is related to [JDK-8315923](https://bugs.openjdk.org/browse/JDK-8315923), which fixes the issue when huge pages are used. But the bug still stands if regular page are used. On linux we can reproduce this bug when /sys/kernel/mm/transparent_hugepage/enabled is madvise or never, but cannot reproduce when it is always.
> 
> A simple way to reproduce, please make sure large pages are NOT used.
> Just run a test with the following parameters
> -XX:+AlwaysPreTouch -Xms31g -Xmx31g -XX:+UseParallelGC
> There is no pre-touching during heap initialing stage. After initialization, RSS of the test process is much smaller than memory committed.
> On the contrary, using -XX:+UseG1GC and run the test again
> -XX:+AlwaysPreTouch -Xms31g -Xmx31g -XX:+UseG1GC
> it takes several seconds to pre-touch heap pages during initialization, and RSS usage after initialization is similar to memory committed. This is the expected behavior of AlwaysPreTouch
> 
> This issue related to [JDK-8283935](https://bugs.openjdk.org/browse/JDK-8283935), which uses alignment() instead of os::vm_page_size() as pre-touching step size. The value of alignment() is usually bigger than OS page size, which causes most heap pages are not pre-touched.

This pull request has now been integrated.

Changeset: 80642dd7
Author:    Wang Zhuo <wzhuo at openjdk.org>
Committer: Denghui Dong <ddong at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/80642dd7af3fcc7c042f11798c5cc899e20b9368
Stats:     83 lines in 2 files changed: 81 ins; 0 del; 2 mod

8324817: Parallel GC does not pre-touch all heap pages when AlwaysPreTouch enabled and large page disabled

Reviewed-by: ayang, tschatzl

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PR: https://git.openjdk.org/jdk/pull/17610