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.akhmad...@gmail.com <mailto:timur.akhmad...@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@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@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@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@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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