RFR: 8315923: pretouch_memory by atomic-add-0 fragments huge pages unexpectedly [v21]

[Thomas Stuefe]

On Thu, 28 Dec 2023 09:26:19 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:
> 
>   Use pthread instead

test/hotspot/jtreg/runtime/os/TestTransparentHugePageUsage.java line 96:

> 94:           .map(e -> Long.valueOf(e.getKey().substring(e.getValue().start(1), e.getValue().end(1))));
> 95:       if (!usage.isPresent()) throw new RuntimeException("The usage of THP was not found.");
> 96:       if (usage.get() == 0) throw new RuntimeException("The usage of THP should not be zero.");

The effect we would see without your patch would be small pages that are then converted to huge pages by khugepaged in its own time, right? So, maybe test that AnonHugePages == Size for the heap VMA?

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PR Review Comment: https://git.openjdk.org/jdk/pull/15781#discussion_r1448478423