RFR 8231264: Disable biased-locking and deprecate all flags related to biased-locking

[Daniel D. Daugherty]

Hi Patricio,

On 11/15/19 9:15 PM, Patricio Chilano wrote:
> Hi all,
>
> Could you review the following patch?
>
> JBS: https://bugs.openjdk.java.net/browse/JDK-8231264
> Webrev: http://cr.openjdk.java.net/~pchilanomate/8231264/v01/webrev

src/hotspot/share/runtime/arguments.cpp
     Is it too early to specify the obsolete_in and expired_in values?
     They could be JDK_Version::undefined() so that all you are doing
     is deprecation in this changeset.

src/hotspot/share/runtime/globals.hpp
     No comments.

test/hotspot/gtest/oops/test_markWord.cpp
     L96:     // Can't test this with biased locking disabled.
         Perhaps (since the comment is inside the if-statement):
              // This sub-test requires biased locking to be enabled.

     L11[135] - Why indent the pre-processor controls? Left most
         column is generally the style used.

     L115:   // Same thread tries to lock it again.
         This comment needs a rewrite. Perhaps:
             // Lock the object using an ObjectLocker helper which
             // will revoke the bias if we happened to use that
             // mechanism above.

     L121:   // This is no longer biased, because ObjectLocker revokes 
the bias.
         This comment needs a rewrite. Perhaps:
             // The object should be unlocked with no hashCode at
             // this point (ObjectLocker dtr has run).

test/jdk/jdk/jfr/event/runtime/TestBiasedLockRevocationEvents.java
     No comments.

Thumbs up! My comments are mostly nits so I don't need to see a new
webrev if you decide to make changes based on my suggestions.

As for the whole "too soon to deprecate" discussion: Deprecation is not
making the code obsolete so this changeset is not taking anything away
other than changing the default of UseBiasedLocking from true to false.
There are things that have been deprecated since JDK8 and they still
have not yet been made obsolete.

Deprecating biased locking is the proper way of saying that we (Oracle)
and/or others think that biased locking should/will go away in a future
release. Yes, there are locking experts outside of Oracle that have said
that biased locking should go away, but I haven't gotten permission to
quote the folks (yet)...

Deprecation is not final. Features can be un-deprecated if some
relevant facts and/or info changes the previous conclusion.

Dan



>
> Biased locking will be disabled by default and all related flags will 
> be deprecated. Performance gains seen when the feature was introduced 
> in the VM are less clear today with modern Java code/processors. 
> Detailed rationale behind the change is included on the description of 
> the bug.
>
> I modified test gtest/oops/test_markWord.cpp so that it still 
> exercises other cases of markword printing.
>
> Tested with mach5, tiers1-6 on all platforms (Linux, macOS, Windows 
> and Solaris).
>
> Thanks,
> Patricio
>
>