RFR: JDK-8280940: gtest os.release_multi_mappings_vm is racy [v2]

[Thomas Stuefe]

On Wed, 9 Feb 2022 07:06:29 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> test/hotspot/gtest/runtime/test_os.cpp line 449:
>> 
>>> 447:   if (MemTracker::tracking_level() > NMT_off) {
>>> 448:     return;
>>> 449:   }
>> 
>> Not a review (I'm not an expert in the relevant area), just a couple drive-by comments.  However, GitHub UI won't let me comment on the parts that I want.  So getting as close as I can.
>> 
>> (1) The `TEST_VM` line should be outdented.
>> 
>> (2) After 8277822, I think the tracking level is always going to be > `NMT_off` in a debug build, so we'll only be testing in product builds.  That seems problematic.
>
> Hi Kim,
> 
> thanks for taking a look!
> 
>> Not a review (I'm not an expert in the relevant area), just a couple drive-by comments. However, GitHub UI won't let me comment on the parts that I want. So getting as close as I can.
>> 
>> (1) The `TEST_VM` line should be outdented.
> 
> Sure.
> 
>> 
>> (2) After 8277822, I think the tracking level is always going to be > `NMT_off` in a debug build, so we'll only be testing in product builds. That seems problematic.
> 
> We run the gtests in all NMT modes (off, summary, default), see:
> 
> https://github.com/openjdk/jdk/blob/072e7b4da0449ab7c1ab1ba0cfbb3db233823e7c/test/hotspot/jtreg/gtest/NMTGtests.java#L30-L36
> 
> so we run these tests in debug builds with NMT=off. The NMT gtests have been introduced with 8256844 and extended to cover the os* tests with 8277822.
> 
> That said, I should take a look at 8263464, see if this still is a problem.
> 
> Cheers, Thomas

Okay, https://bugs.openjdk.java.net/browse/JDK-8263464 is still a problem. This means that even though we can release multiple mappings with os::release_memory, NMT cannot cope with that.

AFAIK the only real example of releasing multiple segments in one go is Windows + UseNUMA, and there os::release_memory(), when called over multiple segments, just recursively releases the segments individually. So I believe this works with NMT. Still, it would be nice to fix this.

@zhengyu123 : do you think https://bugs.openjdk.java.net/browse/JDK-8263464 is difficult to fix? Should I take a stab at it or do you want to take a look?

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PR: https://git.openjdk.java.net/jdk/pull/7288