8293503: gc/metaspace/TestMetaspacePerfCounters.java#Epsilon-64 failed assertGreaterThanOrEqual: expected MMM >= NNN

[tschatzl]

Hi all,

can I have reviews for this change that workarounds some epsilon gc specific behavior in the gc/metaspace/TestMetaspacePerfCounters test?

The test gathers statistics about metaspace to check one by one:

    // The perf counter values are updated during GC and to be able to
    // do the assertions below we need to ensure that the values are from
    // the same GC cycle.
    do {
        gcCountBefore = currentGCCount();

        minCapacity = getMinCapacity(ns);
        maxCapacity = getMaxCapacity(ns);
        capacity = getCapacity(ns);
        used = getUsed(ns);

[...]

For Epsilon GC the assumption stated in the comment in the code does not hold (and in general, it seems more because of knowledge about Hotspot than actual specification) - Epsilon also updates the statistics every X TLAB refills without a pause. Without these updates at TLAB refill, Epsilon would mostly always never update these (it does not do any GC by itself, only out-of-metaspace GC actually performs one).

The problem then is that if there is such an update between getCapacity() and getUsed(), used can be larger than capacity gathered before as both may have changed at the second call. It also assumes that there is sufficient memory visibility related synchronization between these calls.

There is no way to get multiple performance counter values atomically as e.g. in the MemoryMXBean API, so some inconsistency is probably expected.

So the suggested fix here is to get two snapshots of the values, separated by a memory barrier and only continue if both snapshots are equal.

Testing: running the test for 1000 times without issue, fails 10 times or so without this patch

Thanks,
Thomas

---

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Issue

• JDK-8293503: gc/metaspace/TestMetaspacePerfCounters.java#Epsilon-64 failed assertGreaterThanOrEqual: expected MMM >= NNN

Reviewers

• Leo Korinth (@lkorinth - Reviewer) ⚠️ Review applies to 4714b548
• Kim Barrett (@kimbarrett - Reviewer) ⚠️ Review applies to e22cdd54

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[lkorinth]

Looks good to me.

[openjdk]

@tschatzl This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8293503: gc/metaspace/TestMetaspacePerfCounters.java#Epsilon-64 failed assertGreaterThanOrEqual: expected MMM >= NNN

Reviewed-by: lkorinth, kbarrett

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 60 new commits pushed to the master branch:

• f42caef: 8293550: Optionally add get-task-allow entitlement to macos binaries
• 5feca68: 8293840: RISC-V: Remove cbuf parameter from far_call/far_jump/trampoline_call
• 39cd163: 8293578: Duplicate ldc generated by javac
• 7765942: 8290367: Update default value and extend the scope of com.sun.jndi.ldap.object.trustSerialData system property
• 11e7d53: 8293819: sun/util/logging/PlatformLoggerTest.java failed with "RuntimeException: Retrieved backing PlatformLogger level null is not the expected CONFIG"
• 141d5f5: 8293767: AWT test TestSinhalaChar.java has old SCCS markings
• 3beca2d: 8291600: [vectorapi] vector cast op check is not always needed for vector mask cast
• 9a40b76: 8293842: IPv6-only systems throws UnsupportedOperationException for several socket/TCP options
• bb9aa4e: 8293813: ProblemList com/sun/jdi/JdbLastErrorTest.java on windows-x64 in Xcomp mode
• 4cec141: 8291509: Minor cleanup could be done in sun.security
• ... and 50 more: https://git.openjdk.org/jdk/compare/37df5f56259429482cfdbe38e8b6256f1efaf9e8...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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[mlbridge]

Webrevs

• 02: Full - Incremental (6dd3c8be)
• 01: Full - Incremental (e22cdd54)
• 00: Full (06167117)

[kimbarrett]

Looks good, other than the name of the snapshot equivalency check.

[tschatzl]

Thanks @kimbarrett @lkorinth for your reviews
/integrate

[openjdk]

Going to push as commit 04d7b7d.
Since your change was applied there have been 74 commits pushed to the master branch:

• d77c464: 8293891: gc/g1/mixedgc/TestOldGenCollectionUsage.java (still) assumes that GCs take 1ms minimum
• d7c1a76: 8293861: G1: Disable preventive GCs by default
• 43f7f47: 8293499: Provide jmod --compress option
• 26e08cf: 8293844: C2: Verify Location::{oop,normal} types in PhaseOutput::FillLocArray
• 357a2cc: 8293937: x86: Drop LP64 conditions from clearly x86_32 code
• b1ed40a: 8293466: libjsig should ignore non-modifying sigaction calls
• b6ff8fa: 8292073: NMT: remove unused constructor parameter from MallocHeader
• cfd44bb: 8293218: serviceability/tmtools/jstat/GcNewTest.java fails with "Error in the percent calculation"
• 01e7b88: 8290917: x86: Memory-operand arithmetic instructions have too low costs
• 4b8399b: 8293251: Use stringStream::base() instead of as_string() when applicable
• ... and 64 more: https://git.openjdk.org/jdk/compare/37df5f56259429482cfdbe38e8b6256f1efaf9e8...master

Your commit was automatically rebased without conflicts.

[openjdk]

@tschatzl Pushed as commit 04d7b7d.

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