Metaspace - memory allocation

[Timur Akhmadeev]

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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