What is the largest possible heap size with a 64-bit JVM

Question

The theoretical maximum heap value that can be set with -Xmx in a 32-bit system is of course 2 32 bytes  but typically  see  Understanding max JVM heap size - 32bit vs 64bit  one cannot use all 4GB   For a 64-bit JVM running in a 64-bit OS on a 64-bit machine  is there any limit besides the theoretical limit of 2 64 bytes or 16 exabytes   I know that for various reasons  mostly garbage collection   excessively large heaps might not be wise  but in light of reading about servers with terrabytes of RAM  I m wondering what is possible

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I tried -Xmx32255M is accepted by vmargs for compressed oops

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Windows imposes a memory limit per process  you can see what it is for each version here  See    User-mode virtual address space for each 64-bit process  With IMAGE FILE LARGE ADDRESS AWARE set  default   x64   8 TB Intel IPF   7 TB 2 GB with IMAGE FILE LARGE ADDRESS AWARE cleared

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For a 64-bit JVM running in a 64-bit OS on a 64-bit machine  is there any limit besides the theoretical limit of 2 64 bytes or 16 exabytes    You also have to take hardware limits into account  While pointers may be 64bit current CPUs can only address a less than 2 64 bytes worth of virtual memory   With uncompressed pointers the hotspot JVM needs a continuous chunk of virtual address space for its heap  So the second hurdle after hardware is the operating system providing such a large chunk  not all OSes support this   And the third one is practicality  Even if you can have that much virtual memory it does not mean the CPUs support that much physical memory  and without physical memory you will end up swapping  which will adversely affect the performance of the JVM because the GCs generally have to touch a large fraction of the heap   As other answers mention compressed oops  By bumping the object alignment higher than 8 bytes the limits with compressed oops can be increased beyond 32GB

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If you want to use 32-bit references  your heap is limited to 32 GB   However  if you are willing to use 64-bit references  the size is likely to be limited by your OS  just as it is with 32-bit JVM  e g  on Windows 32-bit this is 1 2 to 1 5 GB   Note  you will want your JVM heap to fit into main memory  ideally inside one NUMA region   That s about 1 TB on the bigger machines   If your JVM spans NUMA regions the memory access and the GC in particular will take much longer  If your JVM heap start swapping it might take hours to GC  or even make your machine unusable as it thrashes the swap drive   Note  You can access large direct memory and memory mapped sizes even if you use 32-bit references in your heap  i e  use well above 32 GB   Compressed oops in the Hotspot JVM     Compressed oops represent managed pointers  in many but not all places in the JVM  as 32-bit values which must be scaled by a factor of 8 and added to a 64-bit base address to find the object they refer to  This allows applications to address up to four billion objects  not bytes   or a heap size of up to about 32Gb  At the same time  data structure compactness is competitive with ILP32 mode

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The answer clearly depends on the JVM implementation  Azul claim that their JVM     can scale     to more than a 1 2 Terabyte of memory   By  can scale  they appear to mean  runs wells   as opposed to  runs at all

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In theory everything is possible but reality you find the numbers much lower than you might expect  I have been trying to address huge spaces on servers often and found that even though a server can have huge amounts of memory it surprised me that most software actually never can address it in real scenario s simply because the cpu s are not fast enough to really address them  Why would you say right      Timings thats the endless downfall of every enormous machine which i have worked on  So i would advise to not go overboard by addressing huge amounts just because you can  but use what you think could be used  Actual values are often much lower than what you expected  Ofcourse non of us really uses hp 9000 systems at home and most of you actually ever will go near the capacity of your home system ever   For instance most users do not have more than 16 Gb of memory in their system  Ofcourse some of the casual gamers use work stations for a game once a month but i bet that is a very small percentage   So coming down to earth means i would address on a 8 Gb 64 bit system not much more than 512 mb for heapspace or if you go overboard try 1 Gb  I am pretty sure its even with these numbers pure overkill  I have constant monitored the memory usage during gaming to see if the addressing would make any difference but did not notice any difference at all when i addressed much lower values or larger ones  Even on the server workstations there was no visible change in performance no matter how large i set the values  That does not say some jave users might be able to make use of more space addressed  but this far i have not seen any of the applications needing so much ever  Ofcourse i assume that their would be a small difference in performance if java instances would run out of enough heapspace to work with  This far i have not found any of it at all  however lack of real installed memory showed instant drops of performance if you set too much heapspace  When you have a 4 Gb system you run quickly out of heapspace and then you will see some errors and slowdowns because people address too much space which actually is not free in the system so the os starts to address drive space to make up for the shortage hence it starts to swap

[java] What is the largest possible heap size with a 64-bit JVM?

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