Understanding how memory is structured on the JVM ( Hotspot/OpenJDK) becomes important when you really want to troubleshoot your production environment. In this short talk I will delve into the basics of the topic and demystify some of the misconceptions. Talk given at Java.IL user group

1. Haim Yadid
JVM Memory Structure

2. About Me
• Developing software since 1984
• Basic -> Pascal -> C -> C++ -> Java -> Kotlin
• Developer , architect, group manager
• Independent performance expert for 8 years
• Group manager in
• Java.IL

3. • Founded in the Beginning of 2016
• Disrupt the small businesses insurance field
• Providing online experience which is simple, fast and
transparent
• HQ@Palo Alto / RnD@Kfar Saba (Israel) / Austin-TX
• We started to write real code on May 2016

4. Heap
• -Xms
• -Xmx
Xms Xmx
Max
Init

5. Physical vs Process Memory
0 2^32/2^48Process Address Space
Physical memory0
Page
Swap
Page
Page
Page
Reserved/Virtual
Page
Page
Page
Page
Committed - activeCommitted - inactive

6. Heap structure
• JMX: java.lang.Memory::HeapMemoryUsage
• Continuous address space
Used VirtualCommited
Max
Init

7. JVisualVM Heap

8. Not Only Heap
• Heap
• Code cache
• Meta spaces (Native)
• Native memory
Heap
Code
Cache
0 2^32/2^48
Native
Native
Native
Meta
Meta

9. Meta Space
• Classes byte code
• Part of native memory.
• Need not be continuous in memory or pre reserved.
• Limit by -XX:MaxMetaspaceSize=

10. JVisualVM
JMX : java.lang.MemoryPool.Metaspace::usage

11. Code cache
• JIT’ed hotspot methods (machine code)
• -XX:InitialCodeCacheSize=2555904 (2.5MB)
• -XX:ReservedCodeCacheSize=251658240 (240MB)
• codecache flushing -XX:+UseCodeCacheFlushing

12. JVisualVM
JMX : java.lang.MemoryPool.Code Cache::usage

13. Native Memory
• Thread stacks (1MB for 64 bit OS)
• Native allocations
• Direct byte buffer java.nio.DirectByteBuffer
• Limit by : -XX:MaxDirectMemorySize=
• unsafe.allocateMemory
• JNI malloc()

14. Calculating it All
• Heap Max = 1 GB
• 100 threads = 100 MB
• Code cache 256 MB
• Meta space 100 MB

15. Native Memory Tracking
java -XX:+UnlockDiagnosticVMOptions
-XX:NativeMemoryTracking=summary
-XX:+PrintNMTStatistics

16. jcmd <pid> VM.native_memory summary
Native Memory Tracking:
Total: reserved=5808103KB, committed=1987515KB
- Java Heap (reserved=4194304KB, committed=1647104KB)
(mmap: reserved=4194304KB, committed=1647104KB)
- Class (reserved=1118666KB, committed=77258KB)
(classes #12468)
(malloc=10698KB #17369)
(mmap: reserved=1107968KB, committed=66560KB)
- Thread (reserved=44541KB, committed=44541KB)
(thread #45)
(stack: reserved=44192KB, committed=44192KB)
(malloc=137KB #230)
(arena=212KB #89)
…..
-

17. Docker
-XX:+UnlockExperimentalVMOptions
-XX:+UseCGroupMemoryLimitForHeap
-XX:MaxRAMFraction=

18. Lorem Ipsum Dolor
Questions?