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List:       openjdk-hotspot-dev
Subject:    Re: Compressed-OOP's on =?UTF-8?B?SlZN4oCP?=
From:       Aleksey Shipilev <aleksey.shipilev () oracle ! com>
Date:       2014-06-30 10:08:54
Message-ID: 53B13736.9060507 () oracle ! com
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On 06/30/2014 12:49 PM, Filipp Zhinkin wrote:
> On 06/30/2014 12:48 PM, Andrew Haley wrote:
> > On 28/06/14 13:21, serkan özal wrote:
> > 
> > > In 64 bit JVM with compressed-oops enable and and with minimum heap
> > > size 1G and maximum heap size 1G, object references are 4 byte. In
> > > this case, compressed-oop is real native address.
> > > But in 64 bit JVM with compressed-oops enable and and with minimum
> > > heap size 4G and maximum heap size 8G, object references are 4
> > > byte. But in this case, compressed-oop is needed to be
> > > encoded/decoded (by 3 bit shifting) before getting real native
> > > address.
> > > In both of cases, compressed-oop is enable, but how can I detect
> > > compressed-oops are used as native address or are they need to be
> > > encoded/decoded ?
> > Like this:
> > 
> > $ java -Xmx1G -XX:+UnlockDiagnosticVMOptions
> > -XX:+PrintCompressedOopsMode -XX:HeapBaseMinAddress=1G -version
> > 
> > heap address: 0x0000000040000000, size: 1024 MB, zero based Compressed
> > Oops, 32-bits Oops
> > 
> > Narrow klass base: 0x0000000000000000, Narrow klass shift: 0
> > Compressed class space size: 1073741824 Address: 0x0000000080000000
> > Req Addr: 0x0000000080000000
> > openjdk version "1.8.0-internal"
> > 
> > > If they are encoded/decoded, what is the value of bit shifting ?
> It is determined as binary logarithm of ObjectAlignmentInBytes' value.
> Default value of that option is 8 bytes, so value of bit shifting is 3.

See also:
 http://openjdk.java.net/projects/code-tools/jol/

...and specifically
 http://hg.openjdk.java.net/code-tools/jol/file/b4bc510cbad0/jol-core/src/main/java/org/openjdk/jol/util/VMSupport.java


Hotspot Serviceability Agent (SA) should be able to decode these as well
without guessing.

-Aleksey.


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