Just curious, what is the difference between this fix and an old version
of the code:
http://cr.openjdk.java.net/~shade/8058643/webrev.01/src/java.base/share/classes/java/lang/String.java.sdiff.html
I meant:
"if (h == 0 && value.length > 0) {}"
vs
"if (h != 0) {hash = h;}"
On 13.05.15 14:51, Claes Redestad wrote:
Hi,
9-b33 introduced a sustained regression in SPECjvm2008
xml.transform on a number of our test setups.
JDK-8058643 removed the check on value.length > 0, which
means repeated calls to "".hashCode() now do a store of the
calculated value (0) to the hash field. This has potential to
cause excessive cache coherence traffic in xml.transform[1]
Extracting the code that showed up in profiles to a micro[2] and
running this in multiple threads is up to 100x slower in 9-b33 on
a dual-socket machine.
Adding back the check that value.length > 0 before entering the
calculation loop recuperates the loss in both micro and
xml.transform, but we're seeing as good or better number by
simply guarding the store:
Webrev: http://cr.openjdk.java.net/~redestad/8061254/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8061254 (confidential
due to links to internal systems, sorry!)
This additionally avoids the field store (and potential for pointless
cache traffic) also on non-empty strings whose hash value happen
to equals 0.
Thanks!
/Claes
[1] See
com.sun.org.apache.xml.internal.dtm.ref.ExpandedNameTable#getExpandedTypeID.
[2] http://cr.openjdk.java.net/~redestad/8061254/expandedtypeid-micro.zip
--
Best regards, Sergey.
