RFR(S) 8246019 PerfClassTraceTime slows down VM start-up

[Calvin Cheung]

Hi Ioi,

The change looks good.

thanks,

Calvin

On 6/16/20 1:14 PM, Ioi Lam wrote:
> https://bugs.openjdk.java.net/browse/JDK-8246019
> http://cr.openjdk.java.net/~iklam/jdk16/8246019-avoid-PerfClassTraceTime.v01/ 
>
>
> PerfClassTraceTime is (a rarely used feature) for measuring the time 
> spent during class linking and initialization. However, it's quite 
> expensive and it needs to start and stop a bunch of timers. With CDS, 
> it's quite often for the overhead of the timer itself to be much more 
> than the time it's trying to measure, giving unreliable measurement.
>
> In this patch, when it's clear that the init and linking will be very 
> quick, I disable the timer and count only the number of invocations. 
> This shows a small improvement in start-up
>
> Results of " perf stat -r 100 bin/java -Xshare:on 
> -XX:SharedArchiveFile=jdk2.jsa -Xint -version "
>
> 59623970 59341935 (-282035)   -----  41.774  41.591 ( -0.183) -
> 59623495 59331646 (-291849)   -----  41.696  41.165 ( -0.531) --
> 59627148 59329526 (-297622)   -----  41.249  41.094 ( -0.155) -
> 59612439 59340760 (-271679)   ----   41.773  40.657 ( -1.116) -----
> 59626438 59335681 (-290757)   -----  41.683  40.901 ( -0.782) ----
> 59618436 59338953 (-279483)   -----  41.861  41.249 ( -0.612) ---
> 59608782 59340173 (-268609)   ----   41.198  41.508 (  0.310) +
> 59614612 59325177 (-289435)   -----  41.397  41.738 (  0.341) ++
> 59615905 59344006 (-271899)   ----   41.921  40.969 ( -0.952) ----
> 59635867 59333147 (-302720)   -----  41.491  40.836 ( -0.655) ---
> ================================================
> 59620708 59336100 (-284608)   -----  41.604  41.169 ( -0.434) --
> instruction delta =      -284608    -0.4774%
> time        delta =       -0.434 ms -1.0435%
>
> The number of PerfClassTraceTime's used is reduced from 564 to 116 (so 
> we have an overhead of about 715 instructions per use, yikes!).