Review request for 6977034 Thread.getState() very slow

[Mandy Chung]

   Fix for 6977034: Thread.getState() very slow

Webrev at:
    http://cr.openjdk.java.net/~mchung/6977034/webrev.00/

This is an improvement to map a Thread's threadStatus field to 
Thread.State.  The VM updates the Thread.threadStatus field directly at 
state transition with the value as defined in JVM TI [1].  The 
java.lang.Thread.getState() implementation can directly access the 
threadStatus value and do a direct lookup from an array of 
Thread.State.  The threadStatus value is a bit vector and we would have 
to create an array of a minimum of 1061 (0x425) elements to do direct 
mapping.   I took the approach to use the first highest order bit set to 
1 in the masked threadStatus value as the index to the Thread.State 
element and only caches 32 elements (could be fewer).  I wrote a 
micro-benchmark measuring the Thread.getState of a thread in different 
state that shows 1.7X to 6X speedup (see below).  There is possibly some 
issue with my micro-benchmark that I didn't observe the 14X speed up as 
Doug did in his experiment.  However, I'd like to get this reviewed and 
pushed to the repository so that anyone can do more experiment on the 
performance measurement.

Thanks
Mandy
P.S. The discussion on this thread can be found at [2] [3].

[1] 
http://download.java.net/jdk7/docs/platform/jvmti/jvmti.html#GetThreadState
[2] 
http://mail.openjdk.java.net/pipermail/core-libs-dev/2010-July/004567.html
[3] 
http://mail.openjdk.java.net/pipermail/core-libs-dev/2010-August/004721.html


	JDK 7 b120 (in ms)	With fix (in ms)	Speed up
main		46465	        22772			2.04
NEW		50676		29921			1.69
RUNNABLE	42202		14690			2.87
BLOCKED		72773		12296			5.92
WAITING		48811		13041			3.74
TIMED_WAITING	45737		12849			3.56
TERMINATED	40314		16376			2.46


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