RFR: 8315923: pretouch_memory by atomic-add-0 fragments huge pages unexpectedly [v17]

[Johan Sjölen]

On Thu, 14 Dec 2023 08:12:22 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:
> 
>   Replace to char* when type casting

test/hotspot/gtest/runtime/test_os_linux.cpp line 377:

> 375:   EXPECT_TRUE(os::release_memory(heap, 1 * G));
> 376:   UseTransparentHugePages = useThp;
> 377: }

This seems like it's concurrently running `madvise(..., MADV_POPULATE_WRITE)`, correct? This is not what I meant.

What I meant was having at least 2 threads, where one thread is running `os::pretouch_memory` and another using the memory for something. For example, 1 thread pretouching, the other thread filling out the memory with an incrementing integer array `[0,1,2,3,4,...]`. I think this is what Kim meant also, or am I the one misunderstanding him?

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PR Review Comment: https://git.openjdk.org/jdk/pull/15781#discussion_r1426582129