On 04/28/2011 09:58 PM, Charles Oliver Nutter wrote:
> I'm trying to figure out why polymorphic dispatch is incredibly slow
> in JRuby + indy. Take this benchmark, for example:
>
> class A; def foo; end; end
> class B; def foo; end; end
>
> a = A.new
> b = B.new
>
> 5.times { puts Benchmark.measure { 1000000.times { a, b = b, a; a.foo;
> b.foo } } }
>
> a.foo and b.foo are bimorphic here. Under stock JRuby, using
> CachingCallSite, this benchmark runs in about 0.13s per iteration.
> Using invokedynamic, it takes 9s!!!
>
> This is after a patch I just committed that caches the target method
> handle for direct paths. I believe the only thing created when GWT
> fails now is a new GWT.If you want to emulate a bimorphic cache, you should have two GWTs. So no construction of new GWT after discovering all possible targets for the two callsites. Relying on a mutable MethodHandle, a method handle that change for every call will not work well because the JIT will not be able to inline through this mutable method handle. > Is it expected that rebinding a call site or constructing a GWT would > be very expensive? If yes...I will have to look into having a hard > failover to inline caching or a PIC-like handle chain for polymorphic > cases. That's not necessarily difficult. If no...I'm happy to update > my build and play with patches to see what's happening here. Yes, it's expensive. The target of a CallSite should be stable. So yes it's expensible and yes it's intended. > A sampled profile produced the following output: > > Stub + native Method > 57.6% 0 + 5214 java.lang.invoke.MethodHandleNatives.init > 30.9% 0 + 2798 java.lang.invoke.MethodHandleNatives.init > 2.1% 0 + 189 java.lang.invoke.MethodHandleNatives.getTarget > 0.1% 0 + 7 java.lang.Object.getClass > 0.0% 0 + 3 java.lang.Class.isPrimitive > 0.0% 0 + 3 java.lang.System.arraycopy > 90.7% 0 + 8214 Total stub > > Of course we all know how accurate sampled profiles are, but this is > pretty a pretty dismal result. > > I suspect that this polymorphic cost is a *major* factor in slowing > down some benchmarks under invokedynamic. FWIW, the above benchmark > without the a,b swap runs in 0.06s, better than 2x faster than stock > JRuby (yay!). > > - Charlie Rémi _______________________________________________ mlvm-dev mailing list mlvm-dev@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev
