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List: openjdk-hotspot-runtime-dev
Subject: Re: Question about MarkOop
From: Viacheslav Kovalevskyi <viacheslav () b0noi ! com>
Date: 2014-12-28 21:44:03
Message-ID: 1553A5DB-3341-4D1B-8198-08C4C867AD24 () b0noi ! com
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Hi Filipp,
Thank you VERY MUCH! This make things very clear for me.
---
Viacheslav Kovalevskyi
viacheslav@b0noi.com
> On 28 Dec 2014, at 16:24, Filipp Zhinkin <filipp.zhinkin@oracle.com> wrote:
>
> Hi Viacheslav!
>
> Object header's bit-format has a big-endian layout, but x86 CPU is little-endian.
> JOL reads header byte per byte and don't perform any reordering.
>
> If you'll reorder it yourself (or will just read header as int/long),
> then you'll see that everything is ok: your header's value is 0x0d.
>
> It's a biased lock pattern + age 1 (0000 ... 0000 1101b).
>
> Regards,
> Filipp.
>
> ----- Original Message -----
> From: viacheslav@b0noi.com <mailto:viacheslav@b0noi.com>
> To: hotspot-runtime-dev@openjdk.java.net \
> <mailto:hotspot-runtime-dev@openjdk.java.net>
> Sent: Saturday, December 27, 2014 12:14:21 AM GMT +03:00 Iraq
> Subject: Question about MarkOop
>
> Hi!
>
> My name is Viacheslav, I'm making presentation about Java synchronization to my \
> friends and I realized that I have some gaps in understanding of MarkOop object \
> nature. I was not able to figure out all answers on my own from source code and \
> documentation. If someone can help or at least point the right direction it would \
> be awesome!
> Here is link to markOop.hpp file that is representing revision of JDK (8b132) that \
> I'm using: [1]
> I'm executing stupid and simple program that shows Java object layout on screen: \
> [2] before and during synchronization. I'm using Java Object Layout tool for \
> this([3]).
> Here are results of first 2 outputs that I see in console:
>
> com.b0noi.java.analyzer.AnalyzerTest.Test1 object internals:
> OFFSET SIZE TYPE DESCRIPTION VALUE
> 0 4 (object header) 0d 00 00 00 (0000 1101 0000 0000 0000 \
> 0000 0000 0000) 4 4 (object header) 00 00 00 00 (0000 0000 \
> 0000 0000 0000 0000 0000 0000) 8 4 (object header) 32 62 \
> 01 f8 (0011 0010 0110 0010 0000 0001 1111 1000) …
>
> com.b0noi.java.analyzerAnalyzerTest.Test1 object internals:
> OFFSET SIZE TYPE DESCRIPTION VALUE
> 0 4 (object header) 0d 70 00 1a (0000 1101 0111 0000 0000 \
> 0000 0001 1010) 4 4 (object header) 9b 7f 00 00 (1001 1011 \
> 0111 1111 0000 0000 0000 0000) 8 4 (object header) 32 62 \
> 01 f8 (0011 0010 0110 0010 0000 0001 1111 1000) …
>
> My hardware:
> CPU: i5 x64
> RAM: 16GB
>
> OS:
> Mac OS 10.10
>
> From what I can understand COOPS is switched of on my system. And according to the \
> documentation in markOop file ([1]) my layout should be one from this: // \
> unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object) // \
> JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
> first output of my program should represent this line (object normal, without \
> baised): // unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 \
> (normal object)
> second output of my program should represent this line (object inside \
> synchronization block, should be baised): // JavaThread*:54 epoch:2 unused:1 \
> age:4 biased_lock:1 lock:2 (biased object)
> However in both case real life values looks not as they should according to layout \
> documentation.
> In first case (before synchronization), real life:
> OFFSET SIZE TYPE DESCRIPTION VALUE
> 0 4 (object header) 0d 00 00 00 (0000 1101 0000 0000 0000 \
> 0000 0000 0000) 4 4 (object header) 00 00 00 00 (0000 0000 \
> 0000 0000 0000 0000 0000 0000) …
> according to dict this is:
> // unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
>
> question: why we have this strange first byte (0000 1101)? According to layout doc \
> it should be unused, and I assume field with zeros?
> Second case is even funnier, real life:
> OFFSET SIZE TYPE DESCRIPTION VALUE
> 0 4 (object header) 0d 70 00 1a (0000 1101 0111 0000 0000 \
> 0000 0001 1010) 4 4 (object header) 9b 7f 00 00 (1001 1011 \
> 0111 1111 0000 0000 0000 0000) …
> according to dict this is:
> // JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
>
> question: why first byte remains the same, looks like Thread pointer was written to \
> the different offset then it should (according to the doc). Also the first byte \
> remains the same. Layout is totally inconsistent with documentation.
> Can someone give me a hint or a direction where to dig.
>
> Sorry if this is a wrong mail list to ask this type of question. If this is the \
> case, can someone point me to the right one. Thank you in advance.
> Links:
> [1] http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/oops/markOop.hpp \
> <http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/oops/markOop.hpp \
> ><http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/oops/markOop.hpp \
> > <http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/oops/markOop.hpp>>
> >
> [2] https://github.com/b0noI/JavaAnalyzer/blob/master/src/test/java/com/b0noi/java/analyzer/AnalyzerTest.java \
> <https://github.com/b0noI/JavaAnalyzer/blob/master/src/test/java/com/b0noi/java/anal \
> yzer/AnalyzerTest.java><https://github.com/b0noI/JavaAnalyzer/blob/master/src/test/java/com/b0noi/java/analyzer/AnalyzerTest.java \
> <https://github.com/b0noI/JavaAnalyzer/blob/master/src/test/java/com/b0noi/java/analyzer/AnalyzerTest.java>>
> [3] http://openjdk.java.net/projects/code-tools/jol/ \
> <http://openjdk.java.net/projects/code-tools/jol/> \
> <http://openjdk.java.net/projects/code-tools/jol/ \
> <http://openjdk.java.net/projects/code-tools/jol/>>
> ---
> Viacheslav Kovalevskyi
> viacheslav@b0noi.com <mailto:viacheslav@b0noi.com>
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