Hi Dave,
On 10/05/2013 4:25 PM, David Schlosnagle wrote:
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).
In some absolute sense yes it might be beneficial if someone is also
mis-using StringBuilder.toString; but really the best fix is to change
the bad code to not repeat the toString calls (which was what the
reporter of 6219959 did!).
The intent here to was to provide the minimal set of changes to address
this specific problem, with minimal risk of inadvertently impacting
other uses.
Thanks,
David
-----
Thanks,
Dave
On Fri, May 10, 2013 at 2:03 AM, David Holmes <david.hol...@oracle.com
<mailto:david.hol...@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 :)
