RFR: 8013395 StringBuffer.toString performance regression impacting embedded benchmarks

[David Schlosnagle]

Hi David,

Would it be beneficial to push the toStringCache up to
AbstractStringBuilder so that StringBuilder.toString() benefits from this
cache as well? It seems like this would affect both StringBuilder and
StringBuffer for repeated calls to toString(), although StringBuffer would
obviously have the synchronization overhead as well (assuming the locking
is not elided away by HotSpot).

Thanks,
Dave


On Fri, May 10, 2013 at 2:03 AM, David Holmes <david.holmes at oracle.com>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/<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 :)
>