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Shared classes cache diagnostic utilities

These utilities display information about the contents of a shared classes cache. Run the utilities by specifying them as suboptions of -Xshareclasses. The utilities run on the default cache unless you specify a cache by adding the name=<cache_name> suboption.

printAllStats

-Xshareclasses:printAllStats
-Xshareclasses:printAllStats,name=<cache_name>

Displays the contents of the cache in chronological order. You can use this output to see the history of updates that were made to the cache. For layered caches, some information is shown for the top layer cache only, and some is shown for all layers combined. To see information for the top layer cache only, use printTopLayerStats=all.

Each entry in the output starts with a VM ID, so you can see which VM wrote the associated data. Here are example entries for various types of cache data, with explanations:

Class paths

This example shows one class path with 4 entries:

1: 0x2234FA6C CLASSPATH
        /myVM/Apps/application1.jar
        /myVM/Apps/application2.jar
        /myVM/Apps/application3.jar
        /myVM/Apps/application4.jar
  • 1 : the ID of the VM that wrote this data.
  • 0x2234FA6C : the address where this data is stored.
  • CLASSPATH : the type of data that was written.

ROMClasses

This example shows an entry for a single ROMClass:

1: 0x2234F7DC ROMCLASS: java/lang/Runnable at 0x213684A8
        Index 1 in class path 0x2234FA6C
  • 1 : the ID of the VM that wrote this data.
  • 0x2234F7DC : the address where the metadata about the class is stored.
  • ROMCLASS : the type of data that was stored.
  • java/lang/Runnable : the name of the class.
  • 0x213684A8 : the address where the class was stored.
  • Index 1 : the index in the class path where the class was loaded from.
  • 0x2234FA6C : the address of the class path against which this class is stored.

Stale classes are marked with !STALE!. Any partition or modification context that is used when the class is stored is also shown.

AOT methods

This example shows an entry for one AOT-compiled method:

1: 0x540FBA6A AOT: loadConvert
        for ROMClass java/util/Properties at 0x52345174
  • 1 : the ID of the VM that wrote this data.
  • 0x540FBA6A : the address where the data is stored.
  • AOT : the type of data that was stored.
  • loadConvert : the method for which AOT-compiled code is stored.
  • java/util/Properties : the class that contains the method.
  • 0x52345174 : the address of the class that contains the method.

Stale methods are marked with !STALE!.

URLs and tokens

A Token is a string that is passed to the Java™ SharedClassHelper API. The output for these data types has the same format as that for class paths, but with a single entry.

Zip entry caches

This example shows 4 separate entries for zip entry caches:

1: 0x042FE07C ZIPCACHE: luni-kernel.jar_347075_1272300300_1 Address: 0x042FE094 Size: 7898
1: 0x042FA878 ZIPCACHE: luni.jar_598904_1272300546_1 Address: 0x042FA890 Size: 14195
1: 0x042F71F8 ZIPCACHE: nio.jar_405359_1272300546_1 Address: 0x042F7210 Size: 13808
1: 0x042F6D58 ZIPCACHE: annotation.jar_13417_1272300554_1 Address: 0x042F6D70 Size: 1023
  • 1 : the ID of the VM that wrote this data.
  • 0x042FE07C : the address where the metadata for the zip entry cache is stored.
  • ZIPCACHE : the type of data that was stored.
  • luni-kernel.jar_347075_1272300300_1 : the name of the zip entry cache.
  • 0x042FE094 : the address where the data is stored.
  • 7898 : the size of the stored data, in bytes.

JIT data

Information about JIT data is shown in JITPROFILE and JITHINT entries. For example:
1: 0xD6290368 JITPROFILE: getKeyHash Signature: ()I Address: 0xD55118C0
for ROMClass java/util/Hashtable$Entry at 0xD5511640.
2: 0xD6283848 JITHINT: loadClass Signature: (Ljava/lang/String;)Ljava/lang/Class; Address: 0xD5558F98
for ROMClass com/ibm/oti/vm/BootstrapClassLoader at 0xD5558AE0.

Startup hints

Information about startup hints is shown in STARTUP HINTS KEY and STARTUP HINTS DETAIL. For example:
1: 0x000000002237C6E0 STARTUP HINTS KEY: -Xoptionsfile=jre\bin\compressedrefs\options.default -Xlockword:mode=default -Xjcl:jclse29 -Dcom.ibm.oti.vm.bootstrap.library.path=jre\bin\compressedrefs;jre\bin -Djava.home=jre -Djava.ext.dirs=jre\lib\ext -Duser.dir=bin -Djava.class.path=. -Dsun.java.launcher=SUN_STANDARD Address: 0x000000002237C700 Size: 96
STARTUP HINTS DETAIL Flags: 1 DATA1: 1677721 DATA2: 5033165

printStats

-Xshareclasses:printStats
-Xshareclasses:printStats,name=<cache_name>
-Xshareclasses:printStats=<data_type1>[+<data_type2>][...],name=<cache_name>

Displays summary information about the cache. For layered caches, -Xshareclasses:printStats shows some information for the top layer cache, and summary information (bytes and counts only) for all layers combined. To see information for the top layer cache only, use printTopLayerStats.

You can request more detail about items of a specific data type that are stored in the shared cache by using printStats=<data_type>. Use the plus symbol (+) to separate the data types. For example, use printStats=romclass+url,name=myCache to see information about ROMClass and URL items in all the layer caches of the cache called Cache1. The valid data types are as follows (case insensitive):

  • help (displays the list of valid data types)
  • all (equivalent to printAllStats)
  • classpath
  • url
  • token
  • romclass
  • rommethod
  • aot
  • jitprofile
  • jithint
  • zipcache
  • stale
  • startuphint

Example output for a traditional cache (no cache layers: cache layer = 0), with summary information only:

Current statistics for cache "Cache1":

Cache created with:
        -Xnolinenumbers                      = false
        BCI Enabled                          = true
        Restrict Classpaths                  = false
        Feature                              = cr

Cache contains only classes with line numbers

base address                         = 0x00007F60B807A000
end address                          = 0x00007F60B905E000
allocation pointer                   = 0x00007F60B81BE3A8

cache layer                          = 0
cache size                           = 16776608
softmx bytes                         = 16776608
free bytes                           = 12740572
Reserved space for AOT bytes         = -1
Maximum space for AOT bytes          = -1
Reserved space for JIT data bytes    = -1
Maximum space for JIT data bytes     = -1
Metadata bytes                       = 30440
Metadata % used                      = 0%
Class debug area size                = 1331200
Class debug area used bytes          = 189358
Class debug area % used              = 14%

ROMClass bytes                       = 1328040
AOT bytes                            = 98404
JIT data bytes                       = 168
Zip cache bytes                      = 1133704
Startup hint bytes                   = 0
Data bytes                           = 114080

# ROMClasses                         = 452
# AOT Methods                        = 2
# Classpaths                         = 1
# URLs                               = 0
# Tokens                             = 0
# Zip caches                         = 21
# Startup hints                      = 0
# Stale classes                      = 0
% Stale classes                      = 0%


Cache is 24% full

Cache is accessible to current user = true

Example output for a cache with 2 layers (cache layer = 1), with summary information only:

Current statistics for top layer of cache "Cache1":

Cache created with:
        -Xnolinenumbers                      = false
        BCI Enabled                          = true
        Restrict Classpaths                  = false
        Feature                              = cr


base address                         = 0x00007FCAB2766000
end address                          = 0x00007FCAB374A000
allocation pointer                   = 0x00007FCAB2766000

cache layer                          = 1
cache size                           = 16776608
softmx bytes                         = 16776608
free bytes                           = 15299372
Reserved space for AOT bytes         = -1
Maximum space for AOT bytes          = -1
Reserved space for JIT data bytes    = -1
Maximum space for JIT data bytes     = -1
Class debug area size                = 1331200
Class debug area used bytes          = 0
Class debug area % used              = 0%

Cache is 8% full

Cache is accessible to current user = true
---------------------------------------------------------

Current statistics for all layers of cache "Cache1":

ROMClass bytes                       = 1328040
AOT bytes                            = 128924
JIT data bytes                       = 812
Zip cache bytes                      = 1133704
Startup hint bytes                   = 0
Data bytes                           = 114080

# ROMClasses                         = 452
# AOT Methods                        = 20
# Classpaths                         = 1
# URLs                               = 0
# Tokens                             = 0
# Zip caches                         = 21
# Startup hints                      = 0
# Stale classes                      = 0
% Stale classes                      = 0%

The Cache created with section indicates the options that were used when the cache was created. BCI Enabled relates to the -Xshareclasses:enableBCI option (enabled by default) and Restrict Classpaths relates to the -Xshareclasses:restrictClasspaths option. Feature = cr indicates that the cache is a 64-bit compressed references cache, as described in Creating, populating, monitoring, and deleting a cache.

Line number information for classes in the cache is then shown with one of the following messages:

  • Cache contains only classes with line numbers : VM line number processing was enabled for all the classes that were stored in this shared cache (the -Xlinenumbers option is enabled by default). All classes in the cache contain line numbers if the original classes contained line number data.
  • Cache contains only classes without line numbers : The -Xnolinenumbers option was used to disable VM line number processing for all the classes that were stored in this shared cache, so none of the classes contain line numbers.
  • Cache contains classes with line numbers and classes without line numbers : VM line number processing was enabled for some classes and disabled for others (the -Xnolinenumbers option was specified when some of the classes were added to the cache).

The following summary data is displayed:

baseAddress and endAddress

The boundary addresses of the shared memory area that contains the classes.

allocation pointer

The address where ROMClass data is currently being allocated in the cache.

cache layer

The layer number that the cache stats relate to.

cache size and free bytes

cache size shows the total size of the shared memory area in bytes, and free bytes shows the free bytes that remain.

softmx bytes

The soft maximum size for the cache. For more information, see -Xscmx.

ROMClass bytes

The number of bytes of class data in the cache.

AOT bytes

The number of bytes of AOT-compiled code in the cache.

Reserved space for AOT bytes

The number of bytes reserved for AOT-compiled code in the cache.

Maximum space for AOT bytes

The maximum number of bytes of AOT-compiled code that can be stored in the cache.

JIT data bytes

The number of bytes of JIT-related data stored in the cache.

Reserved space for JIT data bytes

The number of bytes reserved for JIT-related data in the cache.

Maximum space for JIT data bytes

The maximum number of bytes of JIT-related data that can be stored in the cache.

Zip cache bytes

The number of zip entry cache bytes stored in the cache.

Startup hint bytes

The number of bytes of data stored to describe startup hints.

Data bytes

The number of bytes of non-class data stored by the VM.

Metadata bytes

The number of bytes of data stored to describe the cached classes. Note: This field is available only in the top layer cache output or when a cache is not layered.

Metadata % used

The proportion of metadata bytes to class bytes, which indicates how efficiently cache space is being used. The value shown does consider the Class debug area size.

Class debug area size

The size in bytes of the Class Debug Area. This area is reserved to store LineNumberTable and LocalVariableTable class attribute information.

Class debug area bytes used

The size in bytes of the Class Debug Area that contains data.

Class debug area % used

The percentage of the Class Debug Area that contains data.

ROMClasses

The number of classes in the cache. The cache stores ROMClasses (the class data itself, which is read-only) and information about the location from which the classes were loaded. This information is stored in different ways, depending on the Java SharedClassHelper API that was used to store the classes. For more information, see Using the Java Helper API.

AOT methods

Optionally, ROMClass methods can be compiled and the AOT code stored in the cache. The AOT methods information shows the total number of methods in the cache that have AOT code compiled for them. This number includes AOT code for stale classes.

Classpaths, URLs, and Tokens

The number of class paths, URLs, and tokens in the cache. Classes stored from a SharedClassURLClasspathHelper are stored with a Classpath. Classes stored using a SharedClassURLHelper are stored with a URL. Classes stored using a SharedClassTokenHelper are stored with a Token. Most class loaders, including the bootstrap and application class loaders, use a SharedClassURLClasspathHelper. The result is that it is most common to see class paths in the cache.
The number of Classpaths, URLs, and Tokens stored is determined by a number of factors. For example, every time an element of a Classpath is updated, such as when a .jar file is rebuilt, a new Classpath is added to the cache. Additionally, if partitions or modification contexts are used, they are associated with the Classpath, URL, or Token. A Classpath, URL, or Token is stored for each unique combination of partition and modification context. For more information, see SharedClassHelper partitions and Modification contexts.

Zip caches

The number of .zip files that have entry caches stored in the shared cache.

Startup hints

The number of startup hints stored in the cache. There can be a startup hint for each unique set of command line options used to start the JVM.

Stale classes

The number of classes that have been marked as "potentially stale" by the cache code, because of a VM or Java application update. See Understanding dynamic updates.

% Stale classes

The percentage of classes in the cache that are stale.

Cache is XXX% full

The percentage of the cache that is currently used. This line is displayed only if the soft maximum size is not set. This value is calculated as follows:
% Full = (('Cache Size' - 'Free Bytes') * 100) / ('Cache Size')

Cache is XXX% soft full

The percentage of the soft maximum size that is currently used. This line is displayed only if the soft maximum size is set. The free bytes in the cache statistics means the free bytes within the soft maximum limit. This value is calculated as follows:
% soft Full = (('Soft max bytes' - 'Free Bytes') * 100) / ('Soft max  bytes')

For more information about the soft maximum size, see -Xscmx.

Cache is accessible to current user

Whether the current user can access the cache.

printTopLayerStats

Use this utility with a layered cache. This utility works in the same way as printStats. By default, this utility shows information for the top layer cache. To view statistics for a specific layer, use the layer=<number> option. For example, to show statistics for the second layer in a 2-layer cache, run printTopLayerStats,layer=1.

Example output:

Current statistics for cache "Cache1":

Cache created with:
        -Xnolinenumbers                      = false
        BCI Enabled                          = true
        Restrict Classpaths                  = false
        Feature                              = cr


base address                         = 0x00007F234C054000
end address                          = 0x00007F234D038000
allocation pointer                   = 0x00007F234C054000

cache layer                          = 1
cache size                           = 16776608
softmx bytes                         = 16776608
free bytes                           = 15299372
Reserved space for AOT bytes         = -1
Maximum space for AOT bytes          = -1
Reserved space for JIT data bytes    = -1
Maximum space for JIT data bytes     = -1
Metadata bytes                       = 792
Metadata % used                      = 0%
Class debug area size                = 1331200
Class debug area used bytes          = 0
Class debug area % used              = 0%

ROMClass bytes                       = 0
AOT bytes                            = 30520
JIT data bytes                       = 644
Zip cache bytes                      = 0
Startup hint bytes                   = 0
Data bytes                           = 114080

# ROMClasses                         = 0
# AOT Methods                        = 18
# Classpaths                         = 0
# URLs                               = 0
# Tokens                             = 0
# Zip caches                         = 0
# Startup hints                      = 0
# Stale classes                      = 0
% Stale classes                      = 0%


Cache is 8% full

Cache is accessible to current user = true