RFR: 8044107 Add Diagnostic Command to list all ClassLoaders

[Mikael Gerdin]

Hi Staffan,

On Wednesday 28 May 2014 16.50.28 Staffan Larsen wrote:
> This change adds a new Diagnostic Command to list all ClassLoaders and some
> statistics for each classloader. The command is called
> “GC.classloader_stats” and the information listed is:
> 
> * An id for the ClassLoader. This is the pointer to the Klass of the
> ClassLoader. The reason for using the Klass* (instead of the oop) is that
> it is stable across invocations. * The id of the ClassLoader’s parent
> ClassLoader.
> * The pointer to the ClassLoaderData structure in the VM. This can be useful
> for debugging. * The number of classes loaded by the ClassLoader.
> * The total size of all allocated metaspace chunks for the ClassLoader.
> * The total size of all allocated metaspace blocks for the ClassLoader.
> 
> If there are anonymous classes (invokedynamic classes) attributed to the
> ClassLoader, the following additional information is listed: * The number
> of anonymous classes loaded by the ClassLoader.
> * The total size of all allocated metaspace chunks for anonymous classes in
> the ClassLoader. * The total size of all allocated metaspace blocks for
> anonymous classes in the ClassLoader.
> 
> The information is gathered during a safe point to guarantee that the data
> structures are consistent.
> 
> I have added a small test and have run this through jprt. A CCC request has
> been filed.
> 
> webrev: http://cr.openjdk.java.net/~sla/8044107/webrev.00/
> bug: https://bugs.openjdk.java.net/browse/JDK-8044107

I'm going to leave the naming of the anonymous classes and dig into the code a 
bit:

in classLoaderStats.hpp:

Use the (at runtime) known object alignment?
 105     return hash ^ (hash >> 3); // just in case we're dealing with aligned 
ptrs

The &-operator is not needed here:
 108   typedef ResourceHashtable<oop, ClassLoaderStats*,
 109       &ClassLoaderStatsClosure::oop_hash, 
&ClassLoaderStatsClosure::oop_equals> StatsTable;

do_entry does not need to be virtual since the template instantiation binds to 
the symbol during compilation
 129   virtual bool do_entry(oop const& key, ClassLoaderStats* const& cls);


in classLoaderStats.cpp:

Every time you encounter a CLD which reports a ClassLoader oop you will 
overwrite the _cld field of the ClassLoaderStats.
  59   cls->_cld = cld;

I'm not sure if this was your intention or not, but you are relying on the 
subtle fact that since CLD:s are prepended to the linked list in CLDG the _cld 
field will end up pointing to the ClassLoader's CLD instead of any of the 
anonymous classes which also hash to the same ClassLoder oop but have 
different CLDs. 


Can you convert this to an if()-block or add parenthesis for clarity?
  60   cls->_classloader = cl == NULL ? NULL : cl->klass();

The ResourceMark here is unnecessary, all the resource allocations done by the 
ClassLoaderStats-classes are done by the VMThread and are in the scope of a 
ResourceMark covering the call to VM_Operation::doit()
 165   ResourceMark rm;


in ClassLoaderStatsTest.java:
There is at least one other variant of "DummyClassLoader" in the testlibrary, 
can you see if any of them are reusable?

/Mikael

> 
> Example output:
> 
> ClassLoader         Parent              CLData*            Classes   ChunkSz
>   BlockSz  Type 0x00000007c002d908  0x0000000000000000  0x0000000000000000 
>      0         0         0  sun.misc.Launcher$ExtClassLoader
> 0x0000000000000000  0x0000000000000000  0x00007fb239c08de0     761  
> 4694016   4241312  <boot classloader> 37     75776     50928   +
> invokedynamic classes 0x00000007c0061028  0x00000007c0036878 
> 0x00007fb239c2de60       1      6144      1976 
> ClassLoaderStatsTest$DummyClassLoader 1      2048      1288   +
> invokedynamic classes 0x00000007c0036878  0x00000007c002d908 
> 0x00007fb239e10fc0       8     88064     31544 
> sun.misc.Launcher$AppClassLoader Total = 4                                 
>                     808   4866048   4327048 ChunkSz: Total size of all
> allocated metaspace chunks
> BlockSz: Total size of all allocated metaspace blocks (each chunk has
> several blocks)
> 
> 
> Thanks,
> /Staffan