RFR: 8282429: StringBuilder/StringBuffer.toString() skip compressing for UTF16 strings [v4]

Claes Redestad redestad at openjdk.java.net
Tue Mar 22 10:12:39 UTC 2022


On Tue, 22 Mar 2022 08:05:35 GMT, Xin Liu <xliu at openjdk.org> wrote:

>> If AbstractStringBuilder only grow, the inflated value which has been encoded in UTF16 can't be compressed. 
>> toString() can skip compression in this case. This can save an ArrayAllocation in StringUTF16::compress().
>> 
>> java.io.BufferedRead::readLine() is a case that StringBuilder grows only. 
>> 
>> In microbench, we expect to see that allocation/op reduces 20%.  The initial capacity of StringBuilder is S in bytes. When it encounters the 1st character that can't be encoded in LATIN1, it inflates and allocate a new array of 2*S. `toString()` will try to compress that value so it need to allocate S bytes. The last step allocates 2*S bytes because it has to copy the string.  so it requires to allocate 5 * S bytes in total.  By skipping the failed compression, it only allocates 4 * S bytes.  that's 20%. In real execution, we observe 16% allocation reduction, tracked by JMH GC profiler `gc.alloc.rate.norm `.  I think it's because HotSpot can't track all allocations. 
>> 
>> Not only allocation drops, the runtime performance(ns/op) also increases from 3.34% to 18.91%. 
>> 
>> Before: 
>> 
>> $$make test TEST="micro:org.openjdk.bench.java.lang.StringBuilders.toStringWithMixedChars" MICRO="OPTIONS=-prof gc -gc true -o before.log -jvm $HOME/Devel/jdk_baseline/bin/java" 
>> 
>> Benchmark                                                               (MIXED_SIZE)  Mode  Cnt     Score     Error   Units
>> StringBuilders.toStringWithMixedChars                                            128  avgt   15   649.846 ±  76.291   ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate                             128  avgt   15   872.855 ± 128.259  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm                        128  avgt   15   880.121 ±   0.050    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space                    128  avgt   15   707.730 ± 194.421  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm               128  avgt   15   706.602 ±  94.504    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space                128  avgt   15     0.001 ±   0.002  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm           128  avgt   15     0.001 ±   0.001    B/op
>> StringBuilders.toStringWithMixedChars:·gc.count                                  128  avgt   15   113.000            counts
>> StringBuilders.toStringWithMixedChars:·gc.time                                   128  avgt   15    85.000                ms
>> StringBuilders.toStringWithMixedChars                                            256  avgt   15  1316.652 ± 112.771   ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate                             256  avgt   15   800.864 ±  76.869  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm                        256  avgt   15  1648.288 ±   0.162    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space                    256  avgt   15   599.736 ± 174.001  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm               256  avgt   15  1229.669 ± 318.518    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space                256  avgt   15     0.001 ±   0.001  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm           256  avgt   15     0.001 ±   0.002    B/op
>> StringBuilders.toStringWithMixedChars:·gc.count                                  256  avgt   15   133.000            counts
>> StringBuilders.toStringWithMixedChars:·gc.time                                   256  avgt   15    92.000                ms
>> StringBuilders.toStringWithMixedChars                                           1024  avgt   15  5204.303 ± 418.115   ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate                            1024  avgt   15   768.730 ±  72.945  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm                       1024  avgt   15  6256.844 ±   0.358    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space                   1024  avgt   15   655.852 ± 121.602  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm              1024  avgt   15  5315.265 ± 578.878    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space               1024  avgt   15     0.002 ±   0.002  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm          1024  avgt   15     0.014 ±   0.011    B/op
>> StringBuilders.toStringWithMixedChars:·gc.count                                 1024  avgt   15    96.000            counts
>> StringBuilders.toStringWithMixedChars:·gc.time                                  1024  avgt   15    86.000                ms
>> 
>> 
>> After
>> 
>> $make test TEST="micro:org.openjdk.bench.java.lang.StringBuilders.toStringWithMixedChars" MICRO="OPTIONS=-prof gc -gc true -o after.log" 
>> 
>> Benchmark                                                               (MIXED_SIZE)  Mode  Cnt     Score      Error   Units
>> StringBuilders.toStringWithMixedChars                                            128  avgt   15   627.522 ±   54.804   ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate                             128  avgt   15   751.353 ±   83.478  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm                        128  avgt   15   736.120 ±    0.062    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space                    128  avgt   15   589.102 ±  164.083  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm               128  avgt   15   575.425 ±  127.021    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space                128  avgt   15    ≈ 10⁻³             MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm           128  avgt   15    ≈ 10⁻³               B/op
>> StringBuilders.toStringWithMixedChars:·gc.count                                  128  avgt   15   116.000             counts
>> StringBuilders.toStringWithMixedChars:·gc.time                                   128  avgt   15    86.000                 ms
>> StringBuilders.toStringWithMixedChars                                            256  avgt   15  1185.884 ±  137.364   ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate                             256  avgt   15   746.179 ±   92.534  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm                        256  avgt   15  1376.244 ±    0.125    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space                    256  avgt   15   579.137 ±  208.636  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm               256  avgt   15  1055.150 ±  307.010    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space                256  avgt   15     0.002 ±    0.002  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm           256  avgt   15     0.003 ±    0.003    B/op
>> StringBuilders.toStringWithMixedChars:·gc.count                                  256  avgt   15   126.000             counts
>> StringBuilders.toStringWithMixedChars:·gc.time                                   256  avgt   15    97.000                 ms
>> StringBuilders.toStringWithMixedChars                                           1024  avgt   15  4220.415 ±  440.169   ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate                            1024  avgt   15   791.945 ±   75.231  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm                       1024  avgt   15  5217.435 ±    0.543    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space                   1024  avgt   15   657.270 ±  235.803  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm              1024  avgt   15  4232.470 ± 1267.388    B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space               1024  avgt   15     0.005 ±    0.002  MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm          1024  avgt   15     0.033 ±    0.014    B/op
>> StringBuilders.toStringWithMixedChars:·gc.count                                 1024  avgt   15   202.000             counts
>> StringBuilders.toStringWithMixedChars:·gc.time                                  1024  avgt   15   121.000                 ms
>
> Xin Liu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8282429
>  - split StringBuilder.toString() performance test out of StringBuilders.java
>  - Change growOnly to maybeLatin.
>    
>    This patch also copys over the attribute from the other AbstractStringBuilder.
>    Add a unit test to cover methods which cause maybeLatin1 becomes true.
>  - make sure String(StringBuffer) is still synchronized.
>  - Add a microbenchmark.
>  - 8282429:  StringBuilder/StringBuffer.toString() skip compressing for UTF16 strings

Thanks for refactoring the micro to avoid the redundant run overheads, and checking that the we're size neutral on all configurations. 

I have added a few minor comments inline that you can choose to address or ignore. 

Good work!

src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 71:

> 69: 
> 70:     /**
> 71:      *  The attribute indicates {@code value} is in UTF16 but may be compressible.

Since we conservatively set this `true` when deleting anything there are state transitions possible where `maybeLatin1` is set to `true` even though the `coder` is still `LATIN1`. Perhaps clarify that such false positives must be handled gracefully? Since a true doesn't imply UTF16 I also suggest a minor rewording like so: `"Indicates that {@code value} might be compressible to LATIN1 if it's UTF16-encoded."`

src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 75:

> 73:      *  Therefore, set this attribute in all methods that delete chars.
> 74:      */
> 75:     boolean maybeLatin1 = false;

Could remove `= false` (default value).

-------------

Marked as reviewed by redestad (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7671


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