Metaspace - memory allocation
Jon Masamitsu
jon.masamitsu at oracle.com
Thu Feb 4 19:02:09 UTC 2016
On 02/04/2016 06:41 AM, Thomas Stüfe wrote:
> 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.
Thomas,
No disagreement from me. I think you described it
correctly. Note that if you turn off UseCompressedClassPointers,
there will not be a separate space for the class metadata.
Jon
>
> Kind Regards, Thomas
>
> On Thu, Feb 4, 2016 at 2:13 PM, Timur Akhmadeev
> <timur.akhmadeev at gmail.com <mailto: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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