On 13/05/2013 6:39 PM, Peter Levart wrote:
On 05/13/2013 10:07 AM, Remi Forax wrote:
Hi David,
On 05/13/2013 09:12 AM, David Holmes wrote:
Thanks for all the feedback and discussion. I've rewritten the patch
to use Peter's suggestion of caching the char[] instead of the actual
String:
http://cr.openjdk.java.net/~dholmes/8013395/webrev.v3/
I too am concerned that any form of caching that avoids the
array-copy (which is the crux of the issue) is going to incur a 2x
space penalty. I can imagine old but well-behaved code that uses
StringBuffer rather than StringBuilder where very long buffers are
created and toString() only called once - and they may get immediate
or subsequent OOME due to the extra char[]; or excessive GC activity
might be induced if a lot of StringBuffers are used.
So I tried to implement a simple copy-on-write-if-shared (COWIS) scheme:
http://cr.openjdk.java.net/~dholmes/8013395/webrev.v4/
but I must have done something silly as the resulting JDK won't run -
for some reason it causes classes and resources to not be found. :(
I don't know if it's the only error but take a look to the constructor
String(char[], boolean),
you will understand what's wrong.
Hi David, Rémi,
I think the problem is the toString() method (have you had the same
thing in mind, Rémi?):
686 @Override
687 public synchronized String toString() {
688 isShared = true;
689 return new String(value, true);
690 }
...the returned String is always constructed with the whole 'value'
array. It should only take the 1st 'count' chars...
Ah! Yes now I see - I said it was a silly mistake.
I think that toString array caching aspect and copy-on-write aspect can
only be combined in situations when 'count == value.length'. Otherwise
Right so for this simple StringBuffer case we are basically stuck with
the memory waste.
they are different aspects and the copy-on-write aspect which can only
be used when count == value.length, could be useful also in
StringBuilder, so it should perhaps be built into the
AbstractStringBuilder logic. Although it can only be effective when
count == value.length, the performance concious code could benefit (when
logic can predict the final length of resulting String, the building of
it could be performed without extra array copying).
This is going too far afield. The change becomes too significant for
this stage of JDK8 release.
Thanks,
David
For example, to concatenate 2 Strings most optimally, the following
would not be doing any more copies than necessary:
new StringBuilder(s1.length() +
s2.length()).append(s1).append(s2).toString();
Regards, Peter
The other possible source of errors is that the VM has some string
optimizations that
recognize patterns like buffer.append(s).append(s2).toString().
David
-----
Rémi
On 11/05/2013 3:36 AM, Mike Duigou wrote:
The implementation looks OK to me. I like Peter's suggestion of
caching the char[] rather than string.
I do wish that the cache could be in a soft reference but understand
that there are tradeoffs to doing so. I am worried about leaks with
this implementation.
Another possibility, to go totally nuts, is to consider implementing
a form of copy-on-write-following-toString. This would be the exact
opposite of "minimal set of changes to address this specific
problem" and probably not wise to attempt for Java 8.
Just as an FYI, this issue has in-flight conflicts with Martin's
ongoing CharSequence.getChars patch.
Mike
On May 9 2013, at 23:03 , 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 :)
