Metaspace - memory allocation

[Thomas Stüfe]

Hi Timur,

I believe you see the space for the compressed class data (Compressed class
space), which is by default 1GB and controlled by
-XX:CompressedClassSpaceSize . I think this area is semantically one
subarea of the metaspace, the other being non-class data.

I am unsure about the difference between MaxMetaSpaceSize and
CompressedClassSpaceSize, but what I believe is that:

- MaxMetaSpaceSize limits the amount of total committed (not reserved) space
- CompressedClassSpaceSize sets the amount of space to be reserved (not
committed) for compressed classes only.

This would fit your NMT output, which says:

"Class (reserved=1142808KB, committed=106520KB)"

so, ~100M are committed. With a MaxMetaSpaceSize=256M - what you specified
- you would not be able to commit the whole 1GB, but hit the limit far
earlier.

See also:
https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/considerations.htm
(the section about UseCompressedClassesPointers)

Please take this with a grain of salt. I am not sure I understand this
correctly and would value input from other developers.

Kind Regards, Thomas

On Thu, Feb 4, 2016 at 2:13 PM, Timur Akhmadeev <timur.akhmadeev at gmail.com>
wrote:

> Hi,
>
> I'm trying to figure out which parts of memory Hotspot can allocate in
> HugePages on Linux (appears to be heap and code cache by default, plus
> Metaspace with -XX:+UseLargePagesInMetaspace) and noticed something I can't
> explain.
>
> There's 1G allocation on the OS level in 4K pages that is apparently
> attributed to Class metadata, although Metaspace is maxed at a value much
> lower than 1G. Question: why?
>
> [oracle at oel6u7 bin]$ grep -B 11 'KernelPageSize:        4 kB'
> /proc/18906/smaps | grep '^Size:' | head
> Size:                  4 kB
> Size:                  4 kB
> Size:               2764 kB
> Size:              12288 kB
> Size:            1036288 kB
> Size:                128 kB
> Size:                  4 kB
> Size:                  4 kB
> Size:                  4 kB
> Size:                  8 kB
>
> [oracle at oel6u7 bin]$ /home/oracle/jdk1.8.0_72/bin/jcmd 18906
> VM.native_memory summary
> 18906:
>
> Native Memory Tracking:
>
> Total: reserved=2069703KB, committed=1032559KB
> -                 Java Heap (reserved=786432KB, committed=786432KB)
>                             (mmap: reserved=786432KB, committed=786432KB)
>
> -                     Class (reserved=1142808KB, committed=106520KB)
>                             (classes #13624)
>                             (malloc=2072KB #22966)
>                             (mmap: reserved=1140736KB, committed=104448KB)
>
> -                    Thread (reserved=56009KB, committed=56009KB)
>                             (thread #55)
>                             (stack: reserved=55512KB, committed=55512KB)
>                             (malloc=178KB #272)
>                             (arena=319KB #108)
>
> -                      Code (reserved=55407KB, committed=55407KB)
>                             (malloc=5487KB #7167)
>                             (mmap: reserved=49920KB, committed=49920KB)
>
> -                        GC (reserved=2600KB, committed=1748KB)
>                             (malloc=32KB #355)
>                             (mmap: reserved=2568KB, committed=1716KB)
>
> -                  Compiler (reserved=213KB, committed=213KB)
>                             (malloc=82KB #331)
>                             (arena=131KB #3)
>
> -                  Internal (reserved=4921KB, committed=4917KB)
>                             (malloc=4885KB #16137)
>                             (mmap: reserved=36KB, committed=32KB)
>
> -                    Symbol (reserved=17518KB, committed=17518KB)
>                             (malloc=14377KB #172159)
>                             (arena=3141KB #1)
>
> -    Native Memory Tracking (reserved=3617KB, committed=3617KB)
>                             (malloc=149KB #2329)
>                             (tracking overhead=3468KB)
>
> -               Arena Chunk (reserved=178KB, committed=178KB)
>                             (malloc=178KB)
>
> [oracle at oel6u7 bin]$ /home/oracle/jdk1.8.0_72/bin/jcmd 18906
> VM.native_memory detail | grep -A 10 'for Class'
> [0x0000000100000000 - 0x0000000140000000] reserved 1048576KB for Class from
>     [0x00007f71950e2ad2] ReservedSpace::initialize(unsigned long, unsigned
> long, bool, char*, unsigned long, bool)+0xc2
>     [0x00007f71950e2d0b] ReservedSpace::ReservedSpace(unsigned long,
> unsigned long, bool, char*, unsigned long)+0x1b
>     [0x00007f7194ec8300]
> Metaspace::allocate_metaspace_compressed_klass_ptrs(char*, unsigned
> char*)+0x40
>     [0x00007f7194eca7df] Metaspace::global_initialize()+0x4cf
>
> [0x0000000100000000 - 0x0000000100c00000] committed 12288KB from
>             [0x00007f71950e2549] VirtualSpace::expand_by(unsigned long,
> bool)+0x199
>             [0x00007f7194ec6766]
> VirtualSpaceList::expand_node_by(VirtualSpaceNode*, unsigned long, unsigned
> long)+0x76
>             [0x00007f7194ec9580] VirtualSpaceList::expand_by(unsigned
> long, unsigned long)+0xf0
>             [0x00007f7194ec9713] VirtualSpaceList::get_new_chunk(unsigned
> long, unsigned long, unsigned long)+0xb3
> ...
>
> [oracle at oel6u7 bin]$ head ../logs/gc.log
> Java HotSpot(TM) 64-Bit Server VM (25.72-b15) for linux-amd64 JRE
> (1.8.0_72-b15), built on Dec 22 2015 22:00:07 by "java_re" with gcc 4.3.0
> 20080428 (Red Hat 4.3.0-8)
> Memory: 4k page, physical 3082784k(506980k free), swap 1257468k(1148276k
> free)
> CommandLine flags: -XX:InitialCodeCacheSize=50331648
> -XX:InitialHeapSize=49324544 -XX:MaxHeapSize=805306368
> -XX:MaxMetaspaceSize=268435456 -XX:MetaspaceSize=268435456
> -XX:NativeMemoryTracking=detail -XX:+PrintGC -XX:+PrintGCTimeStamps
> -XX:ReservedCodeCacheSize=50331648 -XX:+UseCompressedClassPointers
> -XX:+UseCompressedOops -XX:+UseLargePages -XX:+UseLargePagesInMetaspace
>
> --
> Regards
> Timur Akhmadeev
>
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