RFR: 8276066: Reset LoopPercentProfileLimit for x86 due to suboptimal performance

[Tobias Hartmann]

On Wed, 27 Oct 2021 14:26:49 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">

I'll run this through our performance testing and report back.

-------------

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