Java Memory Management on HPUX Laksh Venkatasubramanian HP Java Labs © 2003 Hewlett-Packard Development Company, L.P.
HP-UX Virtual Memory Layout HP-UX 32 Bit Process • Four 1 GB Quadrants A) SHARE_MAGIC B) EXEC_MAGIC • 0x00000000 A Shared Text 0x40000000 0x00000000Text B DATA (private) 0x40000000 DATA (private) Private Memory Mapped Files Private Memory Mapped Files STACK 0x80000000 Shared Objects (Libraries, Memory Mapped Files) 0xc0000000 Shared Objects (Libraries, Memory Mapped Files) 2/8/2005 (private) (c) Copyright Hewlett Packard Company, 2005 STACK 0x80000000 Shared Objects (Libraries, Memory Mappe
Type of Executables on HP-UX There are 3 magic numbers that can be used for a 32-bit executable (11.00 and greater). /usr/bin/chatr labels the following type of executables in output SHARE_MAGIC: • EXEC_MAGIC: • SHMEM_MAGIC: • shared executable normal executable normal SHMEM_MAGIC executable For 64 bit (11.
Type of Executables on HP-UX • SHARE_MAGIC is the default on 11.0. SHARE_MAGIC is also called DEMAND_MAGIC. With SHARE_MAGIC, quadrant 1 is used for program text, quadrant 2 is used for program data, and quadrants 3 and 4 are for shared items. • EXEC_MAGIC allows a greater process data space by allowing text and data to share quadrant 1. Quadrant 2 is still solely used for data, and quadrants 3 and 4 are also the same as with SHARE_MAGIC executables.
EXEC_MAGIC vs SHARED_MAGIC EXEC_MAGIC Quad 3 Private (q3p) Quad 4 Private (q4p) SHARED_MAGIC Quadrant 1 0x000000000x3FFFFFFF Text starts at the beginning of this space and data starts immediately after the end of the text. Same Same Text only and read only.
Enabling 3rd and 4th quadrants for private data PA-RISC • ‘chatr +q3p enable ’ - an extra 1Gb of private data is made available to a process (Both SHARED_MAGIC and EXEC_MAGIC program can have quadrant 3 and quadrant 4 private). You cannot access shared objects available to other programs in their quadrant 3 when you enable this option. • ‘chatr +q4p enable ’ - this changes quadrants 3 and 4 to be private.
Patches for q3p q4p functionality HP-UX 11.0 PA-RISC Required Patches: PHKL_27282, PHKL_23409, PHKL_28766, PHKL_26136 • 11.0 supports only q3p. It does not support q4p functionality. • HP-UX 11i (11.11) PA-RISC Required Patch: PHKL_28428 (or its superseded patch) • 11.11 supports both q3p and q4p. • HP-UX 11i v1.5 (11.22) ITANIUM • Does not support q3p, q4p functionality HP-UX 11i v2 (11.
Kernel tunables • maxdsiz, maxdsiz_64bit Controls the size of the DATA region. We can call this the Cheap to differentiate this from the JAVA-heap. sbrk(),malloc(), etc. allocate memory in this region. • maxssiz, maxssiz_64bit Controls the size of the primordial thread (main thread) stack. By default, the JVM restricts the size of this stack to 2MB. • maxtsiz, maxtsiz_64bit Controls the size of the TEXT region. This contains the executable.
Kernel tunables • Even though maxdsiz may be set to a large value, the actual available heap space (DATA) might be much lower because the memory mapped (mmap) segments that are mapped private, STACK, TEXT (EXEC_MAGIC case), Java heap, Java threads, etc. also share this address space. • Similarly, even though maxtsiz might be a large value, it consumes only as much physical/virtual memory as the executable requires. • On the contrary, maxssiz consumes as much virtual space as the value it is set to.
Kernel tunables SWAP • swapinfo –mt (displays swap space usage on the system) Mb TYPE AVAIL dev 4096 reserve memory 4089 total 8185 Mb USED 0 266 1313 1579 Mb FREE 4096 -266 2776 6606 PCT START/ Mb USED LIMIT RESERVE PRI NAME 0% 0 - 1 /dev/vg00/lvol2 32% 19% - 0 - Swap is reserved at the time virtual memory is allocated for a process. But when the lazy-swap option is enabled, swap is allocated at the time of actual use of memory.
Glance Memory Regions (/opt/perf/bin/gpm) Main Thread Stack Java Heap - Permanent Java Heap - Old DATA = C-heap Code Cache Java Heap - New JVM Runtime Compiler Threads Java Thread 2/8/2005 (c) Copyright Hewlett Packard Company, 2005 page 11
Glance Memory Regions • RSS (Resident Set Size) - The size (in KB unless otherwise indicated) of the resident memory occupied by a memory region • VSS (Virtual Set Size) - The size (in KB unless otherwise indicated) of the virtual memory occupied by a memory region 2/8/2005 (c) Copyright Hewlett Packard Company, 2005 page 12
Java Memory Regions • The JAVA threads are private mmap segments. The default size for this mmap is 512KB(32bit), 1MB(64bit). • JVM CodeCache (holds compiled JAVA methods) is a private mmap segment. The default size is 32MB. • The JAVA heap is a private mmap (Use -XheapInitialSizes to determine sizes of different generations) segment. The three regions in HotSpot JVM heap (new, old and permanent) are allocated as three different mmap regions in 32bit mode in 1.3.1 or greater JVMs.
java -XheapInitialSizes Defaults when no options are specifiedNewRatio: 3 SurvivorRatio: 8 MaxTenuringThreshold: 32 Survivor size: 589824 Eden size: 5177344 New Size reserved: 22347776 initial: 6356992 Old Size reserved: 44761088 initial: 12779520 Perm Size reserved: 67108864 initial: 1048576 New size will default to around 1/3rd the total heap size if –Xmn is not specified. –Xmn is an alias for –XX:NewSize. If this value is higher than MaxNewSize, MaxNewSize will be set to this value as well.
Large Heap Size with 32-bit Java For Java invoked from the command line, Java will automatically choose an appropriate executable. PA-RISC • For heaps less than 1500MB, the executable is ‘java’ (EXEC_MAGIC executable). For heaps greater than or equal to 1500MB, and less than 2400MB the executable is ‘java_q3p’ (HP-UX 11.00 or greater). • For heaps of 2400MB to 3800MB, the executable is ‘java_q4p’ (HP-UX 11.11 or greater). • ITANIUM • For heaps of 1500MB to 3500MB, the executable is `java_q4p’ (HP-UX 11.
Large Heap Size with 32-bit Java HP-UX 11.11 (PA-RISC) • Because of segmentation in the HP-UX virtual address space, when the Java heap is larger than 3000MB, either new space (Xmn) or old space (-mx minus -Xmn) must be approximately 850MB or less (applicable to 11.11 only). HP-UX 11.00 or greater • You do not need to directly invoke any of the q3p or q4p programs. Just invoke 'java' as usual, and the appropriate program will be run for you.
Components in a JAVA program Virtual Machine is written in C/C++ • JAVA code • JAVA code calling native methods • Native code calling into JAVA code • 2/8/2005 (c) Copyright Hewlett Packard Company, 2005 page 17
Memory Allocation JAVA heap All objects that are created with the ‘new’ keyword in JAVA reside here.
Java Objects • Necessary to make a distinction between live objects and reachable objects Reachable objects- If we can reach an object from the root set through any number of intermediate references, it is termed reachable Live objects- These are reachable objects that are currently being used by the program 2/8/2005 (c) Copyright Hewlett Packard Company, 2005 page 19
Java Objects When JNI references are not cleaned up properly, they could prevent the collection of some unwanted JAVA objects • All objects that are reachable may not be live - objects that are being referenced by some long living objects.
Symptoms of Process Memory Growth Java Heap Object Retention: – Unaccountable growth of the Java Heap • C Heap Memory Leak: – Constantly increasing DATA RSS and VSS – System running out of swap space – Programs failing with out of memory (ENOMEM) errors • 2/8/2005 (c) Copyright Hewlett Packard Company, 2005 page 21
Reasons for Out of Memory Errors Virtual address space limitations • Insufficient java heap • Low values for kernel parameters max_thread_proc Number of threads per process nkthread Total number of threads maxdsiz Data region size nfiles Total number of open files maxfiles Soft limit for number of open files per process maxfiles_lim hard maximum number of file descriptors per process • 2/8/2005 (c) Copyright Hewlett Packard Company, 2005 page 22
Virtual Address Space Usage: Example 1 A) maxtsiz – 1GB (Upper limit for TEXT region) B) maxdsiz – 1GB (Upper limit for DATA region) Address space is reserved for TEXT and DATA in incremental amounts as needed. C) maxssiz – 400MB (Upper limit for STACK region, reserved upfront) D) Java heap - -Xms1GB –Xmx1GB (Perm gen- 64MB default. Not included in mx value.
Virtual Address Space Usage: Example 2 A) maxtsiz – 1GB B) maxdsiz – 1GB C) maxssiz – 400MB D) Java heap - -Xms500m –Xmx1500m (will invoke java_q3p) • • • New Size reserved: 524288000 Old Size reserved: 1048576000 Perm Size reserved: 67108864 initial: 174718976 initial: 349569024 initial: 1048576 E) JVM Code Cache – 32MB F) 300 threads in the application (300 * 512KB = 150MB) Space available for the Java thread stacks Approximate (only significant, greater than 5MB, regions shown in calculation) 3 GB –
OutOfMemoryError: Example 3 Throwable: java.lang.OutOfMemoryError: unable to create new native thread java.lang.OutOfMemoryError: unable to create new native thread at java.lang.Thread.start(Native Method) CHECK • Whether there is enough space for private mmaps for thread stacks. • The number of threads in glance/gpm and see whether max_thread_proc and nkthread are set appropriately.
