RFR: 8259842: Remove Result cache from StringCoding
Peter Levart
plevart at openjdk.java.net
Fri Jan 15 21:42:03 UTC 2021
On Fri, 15 Jan 2021 19:14:06 GMT, Naoto Sato <naoto at openjdk.org> wrote:
>> The `StringCoding.resultCached` mechanism is used to remove the allocation of a `StringCoding.Result` object on potentially hot paths used in some `String` constructors. Using a `ThreadLocal` has overheads though, and the overhead got a bit worse after JDK-8258596 which addresses a leak by adding a `SoftReference`.
>>
>> This patch refactors much of the decode logic back into `String` and gets rid of not only the `Result` cache, but the `Result` class itself along with the `StringDecoder` class and cache.
>>
>> Microbenchmark results:
>> Baseline
>>
>> Benchmark (charsetName) Mode Cnt Score Error Units
>> decodeCharset US-ASCII avgt 15 193.043 ± 8.207 ns/op
>> decodeCharset:·gc.alloc.rate.norm US-ASCII avgt 15 112.009 ± 0.001 B/op
>> decodeCharset ISO-8859-1 avgt 15 164.580 ± 6.514 ns/op
>> decodeCharset:·gc.alloc.rate.norm ISO-8859-1 avgt 15 112.009 ± 0.001 B/op
>> decodeCharset UTF-8 avgt 15 328.370 ± 18.420 ns/op
>> decodeCharset:·gc.alloc.rate.norm UTF-8 avgt 15 224.019 ± 0.002 B/op
>> decodeCharset MS932 avgt 15 328.870 ± 20.056 ns/op
>> decodeCharset:·gc.alloc.rate.norm MS932 avgt 15 232.020 ± 0.002 B/op
>> decodeCharset ISO-8859-6 avgt 15 193.603 ± 12.305 ns/op
>> decodeCharset:·gc.alloc.rate.norm ISO-8859-6 avgt 15 112.010 ± 0.001 B/op
>> decodeCharsetName US-ASCII avgt 15 209.454 ± 9.040 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm US-ASCII avgt 15 112.009 ± 0.001 B/op
>> decodeCharsetName ISO-8859-1 avgt 15 188.234 ± 7.533 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm ISO-8859-1 avgt 15 112.009 ± 0.001 B/op
>> decodeCharsetName UTF-8 avgt 15 399.463 ± 12.437 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm UTF-8 avgt 15 224.019 ± 0.003 B/op
>> decodeCharsetName MS932 avgt 15 358.839 ± 15.385 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm MS932 avgt 15 208.017 ± 0.003 B/op
>> decodeCharsetName ISO-8859-6 avgt 15 162.570 ± 7.090 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm ISO-8859-6 avgt 15 112.009 ± 0.001 B/op
>> decodeDefault N/A avgt 15 316.081 ± 11.101 ns/op
>> decodeDefault:·gc.alloc.rate.norm N/A avgt 15 224.019 ± 0.002 B/op
>>
>> Patched:
>> Benchmark (charsetName) Mode Cnt Score Error Units
>> decodeCharset US-ASCII avgt 15 159.153 ± 6.082 ns/op
>> decodeCharset:·gc.alloc.rate.norm US-ASCII avgt 15 112.009 ± 0.001 B/op
>> decodeCharset ISO-8859-1 avgt 15 134.433 ± 6.203 ns/op
>> decodeCharset:·gc.alloc.rate.norm ISO-8859-1 avgt 15 112.009 ± 0.001 B/op
>> decodeCharset UTF-8 avgt 15 297.234 ± 21.654 ns/op
>> decodeCharset:·gc.alloc.rate.norm UTF-8 avgt 15 224.019 ± 0.002 B/op
>> decodeCharset MS932 avgt 15 311.583 ± 16.445 ns/op
>> decodeCharset:·gc.alloc.rate.norm MS932 avgt 15 208.018 ± 0.002 B/op
>> decodeCharset ISO-8859-6 avgt 15 156.216 ± 6.522 ns/op
>> decodeCharset:·gc.alloc.rate.norm ISO-8859-6 avgt 15 112.010 ± 0.001 B/op
>> decodeCharsetName US-ASCII avgt 15 180.752 ± 9.411 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm US-ASCII avgt 15 112.010 ± 0.001 B/op
>> decodeCharsetName ISO-8859-1 avgt 15 156.170 ± 8.003 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm ISO-8859-1 avgt 15 112.010 ± 0.001 B/op
>> decodeCharsetName UTF-8 avgt 15 370.337 ± 22.199 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm UTF-8 avgt 15 224.019 ± 0.001 B/op
>> decodeCharsetName MS932 avgt 15 312.589 ± 15.067 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm MS932 avgt 15 208.018 ± 0.002 B/op
>> decodeCharsetName ISO-8859-6 avgt 15 173.205 ± 9.647 ns/op
>> decodeCharsetName:·gc.alloc.rate.norm ISO-8859-6 avgt 15 112.009 ± 0.001 B/op
>> decodeDefault N/A avgt 15 303.459 ± 16.452 ns/op
>> decodeDefault:·gc.alloc.rate.norm N/A avgt 15 224.019 ± 0.001 B/op
>>
>> Most variants improve. There's a small added overhead in `String charsetName` variants for some charsets such as `ISO-8859-6` that benefited slightly from the `StringDecoder` cache due avoiding a lookup, but most variants are not helped by this cache and instead see a significant gain from skipping that step. `Charset` variants don't need a lookup and improve across the board.
>>
>> Another drawback is that we need to cram more logic into `String` to bypass the `StringCoding.Result` indirection - but getting rid of two commonly used `ThreadLocal` caches and most cases getting a bit better raw throughput in the process I think more than enough makes up for that.
>>
>> Testing: tier1-4
>
> Nit: copyright year in System.java is 2021.
Hi Claes,
This is quite an undertaking in re-factoring!
I think that decoding logic that you produced can still be kept in StringCoding class though. The private constructor that you created for UTF-8 decoding is unnecessary. The logic can be kept in a static method and then the String instance constructed in it from the value and coder. The only public constructor that remains is then the one taking a Charset parameter. I took your code and moved it back to StringCoding while for the mentioned constructor I tried the following trick which could work (I haven't tested yet with JMH): if you pass an instance of newly constructed object down to a method that is inlined into the caller and that method does not pass that object to any other methods, JIT will eliminate the heap allocation, so I suspect that you can pass results from the called method back in that object and still avoid allocation...
https://github.com/plevart/jdk/commit/691600e3789a4c2b52fae921be87d0d7affa6f0f
WDYT?
-------------
PR: https://git.openjdk.java.net/jdk/pull/2102
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