RFR: 8061254: SPECjvm2008-XML performance regressions in 9-b33 (original) (raw)

Sergey Bylokhov Sergey.Bylokhov at oracle.com
Wed May 13 12:36:11 UTC 2015

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.

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