How many threads can a Java VM support

Question

How many threads can a Java VM support  Does this vary by vendor  by operating system  other factors

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I recall hearing a Clojure talk where he got to run one of his apps on some specialized machine at a trade show with thousands of cores (9000?), and it loaded them all. Unfortunately, I can't find the link right now (help?).

Based on that, I think it's safe to say that the hardware and your code are the limiting factors, not the JVM.

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Year 2017    DieLikeADog class   New thread  92459 Exception in thread  main  java lang OutOfMemoryError  unable to create new native thread  i7-7700 16gb ram

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At least on Mac OS X 10 6 32bit  there is a limit  2560  by the operating system  Check this stackoverflow thread

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After playing around with Charlie s DieLikeACode class  it looks like the Java thread stack size is a huge part of how many threads you can create        -Xss        set java thread stack size   For example     java -Xss100k DieLikeADog   But  Java has the Executor interface   I would use that  you will be able to submit thousands of Runnable tasks  and have the Executor process those tasks with a fixed number of threads

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After reading Charlie Martin s post  I was curious about whether the heap size makes any difference in the number of threads you can create  and I was totally dumbfounded by the result   Using JDK 1 6 0 11 on Vista Home Premium SP1  I executed Charlie s test application with different heap sizes  between 2 MB and 1024 MB   For example  to create a 2 MB heap  I d invoke the JVM with the arguments -Xms2m -Xmx2m   Here are my results   2 mb -- gt  5744 threads 4 mb -- gt  5743 threads 8 mb -- gt  5735 threads 12 mb -- gt  5724 threads 16 mb -- gt  5712 threads 24 mb -- gt  5687 threads 32 mb -- gt  5662 threads 48 mb -- gt  5610 threads 64 mb -- gt  5561 threads 96 mb -- gt  5457 threads 128 mb -- gt  5357 threads 192 mb -- gt  5190 threads 256 mb -- gt  5014 threads 384 mb -- gt  4606 threads 512 mb -- gt  4202 threads 768 mb -- gt  3388 threads 1024 mb -- gt  2583 threads   So  yeah  the heap size definitely matters  But the relationship between heap size and maximum thread count is INVERSELY proportional   Which is weird

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You can process any number of threads  there is no limit  I ran the following code while watching a movie and using NetBeans  and it worked properly without halting the machine  I think you can keep even more threads than this program does   class A extends Thread       public void run             System out println                started                           for double i   0 0  i  lt  500000000000000000 0  i                  System gc                System out println Thread currentThread   getName                       System out println                          finished                                             public class Manager       public static void main String   args            for double j   0 0  j  lt  50000000000 0  j                  A a   new A                a start

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The absolute theoretical maximum is generally a process s user address space divided by the thread stack size  though in reality  if all your memory is reserved for thread stacks  you won t have a working program       So under 32-bit Windows  for example  where each process has a user address space of 2GB   giving each thread a 128K stack size  you d expect an absolute maximum of 16384 threads   2 1024 1024   128   In practice  I find I can start up about 13 000 under XP   Then  I think you re essentially into whether  a  you can manage juggling that many threads in your code and not do obviously silly things  such as making them all wait on the same object then calling notifyAll        and  b  whether the operating system can  In principle  the answer to  b  is  yes  if the answer to  a  is also  yes    Incidentally  you can specify the stack size in the constructor of the Thread  you don t need to  and probably shouldn t  mess about with VM parameters for this

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Maximum number of threads depends on following things  Hardware Configuration like microprocessor  RAM  Operating System like whether it is 32-bit or 64-bit Code inside the run method  If code inside the run method is huge then single thread object will have more memory requirement

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Um  lots  There are several parameters here   The specific VM  plus there are usually run-time parameters on the VM as well   That s somewhat driven by the operating system  what support does the underlying OS have for threads and what limitations does it put on them   If the VM actually uses OS-level threads at all  the good old red thread green thread thing  What  quot support quot  means  is another question   If you write a Java program that is just something like    class DieLikeADog            public static void main String   argv                for                      new Thread new SomeRunaable  start                                      and don t complain about little syntax details  I m on my first cup of coffee  then you should certainly expect to get hundreds or thousands of threads running   But creating a Thread is relatively expensive  and scheduler overhead can get intense  it s unclear that you could have those threads do anything useful  Update Okay  couldn t resist   Here s my little test program  with a couple embellishments  public class DieLikeADog       private static Object s   new Object        private static int count   0      public static void main String   argv           for                  new Thread new Runnable                        public void run                            synchronized s                               count    1                              System err println  quot New thread   quot  count                                                     for                                  try                                   Thread sleep 1000                                 catch  Exception e                                   System err println e                                                                                                    start                       On OS X 10 5 6 on Intel  and Java 6 5  see comments   here s what I got  New thread  2547 New thread  2548 New thread  2549 Can t create thread  5 New thread  2550 Exception in thread  main  java lang OutOfMemoryError  unable to create new native thread         at java lang Thread start0 Native Method          at java lang Thread start Thread java 592          at DieLikeADog main DieLikeADog java 6

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I know this question is pretty old but just want to share my findings    My laptop is able to handle program which spawns 25 000 threads and all those threads write some data in MySql database at regular interval of 2 seconds    I ran this program with 10 000 threads for 30 minutes continuously then also my system was stable and I was able to do other normal operations like browsing  opening  closing other programs  etc   With 25 000 threads system slows down but it remains responsive   With 50 000 threads system stopped responding instantly and I had to restart my system manually   My system details are as follows    Processor   Intel core 2 duo 2 13 GHz RAM   4GB OS   Windows 7 Home Premium JDK Version   1 6   Before running I set jvm argument -Xmx2048m   Hope it helps

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Additional information for modern  systemd  linux systems   There are many resources about this of values that may need tweaking  such as How to increase maximum number of JVM threads  Linux 64bit    however a new limit is imposed by way of the systemd  TasksMax  limit which sets pids max on the cgroup   For login sessions the UserTasksMax default is 33  of the kernel limit pids max  usually 12 288  and can be override in  etc systemd logind conf   For services the DefaultTasksMax default is 15  of the kernel limit pids max  usually 4 915    You can override it for the service by setting TasksMax in  systemctl edit  or update DefaultTasksMax in  etc systemd system conf

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This depends on the CPU you re using  on the OS  on what other processes are doing  on what Java release you re using  and other factors   I ve seen a Windows server have   6500 Threads before bringing the machine down   Most of the threads were not doing anything  of course   Once the machine hit around 6500 Threads  in Java   the whole machine started to have problems and become unstable   My experience shows that Java  recent versions  can happily consume as many Threads as the computer itself can host without problems   Of course  you have to have enough RAM and you have to have started Java with enough memory to do everything that the Threads are doing and to have a stack for each Thread   Any machine with a modern CPU  most recent couple generations of AMD or Intel  and with 1 - 2 Gig of memory  depending on OS  can easily support a JVM with thousands of Threads   If you need a more specific answer than this  your best bet is to profile

[java] How many threads can a Java VM support?

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