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8336911: ZGC: Division by zero in heuristics after JDK-8332717 #21304

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8336911: ZGC: Division by zero in heuristics after JDK-8332717 #21304

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@MBaesken MBaesken commented Oct 2, 2024 •
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When running with ubsan enabled binaries, the following issue is reported,
e.g. in test
compiler/uncommontrap/TestDeoptOOM_ZGenerational.jtr
also in gc/z/TestSmallHeap.jtr

jdk/src/hotspot/share/gc/z/zDirector.cpp:537:84: runtime error: division by zero
    #0 0x7f422495bd1f in calculate_young_to_old_worker_ratio src/hotspot/share/gc/z/zDirector.cpp:537
    #1 0x7f422495bd1f in select_worker_threads src/hotspot/share/gc/z/zDirector.cpp:694
    #2 0x7f42282a0d97 in select_worker_threads src/hotspot/share/gc/z/zDirector.cpp:689
    #3 0x7f42282a0d97 in initial_workers src/hotspot/share/gc/z/zDirector.cpp:784
    #4 0x7f42282a2485 in initial_workers src/hotspot/share/gc/z/zDirector.cpp:795
    #5 0x7f42282a2485 in start_minor_gc src/hotspot/share/gc/z/zDirector.cpp:797
    #6 0x7f42282a2485 in start_gc src/hotspot/share/gc/z/zDirector.cpp:826
    #7 0x7f42282a2485 in ZDirector::run_thread() src/hotspot/share/gc/z/zDirector.cpp:912
    #8 0x7f422840bdd8 in ZThread::run_service() src/hotspot/share/gc/z/zThread.cpp:29
    #9 0x7f4225ab6979 in ConcurrentGCThread::run() src/hotspot/share/gc/shared/concurrentGCThread.cpp:48
    #10 0x7f4227e1137a in Thread::call_run() src/hotspot/share/runtime/thread.cpp:225
    #11 0x7f42274619b1 in thread_native_entry src/hotspot/os/linux/os_linux.cpp:858
    #12 0x7f422c8d36e9 in start_thread (/lib64/libpthread.so.0+0xa6e9) (BuildId: 9a146bd267419cb6a8cf08d7c602953a0f2e12c5)
    #13 0x7f422c1dc58e in clone (/lib64/libc.so.6+0x11858e) (BuildId: f2d1cb1ef49f8c47d43a4053910ba6137673ccce)

The division by 0 leads to 'infinity' on most of our platforms. So instead of relying on this behavior, we can add a small check and set 'infinity' for divisor == 0.

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  • JDK-8336911: ZGC: Division by zero in heuristics after JDK-8332717 (Bug - P4)

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bridgekeeper bot commented Oct 2, 2024

👋 Welcome back mbaesken! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

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@MBaesken 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:

8336911: ZGC: Division by zero in heuristics after JDK-8332717

Reviewed-by: aboldtch, eosterlund

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@openjdk openjdk bot changed the title JDK-8336911: ZGC: Division by zero in heuristics after JDK-8332717 8336911: ZGC: Division by zero in heuristics after JDK-8332717 Oct 2, 2024
@openjdk openjdk bot added the rfr Pull request is ready for review label Oct 2, 2024
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@MBaesken The following label will be automatically applied to this pull request:

  • hotspot-gc

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

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xmas92
xmas92 suggested changes Oct 2, 2024
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I do not think infinity is the solution here. There are more problems with the heuristics when no young collection has reclaimed any memory.

I added a comment about this in an earlier PR (JDK-8339648 / #20888) #20888 (comment).

I proposed a solution to this specific issue that makes more sense to me, and avoid the NaN issues here. But will have to talk it over.

Regardless I think we need to do an overhaul of this code to handle the extreme case of no GC having reclaimed any memory.

Also this must have been an issue before JDK-8332717 as well?

fisk
fisk approved these changes Oct 16, 2024
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I think in the end I'd like to sanitize the input data to all these calculations to have less zeros and hence divisions by zero. But we can do that as a separate cleanup. This change looks good to me.

@openjdk openjdk bot added the ready Pull request is ready to be integrated label Oct 16, 2024
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Thanks for the reviews !

/integrate

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openjdk bot commented Oct 16, 2024

Going to push as commit 1cc3223.
Since your change was applied there have been 208 commits pushed to the master branch:

Your commit was automatically rebased without conflicts.

@openjdk openjdk bot added the integrated Pull request has been integrated label Oct 16, 2024
@openjdk openjdk bot closed this Oct 16, 2024
@openjdk openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Oct 16, 2024
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openjdk bot commented Oct 16, 2024

@MBaesken Pushed as commit 1cc3223.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

@xmas92 xmas92 mentioned this pull request Nov 19, 2024
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