RFR: 8315923: pretouch_memory by atomic-add-0 fragments huge pages unexpectedly [v3]

[Liming Liu]

On Mon, 9 Oct 2023 07:00:06 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>
>
> Liming Liu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Cuddle ptr-operators in pretouch_memory_common

> Side note, does anyone know why we pretouch memory for _explicit_ large pages? I would have thought that memory is already online and as "live" as it can get once it is mmapped.

`UseTransparentHugePages` just gives kernel advice to use transparent huge pages. It is not regular huge pages that need to be allocated explicitly through /sys/kernel/mm/hugepages.

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

PR Comment: https://git.openjdk.org/jdk/pull/15781#issuecomment-1752467996