Performance of Arrays.fill for primitives and references (original) (raw)

Marko Topolnik marko.topolnik at gmail.com
Tue Feb 4 02:08:36 PST 2014

I have encountered a large discrepancy in the performance of Arrays.fill(int[], int) vs. Arrays.fill(Object[], Object). From jmh:

Benchmark Mode Samples Mean Mean error Units fillIntArray avgt 10 802.393 19.323 ns/op fillReferenceArray avgt 10 5323.516 105.982 ns/op

The array size is 8192, which means that filling an int array works at above 10 slots/nanosecond (2.66 GHZ Intel Core i7)---this sound like fantastic performance.

My question is, what is stopping HotSpot from applying the same optimization to a reference array?

One guess was the maintenance of the card table, but I couldn't get enough information to confirm that. A naive view of optimization opportunities seems to indicate that the card table could be updated wholesale either before or after writing out the array.

Printing assembly code, all I can see in the fill(Object[],Object) case is the explicit loop involving the write barrier on each slot write:

lea eax, [edx+ebx*4+0x10] mov [eax], edx shr eax, 9 mov [edi+eax], ah

whereas for fill(int[],int) I see an opaque

mov edx, 0x0000000011145ca0 call edx

for the whole filling operation.

I was testing on 64-Bit Server VM (build 24.0-b56, mixed mode) with default settings.

For reference, this is the code I have used:

@OutputTimeUnit(TimeUnit.NANOSECONDS) @BenchmarkMode(Mode.AverageTime) @Warmup(iterations = 5, time = 1) @Measurement(iterations = 10, time = 2) @State(Scope.Thread) @Threads(1) @Fork(1) public class Writing { static final int TARGET_SIZE = 1<<13;

static final int[] intArray = new int[TARGET_SIZE]; static final Object[] referenceArray = new Object[TARGET_SIZE];

int intVal = 1; @GenerateMicroBenchmark public void fillIntArray() { Arrays.fill(intArray, intVal++); }

@GenerateMicroBenchmark public void fillReferenceArray() { Arrays.fill(referenceArray, new Object()); } } -------------- next part -------------- An HTML attachment was scrubbed... URL: http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/attachments/20140204/96f1e6ba/attachment.html

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