RFR: 8060130: Simplify the synchronization of defining and getting java.lang.Package

[David Holmes]

Hi Claes,

On 11/10/2014 1:10 AM, Claes Redestad wrote:
> Hi all,
>
> please review this patch which attempts to clean up synchronization and
> improve scalability when defining and getting java.lang.Package objects.

Is this a real problem or a theoretical one? I've not previously heard 
of getting Package objects as being critical. ConcurrentHashMap improves 
the contended case but if we aren't normally contended then it will 
degrade performance. How does this perform on real code?

I'm also wondering how this impacts the initialization order of the VM 
as we need a lot more classes to be loaded and initialized when the 
first Package is created.

Thanks,
David

> webrev: http://cr.openjdk.java.net/~redestad/8060130/webrev.02/
> bug: https://bugs.openjdk.java.net/browse/JDK-8060130
>
> Testing: jtreg, UTE vm.parallel_class_loading.testlist, various benchmarks
>
> Torturing the retrieval code with a simple microbenchmark[1] shows that
> the existing code
> could cause bottlenecks, but also that the proposed patch works slightly
> faster even in
> uncontended cases:
>
> Benchmark                           Mode  Samples   Score Score error Units
> baseline, 1 thread:
> o.s.SimpleBench.getClassPackage    thrpt       10 11.621        0.618
> ops/us
> o.s.SimpleBench.getPackage         thrpt       10 41.754        3.381
> ops/us
> o.s.SimpleBench.getPackages        thrpt       10 0.009        0.000
> ops/us
>
> baseline, 2 threads:
> o.s.SimpleBench.getClassPackage    thrpt       10 7.884        1.977
> ops/us
> o.s.SimpleBench.getPackage         thrpt       10 17.013        8.079
> ops/us
> o.s.SimpleBench.getPackages        thrpt       10 0.004        0.001
> ops/us
>
> patch applied, 1 thread:
> o.s.SimpleBench.getClassPackage    thrpt       10 13.519        0.170
> ops/us
> o.s.SimpleBench.getPackage         thrpt       10 59.999        0.988
> ops/us
> o.s.SimpleBench.getPackages        thrpt       10 0.019        0.001
> ops/us
>
> patch applied, 2 threads:
> o.s.SimpleBench.getClassPackage    thrpt       10 19.181        3.688
> ops/us
> o.s.SimpleBench.getPackage         thrpt       10 79.708       31.220
> ops/us
> o.s.SimpleBench.getPackages        thrpt       10 0.025        0.006
> ops/us
>
> /Claes
>
> [1]
> package org.sample;
>
> import org.openjdk.jmh.annotations.*;
>
> @State(Scope.Thread)
> public class SimpleBench {
>
>      @Benchmark
>      public Package[] getPackages() {
>          return Package.getPackages();
>      }
>
>      @Benchmark
>      public Package getClassPackage() {
>          return this.getClass().getPackage();
>      }
>
>      @Benchmark
>      public Package getPackage() {
>          return Package.getPackage("java.util.zip");
>      }
> }
>