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

[Thomas Stuefe]

On Fri, 29 Sep 2023 07:07:00 GMT, Kim Barrett <kbarrett 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>
>
> src/hotspot/share/gc/shared/pretouchTask.cpp line 75:
> 
>> 73:   // initially always use small pages.
>> 74:   page_size = UseTransparentHugePages ? (size_t)os::vm_page_size() : page_size;
>> 75: #endif
> 
> I never liked this, so happy to see it gone.

It was also the wrong place for this fix since it left out naked calls to os::pretouch_memory.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15781#discussion_r1345851483