RFR: 8276066: Reset LoopPercentProfileLimit for x86 due to suboptimal performance [v2]

[Vladimir Kozlov]

On Wed, 3 Nov 2021 07:24:35 GMT, Jie Fu <jiefu at openjdk.org> wrote:

>> Hi all,
>> 
>> I'd like to reset the value of `LoopPercentProfileLimit` (from 30 to the original 10) for x86 to fix performance degradation.
>> 
>> We had observed that for the same Java App, the performance of x86 is slower than that of aarch64.
>> But the x86's performance should not be so worse than the aarch64 according to some SPEC benchmark results.
>> 
>> After some investigation, it seems that the slowness of x86 is caused by the different default settings of `LoopPercentProfileLimit` (30 for x86, but 10 for other platforms).
>> If we change `LoopPercentProfileLimit` from 30 to 10, x86 would run faster.
>> 
>> In JDK-8149421, `LoopPercentProfileLimit` [1] was first added and set to be 30 for x86 and 10 for other platforms.
>> Logically, the default value of `LoopPercentProfileLimit` is 10 for all platforms even before JDK-8149421.
>> This is because when `LoopPercentProfileLimit=10`, `10.0` [2] equals `100.0 / LoopPercentProfileLimit` [3].
>> So if we set `LoopPercentProfileLimit=10`, this unrolling rule [3] would be the same as the original design before JDK-8149421.
>> 
>> One most important fact is that from the very beginning of OpenJDK source code, the default value of `LoopPercentProfileLimit` (logically) is 10 for all platforms.   
>> So I suggest resetting `LoopPercentProfileLimit` to the original value (10) for x86, just as other platforms.
>> 
>> I've noted that the review thread mentioned that JDK-8149421 would be beneficial for some SPECjvm2008 benchmarks [4].
>> Then I run SPECjvm2008 with `LoopPercentProfileLimit=10` finding that there is no performance drop on x86.
>> So it won't revert JDK-8149421's opts for SPECjvm2008.
>> 
>> To show the potential improvement of this change, I've made a jmh test in the patch.
>> Performance can be improved by 1.25x ~ 2.0x according to this micro benchmark.
>> 
>> Any comments?
>> 
>> Thanks.
>> Best regards,
>> Jie
>> 
>>   
>> [1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/loopTransform.cpp#L908
>> [2] https://github.com/openjdk/jdk8u/blob/master/hotspot/src/share/vm/opto/loopTransform.cpp#L673
>> [3] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/loopTransform.cpp#L903
>> [4] https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-February/021205.html
>> 
>> <img width="420" alt="ratio" src="https://user-images.githubusercontent.com/19923746/139084273-aa2e2eae-4a74-4fcb-8430-d2e2e49d5d5c.png">
>> 
>> <img width="615" alt="before" src="https://user-images.githubusercontent.com/19923746/139084508-793f1109-1ce3-4427-a2e3-660c91758a7c.png">
>> 
>> <img width="617" alt="after" src="https://user-images.githubusercontent.com/19923746/139084542-c8a8a705-d7ed-499f-9ceb-7175671c0e3b.png">
>
> Jie Fu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix indentation for Java code by using 4 whitespace

I agree.

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Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6142