[10?] RFR: 8193128: Reduce number of implementation classes returned by List/Set/Map.of()
jbluettduncan at gmail.com
Wed Dec 6 20:58:29 UTC 2017
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
On 6 December 2017 at 20:21, Claes Redestad <claes.redestad at oracle.com>
> please help review this patch to consolidate the number of implementation
> classes returned by the static collection factories:
> 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
> Microbenchmark: http://cr.openjdk.java.net/~re
> 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
> 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, I narrowed in on the following experiment:
> 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 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 #
> ListMorphism.finalSumSizesArrayList thrpt 25 244.595 ± 1.085 ops/us
> ListMorphism.finalSumSizesList thrpt 25 303.351 ± 0.182 ops/us #
> ListMorphism.getFromArrayList thrpt 25 70.631 ± 0.070 ops/us
> ListMorphism.getFromList thrpt 25 73.258 ± 2.955 ops/us #
> 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 #
> 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)
>  Older experiments:
> -- simply merge L0 into LN (still have L1, L2 and LN) - nothing really
> changed, though
> -- L0 merged into LN, drop L2. Substantial performance boost on micros.
> Footprint drop for 2-element lists.
> -- L0+L1+L2+LN merged into one implementation. Same footprint with a
> single class, but loses a lot on throughput in micros.
> -- 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.
>  http://openjdk.java.net/projects/code-tools/jol/
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