On Tue, 12 Nov 2024 10:17:45 GMT, Francesco Nigro <d...@openjdk.org> wrote:
>> @minborg sent me some logs from his machine, and I'm analyzing them now.
>>
>> Basically, I'm trying to see why your Java code is a bit faster than the
>> Loop code.
>>
>> ----------------
>>
>> 44.77% c2, level 4
>> org.openjdk.bench.java.lang.foreign.SegmentBulkRandomFill::heapSegmentFillLoop,
>> version 4, compile id 946
>> 24.43% c2, level 4
>> org.openjdk.bench.java.lang.foreign.SegmentBulkRandomFill::heapSegmentFillLoop,
>> version 4, compile id 946
>> 21.80% c2, level 4
>> org.openjdk.bench.java.lang.foreign.SegmentBulkRandomFill::heapSegmentFillLoop,
>> version 4, compile id 946
>>
>> There seem to be 3 hot regions.
>>
>> **main-loop** (region has 44.77%):
>>
>> ;; B33: # out( B33 B34 ) <- in( B32 B33 ) Loop( B33-B33
>> inner main of N116 strip mined) Freq: 4.62951e+10
>>
>> 0.50% ? 0x00000001149a23c0: sxtw x20, w4
>>
>>
>> ? 0x00000001149a23c4: add x22, x16, x20
>>
>>
>> 0.02% ? 0x00000001149a23c8: str q16, [x22]
>>
>>
>> 16.33% ? 0x00000001149a23cc: str q16, [x22, #16]
>> ;*invokevirtual putByte {reexecute=0 rethrow=0 return_oop=0}
>>
>> ? ; -
>> jdk.internal.misc.ScopedMemoryAccess::putByteInternal@15 (line 534)
>>
>> ? ; -
>> jdk.internal.misc.ScopedMemoryAccess::putByte@6 (line 522)
>>
>> ? ; -
>> java.lang.invoke.VarHandleSegmentAsBytes::set@38 (line 114)
>>
>> ? ; -
>> java.lang.invoke.LambdaForm$DMH/0x000000013a4d5800::invokeStatic@20
>>
>> ? ; -
>> java.lang.invoke.LambdaForm$MH/0x000000013a4d8070::invoke@37 ...
>
> @eme64 not an expert with ARM, but profiling skidding due to modern big
> pipelined OOO CPUs is rather frequent
>
>> with a strange extra add that has some strange looking percentage (profile
>> inaccuracy?):
>
> you should check some instr below it to get the real culprit
>
> More info on this topic are:
> - https://travisdowns.github.io/blog/2019/08/20/interrupts.html for x86
> -
> https://easyperf.net/blog/2018/06/08/Advanced-profiling-topics-PEBS-and-LBR#processor-event-based-sampling-pebs
> - https://ieeexplore.ieee.org/document/10068807 - Intel and AMD PEBS/IBS paper
>
> If you uses Intel/AMD and PEBS/IBS (if supported by your cpu) you can run
> perfasm using precise events via `perfasm:events=cycles:P` IIRC (or adding
> more Ps? @shipilev likely knows) which should have way less skidding and will
> simplify these analysis.
@franz1981 right. That is what I thought. I'm usually working on x64, and am
not used to all the skidding of ARM.
-------------
PR Comment: https://git.openjdk.org/jdk/pull/22010#issuecomment-2470162785
