Java, The Cost of a Single Element Loop

In quite a few cases I have seen myself designing code with listeners and callbacks. It is quite common for a class that emits events, to expose an API to attach listener(s) to it. Those listeners are usually stored in a data structure (see List, Set, Array) and when an event is about to be dispatched the listeners are iterated in a loop and the appropriate callback is called.

Something along the lines of:

public class EventDispatcher {

        private final List<Listener> listeners = new ArrayList<>();

        public void dispatchEvent() {
            final MyEvent event = new MyEvent();
            for (Listener listener : this.listeners) {
                listener.onEvent(event);
            }
        }
        public void attachListener(final Listener listener) {
            this.listeners.add(listener);
        }

        public void removeListener(final Listener listener) {
            this.listeners.remove(listener);
        }
    }

    public static class Listener implements EventListener {

        void onEvent(final MyEvent myEvent) {
            // do staff
        }
    }

    public static class MyEvent {

    }

In many cases I have observed that despite the fact that the class is desinged to accept many listeners, the true is actually that just one listener is attached in the majority of the cases.

Hence I wanted to measure the performance penalty paid in case the class had just one listener vs if the class was initially designed to accept just one listener.

In essence I wanted to check the performance impact on the below two cases.

private Listener listener;
        private final List<Listener> singleElementArray = new ArrayList<Listener>(){
            {add(new Listener());}
        };

        public void dispatch() {
            this.listener.onEvent(new MyEvent());
        }

        public void dispatchInLoop() {
            for (int i = 0; i < 1; i++) {
                this.singleElementArray.get(i).onEvent(new MyEvent());
            }
        }

Assumptions Made Prior To Testing

Before creating a benchmark for the above, I made some assumptions:

  • I assumed the single element (single listener in a data container) loop would be unrolled
  • I (wrongly) assumed that the performane cost will not be significant. As effectively with the loop unrolled I would think the native code produced would more or less look close enough

JMH Benchmark

In order to test my assumptions I created the below benchmark:

SingleElementLoopBenchmark.java

Initial Observations

To my surprise I found out that an invocaiton on a single element list was about ~2,5 slower, based on the below throughput numbers:

Benchmark                                                          Mode  Cnt   Score   Error   Units
          SingleElementLoopBenchmark.directInvocation                       thrpt   10   0.317 ± 0.022  ops/ns
          SingleElementLoopBenchmark.singleElementListLoopInvocation        thrpt   10   0.114 ± 0.010  ops/ns

I couldn’t really understand why and the above seemed a bit too far from my expecations/assumptions.

The first thing that I verified with JVM argument -XX:+PrintCompilation was that both methods were compiled with C2 compiler, which was the case.

I also tried to print the assembly code with -XX:+PrintAssembly but I couldn’t really read/interpret the assembly code.

Resorting to Social Media

I ended up posting a tweet about my findings and asking some pointer on where/how to look for explanations on what I was observing. The answer I got was to try to find the hot methods by using something like perfasm, which would tie the assembly output to the hottest methods of my benchmark.

Which I did with -prof dtraceasm (The benchmark was running on a Mac that’s why I used dtrace). The output was the below:

Direct Invocation

9.56%  ↗  0x000000010b73c950: mov    0x40(%rsp),%r10
  1.00%  │  0x000000010b73c955: mov    0xc(%r10),%r10d                ;*getfield dispatcher {reexecute=0 rethrow=0 return_oop=0}
         │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::directInvocation@1 (line 23)
         │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@17 (line 121)
  0.17%  │  0x000000010b73c959: mov    0xc(%r12,%r10,8),%r11d         ; implicit exception: dispatches to 0x000000010b73ca12
 11.18%  │  0x000000010b73c95e: test   %r11d,%r11d
  0.00%  │  0x000000010b73c961: je     0x000000010b73c9c9             ;*invokevirtual performAction {reexecute=0 rethrow=0 return_oop=0}
         │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invoke@5 (line 40)
         │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::directInvocation@5 (line 23)
         │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@17 (line 121)
 10.69%  │  0x000000010b73c963: mov    %r9,(%rsp)
  0.65%  │  0x000000010b73c967: mov    0x38(%rsp),%rsi
  0.00%  │  0x000000010b73c96c: mov    $0x1,%edx
  0.14%  │  0x000000010b73c971: xchg   %ax,%ax
 10.08%  │  0x000000010b73c973: callq  0x000000010b6c2900             ; ImmutableOopMap{[48]=Oop [56]=Oop [64]=Oop [0]=Oop }
         │                                                            ;*invokevirtual consume {reexecute=0 rethrow=0 return_oop=0}
         │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Listener::performAction@2 (line 53)
         │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invoke@5 (line 40)
         │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::directInvocation@5 (line 23)
         │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@17 (line 121)
         │                                                            ;   {optimized virtual_call}
  1.44%  │  0x000000010b73c978: mov    (%rsp),%r9
  0.19%  │  0x000000010b73c97c: movzbl 0x94(%r9),%r8d                 ;*ifeq {reexecute=0 rethrow=0 return_oop=0}
         │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@30 (line 123)
  9.77%  │  0x000000010b73c984: mov    0x108(%r15),%r10
  0.99%  │  0x000000010b73c98b: add    $0x1,%rbp                      ; ImmutableOopMap{r9=Oop [48]=Oop [56]=Oop [64]=Oop }
         │                                                            ;*ifeq {reexecute=1 rethrow=0 return_oop=0}
         │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_directInvocation_jmhTest::directInvocation_thrpt_jmhStub@30 (line 123)
  0.02%  │  0x000000010b73c98f: test   %eax,(%r10)                    ;   {poll}
  0.28%  │  0x000000010b73c992: test   %r8d,%r8d
  0.00%  ╰  0x000000010b73c995: je     0x000000010b73c950             ;*aload_1 {reexecute=0 rethrow=0 return_oop=0}

Single Element Loop Invocation

         ╭    0x000000011153fa9d: jmp    0x000000011153fad6
  0.19%  │ ↗  0x000000011153fa9f: mov    0x58(%rsp),%r13
  3.55%  │ │  0x000000011153faa4: mov    (%rsp),%rcx
  0.09%  │ │  0x000000011153faa8: mov    0x60(%rsp),%rdx
  0.22%  │ │  0x000000011153faad: mov    0x50(%rsp),%r11
  0.17%  │ │  0x000000011153fab2: mov    0x8(%rsp),%rbx                 ;*if_icmpge {reexecute=0 rethrow=0 return_oop=0}
         │ │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@12 (line 44)
         │ │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
         │ │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  3.55%  │↗│  0x000000011153fab7: movzbl 0x94(%r11),%r8d                ;*goto {reexecute=0 rethrow=0 return_oop=0}
         │││                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@35 (line 44)
         │││                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
         │││                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.16%  │││  0x000000011153fabf: mov    0x108(%r15),%r10
  0.28%  │││  0x000000011153fac6: add    $0x1,%rbx                      ; ImmutableOopMap{r11=Oop rcx=Oop rdx=Oop r13=Oop }
         │││                                                            ;*ifeq {reexecute=1 rethrow=0 return_oop=0}
         │││                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@30 (line 123)
  0.19%  │││  0x000000011153faca: test   %eax,(%r10)                    ;   {poll}
  4.00%  │││  0x000000011153facd: test   %r8d,%r8d
         │││  0x000000011153fad0: jne    0x000000011153fbe9             ;*aload_1 {reexecute=0 rethrow=0 return_oop=0}
         │││                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@33 (line 124)
  0.07%  ↘││  0x000000011153fad6: mov    0xc(%rcx),%r8d                 ;*getfield dispatcher {reexecute=0 rethrow=0 return_oop=0}
          ││                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@1 (line 28)
          ││                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.22%   ││  0x000000011153fada: mov    0x10(%r12,%r8,8),%r10d         ;*getfield singleListenerList {reexecute=0 rethrow=0 return_oop=0}
          ││                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@4 (line 44)
          ││                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
          ││                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
          ││                                                            ; implicit exception: dispatches to 0x000000011153ff2a
  0.21%   ││  0x000000011153fadf: mov    0x8(%r12,%r10,8),%edi          ; implicit exception: dispatches to 0x000000011153ff3e
  4.39%   ││  0x000000011153fae4: cmp    $0x237565,%edi                 ;   {metadata(&apos;com/nikoskatsanos/benchmarks/loops/SingleElementLoopBenchmark$Dispatcher$1&apos;)}
          ││  0x000000011153faea: jne    0x000000011153fc92
  0.33%   ││  0x000000011153faf0: lea    (%r12,%r10,8),%r9              ;*invokeinterface size {reexecute=0 rethrow=0 return_oop=0}
          ││                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@7 (line 44)
          ││                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
          ││                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.09%   ││  0x000000011153faf4: mov    0x10(%r9),%r9d
  0.14%   ││  0x000000011153faf8: test   %r9d,%r9d
          ╰│  0x000000011153fafb: jle    0x000000011153fab7             ;*if_icmpge {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@12 (line 44)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  3.98%    │  0x000000011153fafd: lea    (%r12,%r8,8),%rdi              ;*getfield dispatcher {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@1 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.06%    │  0x000000011153fb01: xor    %r9d,%r9d                      ;*aload_0 {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@15 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.09%    │  0x000000011153fb04: mov    0x8(%r12,%r10,8),%esi          ; implicit exception: dispatches to 0x000000011153ff4e
  0.06%    │  0x000000011153fb09: cmp    $0x237565,%esi                 ;   {metadata(&apos;com/nikoskatsanos/benchmarks/loops/SingleElementLoopBenchmark$Dispatcher$1&apos;)}
  0.00%    │  0x000000011153fb0f: jne    0x000000011153fcc2
  3.93%    │  0x000000011153fb15: lea    (%r12,%r10,8),%rax             ;*invokeinterface get {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.06%    │  0x000000011153fb19: mov    0x10(%rax),%r10d               ;*getfield size {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - java.util.ArrayList::get@2 (line 458)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.11%    │  0x000000011153fb1d: test   %r10d,%r10d
           │  0x000000011153fb20: jl     0x000000011153fcf6             ;*invokestatic checkIndex {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - java.util.Objects::checkIndex@3 (line 372)
           │                                                            ; - java.util.ArrayList::get@5 (line 458)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.28%    │  0x000000011153fb26: cmp    %r10d,%r9d
  0.00%    │  0x000000011153fb29: jae    0x000000011153fc1c
  3.97%    │  0x000000011153fb2f: mov    0x14(%rax),%r10d               ;*getfield elementData {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - java.util.ArrayList::elementData@1 (line 442)
           │                                                            ; - java.util.ArrayList::get@11 (line 459)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.05%    │  0x000000011153fb33: mov    %r9d,%ebp                      ;*invokestatic checkIndex {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - java.util.Objects::checkIndex@3 (line 372)
           │                                                            ; - java.util.ArrayList::get@5 (line 458)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.08%    │  0x000000011153fb36: mov    0xc(%r12,%r10,8),%esi          ; implicit exception: dispatches to 0x000000011153ff62
  1.27%    │  0x000000011153fb3b: cmp    %esi,%ebp
           │  0x000000011153fb3d: jae    0x000000011153fc5a
  3.94%    │  0x000000011153fb43: shl    $0x3,%r10
  0.05%    │  0x000000011153fb47: mov    0x10(%r10,%rbp,4),%r9d         ;*aaload {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - java.util.ArrayList::elementData@5 (line 442)
           │                                                            ; - java.util.ArrayList::get@11 (line 459)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@20 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  1.71%    │  0x000000011153fb4c: mov    0x8(%r12,%r9,8),%r10d          ; implicit exception: dispatches to 0x000000011153ff72
 17.85%    │  0x000000011153fb51: cmp    $0x237522,%r10d                ;   {metadata(&apos;com/nikoskatsanos/benchmarks/loops/SingleElementLoopBenchmark$Listener&apos;)}
  0.00%    │  0x000000011153fb58: jne    0x000000011153fef6             ;*checkcast {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@25 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  3.79%    │  0x000000011153fb5e: mov    %rdi,0x18(%rsp)
  0.02%    │  0x000000011153fb63: mov    %r8d,0x10(%rsp)
  0.02%    │  0x000000011153fb68: mov    %rbx,0x8(%rsp)
  0.19%    │  0x000000011153fb6d: mov    %r11,0x50(%rsp)
  3.95%    │  0x000000011153fb72: mov    %rdx,0x60(%rsp)
  0.02%    │  0x000000011153fb77: mov    %rcx,(%rsp)
  0.03%    │  0x000000011153fb7b: mov    %r13,0x58(%rsp)
  0.36%    │  0x000000011153fb80: mov    %rdx,%rsi
  3.78%    │  0x000000011153fb83: mov    $0x1,%edx
  0.01%    │  0x000000011153fb88: vzeroupper
  4.05%    │  0x000000011153fb8b: callq  0x00000001114c2900             ; ImmutableOopMap{[80]=Oop [88]=Oop [96]=Oop [0]=Oop [16]=NarrowOop [24]=Oop }
           │                                                            ;*invokevirtual consume {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Listener::performAction@2 (line 53)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@29 (line 45)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
           │                                                            ;   {optimized virtual_call}
  0.98%    │  0x000000011153fb90: mov    0x10(%rsp),%r8d
  3.61%    │  0x000000011153fb95: mov    0x10(%r12,%r8,8),%r10d         ;*getfield singleListenerList {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@4 (line 44)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.24%    │  0x000000011153fb9a: mov    0x8(%r12,%r10,8),%r9d          ; implicit exception: dispatches to 0x000000011153ff9e
  0.74%    │  0x000000011153fb9f: inc    %ebp                           ;*iinc {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@32 (line 44)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.04%    │  0x000000011153fba1: cmp    $0x237565,%r9d                 ;   {metadata(&apos;com/nikoskatsanos/benchmarks/loops/SingleElementLoopBenchmark$Dispatcher$1&apos;)}
  0.00%    │  0x000000011153fba8: jne    0x000000011153fd36
  3.60%    │  0x000000011153fbae: lea    (%r12,%r10,8),%r11             ;*invokeinterface size {reexecute=0 rethrow=0 return_oop=0}
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@7 (line 44)
           │                                                            ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
           │                                                            ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)
  0.11%    │  0x000000011153fbb2: mov    0x10(%r11),%r9d
  0.35%    │  0x000000011153fbb6: cmp    %r9d,%ebp
           ╰  0x000000011153fbb9: jge    0x000000011153fa9f             ;*if_icmpge {reexecute=0 rethrow=0 return_oop=0}
                                                                        ; - com.benchmarks.loops.SingleElementLoopBenchmark$Dispatcher::invokeInLoop@12 (line 44)
                                                                        ; - com.benchmarks.loops.SingleElementLoopBenchmark::singleElementLoopInvocation@5 (line 28)
                                                                        ; - com.benchmarks.loops.generated.SingleElementLoopBenchmark_singleElementLoopInvocation_jmhTest::singleElementLoopInvocation_thrpt_jmhStub@17 (line 121)

As I said I am not really able to read/interpret assembly code, but in between the lines I could see that:

  • The loop was indeed unrolled
  • A penalty was paid to cast the item to the expected type (17.85% of the CPU instructions)
  • A penalty was paid to fetch the item from the list, underlying array

In order to get some advice from someone knowledgable on this I posted the below question on StackOverflow. The answer is pretty comprehensive, as the person who answered is one of the most prominent names in JVM community

StackOveflow: Java Method Direct Invocation vs Single Element Loop

Conclusion/Observations

In summary:

  • The loop was indeed unrolled, as expected and as seen from the assembly code
  • The main penalty paid is for fetching the element from the list and casting it to the expected type
  • Some cost is also because of checks performed on the data container itself (i.e. size)
  • In general the extra cost been paid is memory access cost, rather than CPU instructions cost

As seen in the SO answer, Andrei makes the point that invoking the object’s method from inside the loop is not ~2,5 times slower, but rather 3 ns slower, if we look it from a perspective of latency (ns/op) rather than throughput (ops/ns). This is a valid point, but I am not sure If i aggree 100%, as in some applications, depending on the nature, that extra cost will actually translate in ~2,5.

Finally, I have added in the JMH Benchmark test, tests for different data container types:

  • Array
  • List
  • Set

Observing the numbers of those, and as expected, an array is faster than the rest. The array is typed, hence the casting cost is not paid. The array underlying an array list is of type Object, hence the need for casting to the list’s type.