On Oct 17, 2011, at 4:14 PM, Charles Oliver Nutter wrote: > Yeah that's a pretty big problem :) Indy almost becomes a nonstarter if these > degraded cases are not made a lot better. The performance on this example > becomes terrible, and it's common for Ruby methods to be larger and more > complex than this, too. > Yeah, I know. > You say you don't know how to fix it, so perhaps we can brainstorm a bit? If > we can't inline the call we ate least want it to perform like an uninlined > method call. Could we still compile the MH chain and CALL it, so at least > we're not doing c2i? Or perhaps we should always treat the MH chain as > inlinable regardless of budgets, but not inline the eventual call when > budgets are exceeded? The latter option sounds more correct to me; the MH > chain should be considered a non-removable part of the invokedynamic > operation and always inlined. That would avoid the degradation without > blowing up code size. > I didn't say I don't know how to fix it but rather what's the best approach. Internally we (Tom, John and I) already talked about this and I tried to do something what you suggest above: compile the MH chain and call it. It turned out it's not that easy (as I thought) and needs some special handling.
An interesting idea is to always inline the "adaption" code of the MH chain but not the eventual call (or all calls in the chain). I would guess that for normal usage the added code size is insignificant but there can definitely be pathological cases. I try to get some data on that. -- Chris > - Charlie (mobile) > > On Oct 17, 2011 4:58 AM, "Christian Thalinger" > <christian.thalin...@oracle.com> wrote: > > On Oct 15, 2011, at 2:56 PM, Charles Oliver Nutter wrote: > > > I'm seeing something peculiar and wanted to run it by you folks. > > > > There are a few values that JRuby's compiler had previously been > > loading from instance fields every time they're needed. Specifically, > > fields like ThreadContext.runtime (the current JRuby runtime), > > Ruby.falseObject, Ruby.trueObject, Ruby.nilObject (false, true, and > > nil values). I figured I'd make a quick change today and have those > > instead be constant method handles bound into a mutable call site. > > > > Unfortunately, performance seems to be worse. > > > > The logic works like this: > > > > * ThreadContext is loaded to stack > > * invokedynamic, bootstrap just wires up an initialization method into > > a MutableCallSite > > * initialization method rebinds call site forever to a constant method > > handle pointing at the value (runtime/true/false/nil objects) > > > > My expectation was that this would be at least no slower (and > > potentially a tiny bit faster) but also less bytecode (in the case of > > true/false/nil, it was previously doing > > ThreadContext.runtime.getNil()/getTrue()/getFalse()). It seems like > > it's actually slower than walking those references, though, and I'm > > not sure why. > > > > Here's a couple of the scenarios in diff form showing bytecode before > > and bytecode after: > > > > Loading "runtime" > > > > ALOAD 1 > > - GETFIELD org/jruby/runtime/ThreadContext.runtime : Lorg/jruby/Ruby; > > + INVOKEDYNAMIC getRuntime > > (Lorg/jruby/runtime/ThreadContext;)Lorg/jruby/Ruby; > > [org/jruby/runtime/invokedynamic/InvokeDynamicSupport.getObjectBootstrap(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/St > > ring;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; (6)] > > > > Loading "false" > > > > ALOAD 1 > > - GETFIELD org/jruby/runtime/ThreadContext.runtime : Lorg/jruby/Ruby; > > - INVOKEVIRTUAL org/jruby/Ruby.getFalse ()Lorg/jruby/RubyBoolean; > > + INVOKEDYNAMIC getFalse > > (Lorg/jruby/runtime/ThreadContext;)Lorg/jruby/RubyBoolean; > > [org/jruby/runtime/invokedynamic/InvokeDynamicSupport.getObjectBootstrap(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite; > > (6)] > > > > I think because these are now seen as invocations, I'm hitting some > > inlining budget limit I didn't hit before (and which isn't being > > properly discounted). The benchmark I'm seeing degrade is > > bench/language/bench_flip.rb, and it's a pretty significant > > degradation. Only the "heap" version shows the degradation, and it > > definitely does have more bytecode...but the bytecode with my patch > > differs only in the way these values are being accessed, as shown in > > the diffs above. > > > > Before: > > user system > > total real > > 1m x10 while (a)..(!a) (heap) 0.951000 0.000000 > > 0.951000 ( 0.910000) > > user system > > total real > > 1m x10 while (a)..(!a) (heap) 0.705000 0.000000 > > 0.705000 ( 0.705000) > > user system > > total real > > 1m x10 while (a)..(!a) (heap) 0.688000 0.000000 > > 0.688000 ( 0.688000) > > user system > > total real > > > > After: > > user system > > total real > > 1m x10 while (a)..(!a) (heap) 2.350000 0.000000 > > 2.350000 ( 2.284000) > > user system > > total real > > 1m x10 while (a)..(!a) (heap) 2.128000 0.000000 > > 2.128000 ( 2.128000) > > user system > > total real > > 1m x10 while (a)..(!a) (heap) 2.115000 0.000000 > > 2.115000 ( 2.116000) > > user system > > total real > > > > You can see the degradation is pretty bad. > > > > I'm concerned because I had hoped that invokedynamic + mutable call > > site + constant handle would always be faster than a field > > access...since it avoids excessive field accesses and makes it > > possible for Hotspot to fold those constants away. What's going on > > here? > > I looked into this and the main issue here is an old friend: slow invokes of > non-inlined MH call sites. The problem is that you trade a normal invoke (to > a field load?) with a MH invoke. If the normal invoke doesn't get inlined > we're good but if the MH invoke doesn't get inlined we're screwed (since we > are still doing the C2I-I2C dance). > > I refactored the benchmark a little (moved stack and heap loops into its own > methods and only do 5 while-loops instead of 11; that inlines all calls in > that method) and the performance is like you had expected (a little faster): > > 32-bit: > > before: > > 1m x10 while (a)..(!a) (stack) 0.214000 0.000000 0.214000 ( > 0.214000) > 1m x10 while (a)..(!a) (heap) 0.249000 0.000000 0.249000 ( > 0.250000) > > after: > > 1m x10 while (a)..(!a) (stack) 0.203000 0.000000 0.203000 ( > 0.203000) > 1m x10 while (a)..(!a) (heap) 0.234000 0.000000 0.234000 ( > 0.234000) > > 64-bit: > > before: > > 1m x10 while (a)..(!a) (stack) 0.248000 0.000000 0.248000 ( > 0.248000) > 1m x10 while (a)..(!a) (heap) 0.257000 0.000000 0.257000 ( > 0.257000) > > after: > > 1m x10 while (a)..(!a) (stack) 0.226000 0.000000 0.226000 ( > 0.226000) > 1m x10 while (a)..(!a) (heap) 0.244000 0.000000 0.244000 ( > 0.244000) > > We have to fix that but I'm not sure yet what's the best approach. Sorry I > don't have better news for now. > > -- Chris > > > > > Patch for the change (apply to JRuby master) is here: > > https://gist.github.com/955976b52b0c4e3f611e > > > > - Charlie > > _______________________________________________ > > mlvm-dev mailing list > > mlvm-dev@openjdk.java.net > > http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev > > _______________________________________________ > mlvm-dev mailing list > mlvm-dev@openjdk.java.net > http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev > _______________________________________________ > mlvm-dev mailing list > mlvm-dev@openjdk.java.net > http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev
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