RFR: 8293972: runtime/NMT/NMTInitializationTest.java#default_long-off failed with "Suspiciously long bucket chains in lookup table." [v4]

[Gerard Ziemski]

On Wed, 19 Jul 2023 20:43:02 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

>> We initialize `NMT` earlier in the VM startup sequence, so that `NMTPreInit` does not have to deal with a variable number of VM arguments.
>> 
>> Since the `NMTPreInit` lookup table is of a fixed size (it only needs to hold about 56 items) we do not need to worry about it getting out of balance. 
>> 
>> Since the time window that `NMTPreInit` is needed is now much shorter, we do not "leak" as much memory. We also do not need to worry about `realloc`.
>> 
>> As a result, we do not need to test is as extensively as we used to and are being able to simplify `NMTPreInit` quite a bit, which allows us to remove a good chunk of code.
>> 
>> In addition, we improve the hash function, by using Mersenne prime (2^x - 1) i.e. 127, as the table size, which when combined with "modulo" operation, provides a uniform bit distribution.
>> 
>> To help with testing this change to make sure we improve the hash function, I collected `NMTPreInitAllocation` data on 3 platforms: macOS, Linux and Windows, using 100 runs of `test/hotspot/jtreg/runtime/NMT/NMTInitializationTest.java` and computed `stdev` as well as average `max chain`:
>> 
>> 
>> mac OS data:
>>  BEFORE total: max chain: 676, empties: 210892, ranges: 676, std_devs: 112.771
>>   AFTER total: max chain: 622, empties: 230762, ranges: 622, std_devs: 111.094
>>  BEFORE avg: max chain: 5.061, empties: 18.695, ranges: 5.061, std_devs: 0.010
>>   AFTER avg: max chain: 4.181, empties: 16.970, ranges: 4.161, std_devs: 0.009
>> 	
>> Linux data:
>>  BEFORE total: max chain: 523, empties: 193174, ranges: 523, std_devs: 103.243
>>   AFTER total: max chain: 432, empties: 175347, ranges: 430, std_devs: 90.670
>>  BEFORE avg: max chain: 6.540, empties: 20.402, ranges: 6.540, std_devs: 0.011
>>   AFTER avg: max chain: 6.017, empties: 22.324, ranges: 6.017, std_devs: 0.011
>> 
>> Windows data:
>>  BEFORE total: max chain: 578, empties: 201224, ranges: 578, std_devs: 108.988
>>   AFTER total: max chain: 491, empties: 218653, ranges: 491, std_devs: 104.879
>>  BEFORE avg: max chain: 5.602, empties: 19.504, ranges: 5.602, std_devs: 0.011
>>   AFTER avg: max chain: 4.759, empties: 21.193, ranges: 4.759, std_devs: 0.010
>> 
>> 
>> Note: the lower the number, the better.
>> Note: the `stdedev` and `max chain` are the most important numbers here.
>> 
>> We also, mark output with PID to help identify which test process it is coming from as the attached log was very confusing to read.
>> 
>> Tested manually with `jtreg -nr -va -jdk:./build/macosx-x86_64-server-fastdebug/images/j...
>
> Gerard Ziemski has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:
> 
>  - init NMT earlier to decrease the reliance on NMTPreInit lookup table, which lets us simplify the code
>  - Merge remote-tracking branch 'upstream/master' into JDK-8293972
>  - do not clip bits to 32, we need full 64 bits
>  - ^ is XOR, not pow
>  - remove tabs
>  - mark output with PID to help identify where it comes from
>  - use Mersenne prime and mod for uniform bit distribution of hash

I'm leaning towards adding back the realloc to `NMTPreInit` for now.

I will tackle replacing `NMTPreInit` with `NMT` in **summary** mode in a different issue.

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PR Comment: https://git.openjdk.org/jdk/pull/14607#issuecomment-1644483822