[10?] RFR: 8193128: Reduce number of implementation classes returned by List/Set/Map.of() (original) (raw)

Jonathan Bluett-Duncan jbluettduncan at gmail.com
Wed Dec 6 20:58:29 UTC 2017

Hi Claes,

Looking at http://cr.openjdk.java.net/~redestad/8193128/open.00/src/java.base/share/classes/java/util/ImmutableCollections.java.cdiff.html, there are sections labelled --- 646,657 ---- and --- 834,845 ---- where lines like Objects.requireNonNull(0 /* zero */); are written. I believe that they were supposed to be either removed or made to be written like Objects.requireNonNull(o /* lowercase o */). Is my belief/understanding correct?

Cheers, Jonathan

On 6 December 2017 at 20:21, Claes Redestad <claes.redestad at oracle.com> wrote:

Hi,

please help review this patch to consolidate the number of implementation classes returned by the static collection factories: http://cr.openjdk.java.net/~redestad/8193128/open.00/ I set out to explore options for addressing small inefficiencies we've been running into, the latest after replacing use of @Stable arrays in java.lang.invoke with List.of() (JDK-8184777): - List.indexOf deferred to the iterator in AbstractList, which check for concurrent modification - Warmup takes a bit longer due to having to warm up four different classes and associated methods - Slowdowns in certain code paths attributable to certain calls becoming megamorphic Microbenchmark: http://cr.openjdk.java.net/~re destad/scratch/lessimmutables/ListMorphism.java http://cr.openjdk.java.net/~redestad/scratch/lessimmutables /benchmarks.jar The benchmark explores how call-sites behave when performing some naive operations on a few different Lists. For every benchmark using List.of() there's a variant using ArrayList for comparison: Baseline: Benchmark Mode Cnt Score Error Units ListMorphism.finalGetFromArrayList thrpt 25 92.659 ± 3.058 ops/us ListMorphism.finalGetFromList thrpt 25 335.245 ± 0.244 ops/us # 3.6x ListMorphism.finalSumSizesArrayList thrpt 25 245.020 ± 0.106 ops/us ListMorphism.finalSumSizesList thrpt 25 335.107 ± 0.439 ops/us # 1.4x ListMorphism.getFromArrayList thrpt 25 70.343 ± 0.972 ops/us ListMorphism.getFromList thrpt 25 37.121 ± 0.135 ops/us # 0.53x ListMorphism.getFromArrayList12 thrpt 25 109.890 ± 0.286 ops/us ListMorphism.getFromList12 thrpt 25 109.552 ± 6.972 ops/us # 1.0x ListMorphism.sumSizesArrayList thrpt 25 131.467 ± 4.672 ops/us ListMorphism.sumSizesList thrpt 25 57.877 ± 0.102 ops/us # 0.45x ListMorphism.sumSizesArrayList12 thrpt 25 208.652 ± 11.294 ops/us ListMorphism.sumSizesList12 thrpt 25 227.269 ± 0.961 ops/us # 1.1x The good: When dealing with List literals (the final* benchmarks), List.of() allows really nice speed-ups compared to ArrayList. The bad: When not used as a literal, List.of() implementations at call-sites can cause a substantial slowdown (megamorphic dispatch) The ugly: After some explorations[1], I narrowed in on the following experiment: http://cr.openjdk.java.net/~redestad/scratch/lessimmutables/webrev/ Basically: Merge List1 and List2 into a single class, List12, merge List0 into ListN (List0 has a singleton instance, so might as well be an instance of ListN). Same for Set0,1,2,N. Map0 is merged into MapN. This strikes a balance between throughput, footprint and slightly better startup/warmup behavior. According to jol estimates[2] the size of List12 is the same as both List1 and List2 in the current JDK implementation. Set12 is footprint neutral compared to Set2 on all platforms but lose a few bytes on 64-bit VMs compared to Set1. Benchmark Mode Cnt Score Error Units ListMorphism.finalGetFromArrayList thrpt 25 93.046 ± 0.569 ops/us ListMorphism.finalGetFromList thrpt 25 335.280 ± 0.154 ops/us # 3.6x ListMorphism.finalSumSizesArrayList thrpt 25 244.595 ± 1.085 ops/us ListMorphism.finalSumSizesList thrpt 25 303.351 ± 0.182 ops/us # 1.2x ListMorphism.getFromArrayList thrpt 25 70.631 ± 0.070 ops/us ListMorphism.getFromList thrpt 25 73.258 ± 2.955 ops/us # 1.04x ListMorphism.getFromArrayList12 thrpt 25 109.921 ± 0.096 ops/us ListMorphism.getFromList12 thrpt 25 127.392 ± 0.088 ops/us # 1.16x ListMorphism.sumSizesArrayList thrpt 25 131.393 ± 4.882 ops/us ListMorphism.sumSizesList thrpt 25 107.686 ± 5.286 ops/us # 0.82x ListMorphism.sumSizesArrayList12 thrpt 25 212.350 ± 0.134 ops/us ListMorphism.sumSizesList12 thrpt 25 198.778 ± 0.479 ops/us # 0.94x The experiment has a flag to change number of specialized List/Set/Map classes (-Djdk.ImmutableCollections.specializations=0|1|2, default=2). 1 specialization (List1 + ListN, Set1 + SetN) is more or less the same as 2, some ~1-2% improvements, mainly in sumSizes micros. 0 specializations (List-, Set, MapN only) achieves a small increase in throughput on some micros by ensuring callsites stay monomorphic, but it's not very substantial by my measures (~5%, but mostly in sumSizes micros). Keeping the footprint more or less the same, while evening out a few rough edges and improving startup and static footprint seems like the overall best option. An alternative would be to keep the experimental flag, but I don't think a 5% gain on micros warrants the extra maintenance burden and testing that entails. The proposed patch is more or less this experiment with 2 specializations, but having removed the flag and code movement needed to support it removed (along with a fix in the writeReplace methods of List12/Set12) Thanks! /Claes [1] Older experiments: http://cr.openjdk.java.net/~redestad/immutable/list12N.0/ -- simply merge L0 into LN (still have L1, L2 and LN) - nothing really changed, though http://cr.openjdk.java.net/~redestad/immutable/list1N.0/ -- L0 merged into LN, drop L2. Substantial performance boost on micros. Footprint drop for 2-element lists. http://cr.openjdk.java.net/~redestad/immutable/listNdouble.0/ -- L0+L1+L2+LN merged into one implementation. Same footprint with a single class, but loses a lot on throughput in micros. http://cr.openjdk.java.net/~redestad/immutable/listNsingle.0/ -- L0+L1+LN merged, drop L2. Simplification of the previous. Like the list1N.0 experiment in footprint, but a loss in throughput on all measures. No approach seemed a win across the board here: we either had to accept a footprint reduction, or see throughput suffer dramatically. [2] http://openjdk.java.net/projects/code-tools/jol/

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