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

[Johan Sjölen]

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>

Some nits, but looks good to me.

src/hotspot/share/runtime/os.hpp line 230:

> 228:   // Some platforms may have special treatments for pretouch, while most
> 229:   // platforms do the same. So the common part of the code was extract here to
> 230:   // avoid copying it around.

Small grammar fixes:
- treatment in singular
- do the same *thing*
- was extract*ed*
- Missing comma after "So"

```c++
 // Some platforms may have special treatment for pretouch, while most
 // platforms do the same thing. So, the common part of the code was extracted here to
 // avoid copying it around.

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PR Review: https://git.openjdk.org/jdk/pull/15781#pullrequestreview-1652473163
PR Review Comment: https://git.openjdk.org/jdk/pull/15781#discussion_r1342517099