On Apr 28, 2011, at 9:58 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.
>
> Is it expected that rebinding a call site or constructing a GWT would
> be very expensive?
Looking at the compiled methods, it seems so. There is a lot going on when
creating a new GWT.
> 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.
>
> 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.
But that seems to be correct.
java.lang.invoke.MethodHandleImpl$GuardWithTest::<init> gets compiled and the
inline tree is:
8892 135
java.lang.invoke.MethodHandleImpl$GuardWithTest::<init> (22 bytes)
@ 2 java.lang.invoke.BoundMethodHandle::<init>
(37 bytes) inline (hot)
@ 2 java.lang.invoke.MethodHandle::type (5
bytes) inline (hot)
@ 7
java.lang.invoke.MethodType::dropParameterTypes (162 bytes) already compiled
into a big method
@ 10 java.lang.invoke.MethodHandle::<init> (15
bytes) inline (hot)
@ 1 java.lang.Object::<init> (1 bytes)
inline (hot)
@ 5 java.lang.Object::getClass (0 bytes)
(intrinsic)
@ 20 java.lang.invoke.MethodHandle::type (5
bytes) inline (hot)
@ 24
java.lang.invoke.MethodType::parameterSlotDepth (30 bytes) inline (hot)
@ 26
java.lang.invoke.MethodTypeForm::parameterToArgSlot (9 bytes) inline (hot)
@ 33
java.lang.invoke.BoundMethodHandle::initTarget (7 bytes) inline (hot)
@ 3
java.lang.invoke.MethodHandleNatives::init (0 bytes) native method
Obviously that is VERY expensive.
-- Christian
>
> 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
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