RFR: 8315923: pretouch_memory by atomic-add-0 fragments huge pages unexpectedly

[David Holmes]

On Mon, 18 Sep 2023 07:37:26 GMT, Liming Liu <duke at openjdk.org> wrote:

> As described at [JDK-8315923](https://bugs.openjdk.org/browse/JDK-8315923), this patch uses madvise with MADV_POPULATE_WRITE to pretouch memory when supported (since kernel 5.14).
> 
> Ran the newly added jtreg test on 64c Neoverse-N1 machines with kernel 4.18, 5.13 and 6.1, and observed that transparent huge pages formed right after pretouch on kernel 6.1. Recorded the time spent on the test in *seconds* with `VERBOSE=time` as the table below, and got that the patch takes improvements when the system call is supported, while does not hurt if not supported:
> 
> <table>
>   <tr>
>     <th>Kernel</th>
>     <th colspan="2"><tt>-XX:-TransparentHugePages</tt></th>
>     <th colspan="2"><tt>-XX:+TransparentHugePages</tt></th>
>   </tr>
>   <tr><td></td><td>Unpatched</td><td>Patched</td><td>Unpatched</td><td>Patched</td></tr>
>   <tr><td>4.18</td><td>11.30</td><td>11.30</td><td>0.25</td><td>0.25</td></tr>
>   <tr><td>5.13</td><td>0.22</td><td>0.22</td><td>3.42</td><td>3.42</td></tr>
>   <tr><td>6.1</td><td>0.27</td><td>0.33</td><td>3.54</td><td>0.33</td></tr>
> </table>

I can't comment on the actual use of MADV_POPULATE_WRITE (probably need to wait for @tstuefe to get back for that) nor the flags usage in the test, but the general refactoring looks okay.

Thanks

test/hotspot/jtreg/gc/parallel/TestParallelAlwaysPreTouch.java line 29:

> 27:  * @requires vm.gc.Parallel & os.family == "linux" & os.maxMemory > 30G
> 28:  * @summary Check if parallel pretouch performs normally with and without THP.
> 29:  * @comment The test is not ParallelGC-specific, but a multi-threaded GC is    \

No need for a line continuation character - \

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PR Review: https://git.openjdk.org/jdk/pull/15781#pullrequestreview-1647974313
PR Review Comment: https://git.openjdk.org/jdk/pull/15781#discussion_r1339579908