RFR: 8013395 StringBuffer.toString performance regression impacting embedded benchmarks

[Peter Levart]

Hi David,

One remote incompatibility note: the String returned from 
StringBuffer.toString() is retained by StringBuffer until the next call 
to StringBuffer mutating method. This can be observed for example if the 
returned String object is wrapped by a WeakReference. This is really a 
remotely incompatible difference in external behaviour, but it could be 
fixed by the following variation of toString():

     private transient char[] toStringCache;

     @Override
     public synchronized String toString() {
         if (toStringCache == null) {
             toStringCache = Arrays.copyOfRange(value, 0, count);
         }
         return new String(toStringCache, true);
     }


Regards, Peter

On 05/10/2013 08:03 AM, David Holmes wrote:
> Short version:
>
> Cache the value returned by toString and use it to copy-construct a 
> new String on subsequent calls to toString(). Clear the cache on any 
> mutating operation.
>
> webrev: http://cr.openjdk.java.net/~dholmes/8013395/webrev.v2/
>
> Testing: microbenchmark for toString performance; new regression test 
> for correctness; JPRT testset core as a sanity check
>
> Still TBD - full SE benchmark (?)
>
> Thanks,
> David
> ---------
>
> Long version:
>
> One of the goals for JDK8 is to provide a path from Java ME CDC to 
> Java SE (or SE Embedded). In the embedded space some pretty old 
> benchmarks still get used for doing comparisons between JRE's. One of 
> which makes heavy use of StringBuffer.toString, without modifying the 
> StringBuffer in between.
>
> Up to Java 1.4.2 a StringBuffer and a String could share the 
> underlying char[]. This meant that toString simply needed to create a 
> new String that referenced the StringBuffer's char[] with no copying 
> of the array needed. In Java 5 the String/StringBuffer implementations 
> were completely revised: StringBuilder was introduced for 
> non-synchronized use, and a new AbstractStringBuilder base class added 
> for it and StringBuffer. In that implementation toString now has to 
> copy the StringBuffer's char[]. This resulted in a significant 
> performance regression for toString() and a bug - 6219959 - was 
> opened. There is quite an elaborate evaluation in that bug report but 
> bottom line was that "real code doesn't depend on this - won't fix".
>
> At some stage ME also updated to the new Java 5 code and they also 
> noticed the problem. As a result CDC6 included a variation of the 
> caching strategy that is proposed here.
>
> Going forward because we want people to be able to compare ME and SE 
> with their familiar benchmarks, we would like to address this corner 
> case and fix it using the caching strategy outlined. As a data point 
> an 8K StringBuffer that takes ~1ms to be converted to a String 
> initially, can process subsequent toString() calls in a few 
> microseconds. So that performance issue is addressed.
>
> However we've added a write to a field in all the mutating methods 
> which obviously adds some additional computational effort - though I 
> have no doubt it is lost in the noise for all but the smallest of 
> mutating methods. Even so this should be run against regular SE 
> benchmarks to ensure there are no performance regressions there - so 
> if anyone has a suggestion as to the best benchmark to run to exercise 
> StringBuffer (if it exists), please let me know.
>
> Thanks for reading this far :)