Maximum Java heap size of a 32-bit JVM on a 64-bit OS

Question

The question is not about the maximum heap size on a 32-bit OS  given that 32-bit OSes have a maximum addressable memory size of 4GB  and that the JVM s max heap size depends on how much contiguous free memory can be reserved   I m more interested in knowing the maximum  both theoretical and practically achievable  heap size for a 32-bit JVM running in a 64-bit OS  Basically  I m looking at answers similar to the figures in a related question on SO   As to why a 32-bit JVM is used instead of a 64-bit one  the reason is not technical but rather administrative bureaucratic - it is probably too late to install a 64-bit JVM in the production environment

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The limitations of a 32-bit JVM on a 64-bit OS will be exactly the same as the limitations of a 32-bit JVM on a 32-bit OS. After all, the 32-bit JVM will be running In a 32-bit virtual machine (in the virtualization sense) so it won't know that it's running on a 64-bit OS/machine.

The one advantage to running a 32-bit JVM on a 64-bit OS versus a 32-bit OS is that you can have more physical memory, and therefore will encounter swapping/paging less frequently. This advantage is only really fully realized when you have multiple processes, however.

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Should be a lot better  For a 32-bit JVM running on a 64-bit host  I imagine what s left over for the heap will be whatever unfragmented virtual space is available after the JVM  it s own DLL s  and any OS 32-bit compatibility stuff has been loaded  As a wild guess I would think 3GB should be possible  but how much better that is depends on how well you are doing in 32-bit-host-land   Also  even if you could make a giant 3GB heap  you might not want to  as this will cause GC pauses to become potentially troublesome  Some people just run more JVM s to use the extra memory rather than one giant one  I imagine they are tuning the JVM s right now to work better with giant heaps   It s a little hard to know exactly how much better you can do  I guess your 32-bit situation can be easily determined by experiment  It s certainly hard to predict abstractly  as a lot of things factor into it  particularly because the virtual space available on 32-bit hosts is rather constrained   The heap does need to exist in contiguous virtual memory  so fragmentation of the address space for dll s and internal use of the address space by the OS kernel will determine the range of possible allocations   The OS will be using some of the address space for mapping HW devices and it s own dynamic allocations  While this memory is not mapped into the java process address space  the OS kernel can t access it and your address space at the same time  so it will limit the size of any program s virtual space   Loading DLL s depends on the implementation and the release of the JVM  Loading the OS kernel depends on a huge number of things  the release  the HW  how many things it has mapped so far since the last reboot  who knows     In summary  I bet you get 1-2 GB in 32-bit-land  and about 3 in 64-bit  so an overall improvement of about 2x

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From 4 1 2 Heap Sizing       For a 32-bit process model  the maximum virtual address size of the   process is typically 4 GB  though some operating systems limit this to   2 GB or 3 GB  The maximum heap size is typically -Xmx3800m  1600m  for   2 GB limits   though the actual limitation is application dependent    For 64-bit process models  the maximum is essentially unlimited     Found a pretty good answer here  Java maximum memory on Windows XP

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Theoretical 4gb  but in practice  for IBM JVM    Win 2k8 64  IBM Websphere Application Server 8 5 5 32bit  C  IBM WebSphere AppServer bin gt managesdk bat -listAvailable -verbose CWSDK1003I            SDK  CWSDK1005I      SDK  1 6 32 - com ibm websphere sdk version 1 6 32 1 6 - com ibm websphere sdk bits 1 6 32 32 - com ibm websphere sdk location 1 6 32   WAS INSTALL ROOT  java - com ibm websphere sdk platform 1 6 32 windows - com ibm websphere sdk architecture 1 6 32 x86 32 - com ibm websphere sdk nativeLibPath 1 6 32   WAS INSTALL ROOT  lib native win  x86 32  CWSDK1001I         managesdk                    C  IBM WebSphere AppServer java bin gt java -Xmx2036 MaxMemory JVMJ9GC017E -Xmx                                     1 M      JVMJ9VM015W                                     j9gc26 2                                Could not create the Java virtual machine  C  IBM WebSphere AppServer java bin gt java -Xmx2047M MaxMemory Total Memory  4194304  4 0 MiB  Max Memory    2146435072  2047 0 MiB  Free Memory   3064536  2 9225692749023438 MiB  C  IBM WebSphere AppServer java bin gt java -Xmx2048M MaxMemory JVMJ9VM015W                                     j9gc26 2                                                 2G Could not create the Java virtual machine   RHEL 6 4 64  IBM Websphere Application Server 8 5 5 32bit    bin   java  -Xmx3791M MaxMemory Total Memory  4194304  4 0 MiB  Max Memory    3975151616  3791 0 MiB  Free Memory   3232992  3 083221435546875 MiB   root nagios1p bin     java  -Xmx3793M MaxMemory Total Memory  4194304  4 0 MiB  Max Memory    3977248768  3793 0 MiB  Free Memory   3232992  3 083221435546875 MiB   bin    opt IBM WebSphere AppServer bin managesdk sh -listAvailable -verbose CWSDK1003I  Available SDKs   CWSDK1005I  SDK name  1 6 32  - com ibm websphere sdk version 1 6 32 1 6  - com ibm websphere sdk bits 1 6 32 32  - com ibm websphere sdk location 1 6 32   WAS INSTALL ROOT  java  - com ibm websphere sdk platform 1 6 32 linux  - com ibm websphere sdk architecture 1 6 32 x86 32  -com ibm websphere sdk nativeLibPath 1 6 32   WAS INSTALL ROOT  lib native linux x86 32  CWSDK1001I  Successfully performed the requested managesdk task

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The limitation also comes from the fact that for a 32 bit VM  the heap itself has to start at address zero if you want all those 4GB    Think about it  if you want to reference something via   0000    0001   i e   a reference that has this particular bits representation  it means you are trying to access the very first memory from the heap  For that to be possible  the heap has to start at address zero  But that never happens  it starts at some offset from zero                                  heap start      heap end         -- gt   this can t be referenced   lt --   Because heap never starts from address zero in an OS  there are quite a few bits that are never used from a 32 bits reference  and as such the heap that can be referenced is lower

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32-bit JVMs which expect to have a single large chunk of memory and use raw pointers cannot use more than 4 Gb  since that is the 32 bit limit which also applies to pointers    This includes Sun and - I m pretty sure - also IBM implementations   I do not know if e g  JRockit or others have a large memory option with their 32-bit implementations   If you expect to be hitting this limit you should strongly consider starting a parallel track validating a 64-bit JVM for your production environment so you have that ready for when the 32-bit environment breaks down   Otherwise you will have to do that work under pressure  which is never nice     Edit 2014-05-15   Oracle FAQ   The maximum theoretical heap limit for the 32-bit JVM is 4G  Due to various additional constraints such as available swap  kernel address space usage  memory fragmentation  and VM overhead  in practice the limit can be much lower  On most modern 32-bit Windows systems the maximum heap size will range from 1 4G to 1 6G  On 32-bit Solaris kernels the address space is limited to 2G  On 64-bit operating systems running the 32-bit VM  the max heap size can be higher  approaching 4G on many Solaris systems    http   www oracle com technetwork java hotspotfaq-138619 html gc heap 32bit

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You can ask the Java Runtime   public class MaxMemory       public static void main String   args            Runtime rt   Runtime getRuntime            long totalMem   rt totalMemory            long maxMem   rt maxMemory            long freeMem   rt freeMemory            double megs   1048576 0           System out println   Total Memory      totalMem           totalMem megs      MiB             System out println   Max Memory        maxMem           maxMem megs      MiB             System out println   Free Memory       freeMem           freeMem megs      MiB               This will report the  Max Memory  based upon default heap allocation  So you still would need to play with -Xmx  on HotSpot    I found that running on Windows 7 Enterprise 64-bit  my 32-bit HotSpot JVM can allocate up to 1577MiB     C scratch   java -Xmx1600M MaxMemory Error occurred during initialization of VM Could not reserve enough space for object heap Could not create the Java virtual machine   C scratch   java -Xmx1590M MaxMemory Total Memory  2031616  1 9375 MiB  Max Memory    1654456320  1577 8125 MiB  Free Memory   1840872  1 75559234619 MiB   C scratch     Whereas with a 64-bit JVM on the same OS  of course it s much higher  about 3TiB     C scratch   java -Xmx3560G MaxMemory Error occurred during initialization of VM Could not reserve enough space for object heap  C scratch   java -Xmx3550G MaxMemory Total Memory  94240768  89 875 MiB  Max Memory    3388252028928  3184151 84297 MiB  Free Memory   93747752  89 4048233032 MiB   C scratch     As others have already mentioned  it depends on the OS    For 32-bit Windows   it ll be  lt 2GB   Windows internals book says 2GB for user processes  For 32-bit BSD   Linux    lt 3GB        from the Devil Book  For 32-bit MacOS X   lt 4GB             from Mac OS X internals book  Not sure about 32-bit Solaris  try the above code and let us know    For a 64-bit host OS  if the JVM is 32-bit  it ll still depend  most likely like above as demonstrated   -- UPDATE 20110905   I just wanted to point out some other observations   details    The hardware that I ran this on was 64-bit with 6GB of actual RAM installed   The operating system was Windows 7 Enterprise  64-bit The actual amount of Runtime MaxMemory that can be allocated also depends on the operating system s working set   I once ran this while I also had VirtualBox running and found I could not successfully start the HotSpot JVM with -Xmx1590M and had to go smaller   This also implies that you may get more than 1590M depending upon your working set size at the time  though I still maintain it ll be under 2GiB for 32-bit because of Windows  design

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I was having the same problems with the JVM that App Inventor for Android Blocks Editor uses  It sets the heap at 925m max  This is not enough but I couldn t set it more than about 1200m  depending on various random factors on my machine   I downloaded Nightly  the beta 64-bit browser from Firefox  and also JAVA 7 64 bit version   I haven t yet found my new heap limit  but I just opened a JVM with a heap size of 5900m  No problem   I am running Win 7 64 bit Ultimate on a machine with 24gb RAM

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This is heavy tunning  but you can get a 3gb heap   http   www microsofttranslator com bv aspx from  amp to en amp a http   forall ru-board com egor23 online FAQ Virtual Memory Limits Virtual Memory html

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Here is some testing under Solaris and Linux 64-bit  Solaris 10 - SPARC - T5220 machine with 32 GB RAM  and about 9 GB free     java -XX PermSize 128M -XX MaxPermSize 256M -Xms512m -Xmx3750m MaxMemory Error occurred during initialization of VM Could not reserve space for ObjectStartArray   java -XX PermSize 128M -XX MaxPermSize 256M -Xms512m -Xmx3700m MaxMemory Total Memory  518520832  494 5 MiB  Max Memory    3451912192  3292 0 MiB  Free Memory   515815488  491 91998291015625 MiB  Current PID is  28274 Waiting for user to press Enter to finish        java -version java version  1 6 0 30  Java TM  SE Runtime Environment  build 1 6 0 30-b12  Java HotSpot TM  Server VM  build 20 5-b03  mixed mode     which java  usr bin java   file  usr bin java  usr bin java  ELF 32-bit MSB executable SPARC Version 1  dynamically linked  not stripped  no debugging information available    prstat -p 28274    PID USERNAME  SIZE   RSS STATE  PRI NICE      TIME  CPU PROCESS NLWP 28274 user1     670M   32M sleep   59    0   0 00 00 0 0  java 35   BTW  Apparently Java does not allocate much actual memory with the startup  It seemed to take only about 100 MB per instance started  I started 10   Solaris 10 - x86 - VMWare VM with 8 GB RAM  about 3 GB free    The 3 GB free RAM is not really true  There is a large chunk of RAM that ZFS caches use  but I don t have root access to check how much exactly    java -XX PermSize 128M -XX MaxPermSize 256M -Xms512m -Xmx3650m MaxMemory Error occurred during initialization of VM Could not reserve enough space for object heap Could not create the Java virtual machine     java -XX PermSize 128M -XX MaxPermSize 256M -Xms512m -Xmx3600m MaxMemory Total Memory  516423680  492 5 MiB  Max Memory    3355443200  3200 0 MiB  Free Memory   513718336  489 91998291015625 MiB  Current PID is  26841 Waiting for user to press Enter to finish        java -version java version  1 6 0 41  Java TM  SE Runtime Environment  build 1 6 0 41-b02  Java HotSpot TM  Server VM  build 20 14-b01  mixed mode     which java  usr bin java    file  usr bin java  usr bin java   ELF 32-bit LSB executable 80386 Version 1  FPU   dynamically linked  not stripped  no debugging information available    prstat -p 26841    PID USERNAME  SIZE   RSS STATE  PRI NICE      TIME  CPU PROCESS NLWP 26841 user1     665M   22M sleep   59    0   0 00 00 0 0  java 12   RedHat 5 5 - x86 - VMWare VM with 4 GB RAM  about 3 8 GB used - 200 MB in buffers and 3 1 GB in caches  so about 3 GB free     alias java   HOME jre jre1 6 0 34 bin java     java -XX PermSize 128M -XX MaxPermSize 256M -Xms512m -Xmx3500m MaxMemory Error occurred during initialization of VM Could not reserve enough space for object heap Could not create the Java virtual machine     java -XX PermSize 128M -XX MaxPermSize 256M -Xms512m -Xmx3450m MaxMemory Total Memory  514523136  490 6875 MiB  Max Memory    3215654912  3066 6875 MiB  Free Memory   511838768  488 1274871826172 MiB  Current PID is  21879 Waiting for user to press Enter to finish        java -version java version  1 6 0 34  Java TM  SE Runtime Environment  build 1 6 0 34-b04  Java HotSpot TM  Server VM  build 20 9-b04  mixed mode     file  HOME jre jre1 6 0 34 bin java  home user1 jre jre1 6 0 34 bin java  ELF 32-bit LSB executable  Intel 80386  version 1  SYSV   for GNU Linux 2 2 5  dynamically linked  uses shared libs   for GNU Linux 2 2 5  not stripped    cat  proc 21879 status   grep  Vm VmPeak   3882796 kB VmSize   3882796 kB VmLck          0 kB VmHWM      12520 kB VmRSS      12520 kB VmData   3867424 kB VmStk         88 kB VmExe         40 kB VmLib      14804 kB VmPTE         96 kB   Same machine using JRE 7    alias java   HOME jre jre1 7 0 21 bin java     java -XX PermSize 128M -XX MaxPermSize 256M -Xms512m -Xmx3500m MaxMemory Error occurred during initialization of VM Could not reserve enough space for object heap Error  Could not create the Java Virtual Machine  Error  A fatal exception has occurred  Program will exit     java -XX PermSize 128M -XX MaxPermSize 256M -Xms512m -Xmx3450m MaxMemory Total Memory  514523136  490 6875 MiB  Max Memory    3215654912  3066 6875 MiB  Free Memory   511838672  488 1273956298828 MiB  Current PID is  23026 Waiting for user to press Enter to finish        java -version java version  1 7 0 21  Java TM  SE Runtime Environment  build 1 7 0 21-b11  Java HotSpot TM  Server VM  build 23 21-b01  mixed mode     file  HOME jre jre1 7 0 21 bin java  home user1 jre jre1 7 0 21 bin java  ELF 32-bit LSB executable  Intel 80386  version 1  SYSV   for GNU Linux 2 6 9  dynamically linked  uses shared libs   for GNU Linux 2 6 9  not stripped    cat  proc 23026 status   grep  Vm VmPeak   4040288 kB VmSize   4040288 kB VmLck          0 kB VmHWM      13468 kB VmRSS      13468 kB VmData   4024800 kB VmStk         88 kB VmExe          4 kB VmLib      10044 kB VmPTE        112 kB

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As to why a 32-bit JVM is used instead of a 64-bit one  the reason is not technical but rather administrative bureaucratic       When I was working for BEA  we found that the average application actually ran slower in a 64-bit JVM  then it did when running in a 32-bit JVM   In some cases  the performance hit was as high as 25  slower   So  unless your application really needs all that extra memory  you were better off setting up more 32-bit servers   As I recall  the three most common technical justifications for using a 64-bit that BEA professional services personnel ran into were    The application was manipulating multiple massive images   The application was doing massive number crunching   The application had a memory leak  the customer was the prime on a government contract  and they didn t want to take the time and the expense of tracking down the memory leak   Using a massive memory heap would increase the MTBF and the prime would still get paid

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The JROCKIT JVM currently owned by Oracle supports non-contiguous heap usage  thus allowing the 32 bit JVM to access more then 3 8 GB of memory when the JVM is running on a 64 bit windows OS   2 8 GB when running on a 32 bit OS    http   blogs oracle com jrockit entry how to get almost 3 gb heap on windows  The JVM can be freely downloaded  registration required  at  http   www oracle com technetwork middleware jrockit downloads index html

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I have tried setting the heap size upto 2200M on 32bit Linux machine and JVM worked fine  The JVM didnt start when I set it to 2300M

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We recently had some experience with this  We have ported from Solaris  x86-64 Version 5 10  to Linux  RedHat x86-64  recently and have realized that we have less memory available for a 32 bit JVM process on Linux than Solaris   For Solaris this almost comes around to 4GB  http   www oracle com technetwork java hotspotfaq-138619 html gc heap 32bit     We ran our app with -Xms2560m -Xmx2560m -XX MaxPermSize 512m -XX PermSize 512m with no issues on Solaris for past couple of years  Tried to move it to linux and we had issues with random out of memory errors on start up  We could only get it to consistently start up on -Xms2300 -Xmx2300  Then we were advised of this by support       A 32 bit process on Linux has a   maximum addressable  address space of   3gb  3072mb  whereas on Solaris it is   the full 4gb  4096mb

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You don t specify which OS   Under Windows  for my application - a long running risk management application  we observed that we could go no further than 1280MB on Windows 32bit  I doubt that running a 32bit JVM under 64bit would make any difference   We ported the app to Linux and we are running a 32bit JVM on 64bit hardware and have had a 2 2GB VM running pretty easily   The biggest problem you may have is GC depending on what you are using memory for

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one more point here for hotspot 32-bit JVM - the native heap capacity   4 Gig     Java Heap - PermGen   It can get especially tricky for 32-bit JVM since the Java Heap and native Heap are in a race  The bigger your Java Heap  the smaller the native Heap  Attempting to setup a large Heap for a 32-bit VM e g  2 5 GB  increases risk of native OutOfMemoryError depending of your application s  footprint  number of Threads etc

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On Solaris the limit has been about 3 5 GB since Solaris 2 5   about 10 years ago

[java] Maximum Java heap size of a 32-bit JVM on a 64-bit OS

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