RFR: 8331117: [PPC64] secondary_super_cache does not scale well

[Martin Doerr]

On Mon, 27 May 2024 17:11:41 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> PPC64 implementation of [JDK-8180450](https://bugs.openjdk.org/browse/JDK-8180450). Please review!
>> I noticed that `r_array_length` is sometimes 0 and I don't see code for that on x86. Any idea?
>> How can we verify it? By comparing the performance using the micro benchmarks?
>> 
>> Micro benchmark results without patch (measured on Power10 with 2*8 hardware threads):
>> 
>> Original 
>> SecondarySuperCacheHits: 13.033 ±(99.9%) 0.058 ns/op [Average]
>> SecondarySuperCacheInterContention.test     avgt   15  432.366 ±  8.364  ns/op
>> SecondarySuperCacheInterContention.test:t1  avgt   15  432.310 ±  8.460  ns/op
>> SecondarySuperCacheInterContention.test:t2  avgt   15  432.422 ± 10.819  ns/op
>> SecondarySuperCacheIntraContention.test  avgt   15  355.192 ± 3.597  ns/op
>> SecondarySupersLookup.testNegative00  avgt   15  12.274 ± 0.026  ns/op
>> SecondarySupersLookup.testNegative01  avgt   15  12.300 ± 0.039  ns/op
>> SecondarySupersLookup.testNegative02  avgt   15  12.304 ± 0.034  ns/op
>> SecondarySupersLookup.testNegative03  avgt   15  12.276 ± 0.050  ns/op
>> SecondarySupersLookup.testNegative04  avgt   15  12.235 ± 0.044  ns/op
>> SecondarySupersLookup.testNegative05  avgt   15  12.308 ± 0.156  ns/op
>> SecondarySupersLookup.testNegative06  avgt   15  12.291 ± 0.048  ns/op
>> SecondarySupersLookup.testNegative07  avgt   15  12.307 ± 0.052  ns/op
>> SecondarySupersLookup.testNegative08  avgt   15  12.398 ± 0.075  ns/op
>> SecondarySupersLookup.testNegative09  avgt   15  12.552 ± 0.122  ns/op
>> SecondarySupersLookup.testNegative10  avgt   15  12.490 ± 0.083  ns/op
>> SecondarySupersLookup.testNegative16  avgt   15  12.565 ± 0.092  ns/op
>> SecondarySupersLookup.testNegative20  avgt   15  19.059 ± 0.958  ns/op
>> SecondarySupersLookup.testNegative30  avgt   15  19.268 ± 0.124  ns/op
>> SecondarySupersLookup.testNegative32  avgt   15  20.059 ± 0.114  ns/op
>> SecondarySupersLookup.testNegative40  avgt   15  25.117 ± 0.368  ns/op
>> SecondarySupersLookup.testNegative50  avgt   15  32.735 ± 0.359  ns/op
>> SecondarySupersLookup.testNegative55  avgt   15  34.866 ± 0.152  ns/op
>> SecondarySupersLookup.testNegative56  avgt   15  35.492 ± 0.276  ns/op
>> SecondarySupersLookup.testNegative57  avgt   15  36.620 ± 0.334  ns/op
>> SecondarySupersLookup.testNegative58  avgt   15  37.226 ± 0.180  ns/op
>> SecondarySupersLookup.testNegative59  avgt   15  37.774 ± 0.241  ns/op
>> SecondarySupersLookup.testNegative60  avgt   15  38.627 ± 1.451  ns/op
>> SecondarySupersLookup.testNegative61  avgt   15 ...
>
>> PPC64 implementation of [JDK-8180450](https://bugs.openjdk.org/browse/JDK-8180450). Please review! I noticed that `r_array_length` is sometimes 0 and I don't see code for that on x86. Any idea? How can we verify it? By comparing the performance using the micro benchmarks?
> 
> Run all of tier1 with `-XX:+VerifySecondarySupers`

> > @theRealAph: It would be great if you could take a look and see if you can spot any bug. Especially, I wonder why `r_array_length` happens to be 0 in some cases, but x86 doesn't check.
> 
> Why would it not be zero? Some classes don't have secondary super types. In addition, 12ns is very slow. I don't understand that.

I had to check for `r_array_length  >= 0` here: https://github.com/openjdk/jdk/pull/19368/files#diff-0f708565c9e138b8013165540634368334f5d1df2ba437e39696e9791440050dR2312
The x86 implementation doesn't do that and I wonder why. Doesn't it access stale memory (https://github.com/openjdk/jdk/blob/be1d374bc54d43aae3b3c1feace22d38fe2156b6/src/hotspot/cpu/x86/macroAssembler_x86.cpp#L4967)?

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PR Comment: https://git.openjdk.org/jdk/pull/19368#issuecomment-2133849808