Java very limited on max number of threads?

Question

We have a small text box with 512Mb of ram. We wanted to see how many threads we can create in Java in this box. To our surprise, we can't create many. Essentially the minimum stack size you can set with -Xss is 64k. Simple math will tell you that 64*7000 will consume 430Mb so we were only able to get it up to around 7000 threads or so and then we encountered this error:

java.lang.OutOfMemoryError: unable to create new native thread. 

Is this the true limit with Java? Per 512Mb of ram we can only squeeze in 7k number of threads or so?

Answer 1

It's not the programming language, it's on the operating system level.

More reading about it, for Windows:

• Does Windows have a limit of 2000 threads per process?
• Pushing the Limits of Windows: Processes and Threads (by Mark Russinovich)

Answer 2

Keep in mind that you will never be able to dedicate 100% of the RAM to running Java threads. Some RAM is used by the OS and other running applications, meaning you will never have the full 512 Mb available.

Answer 3

Try setting the maximum memory allowed -Xmx to a lower value and see whether the thread count can be increased. In a project at work I could allocate around 2,5k threads with -Xmx512m and about 4k threads with -Xmx96m.

The bigger your heap the smaller your thread stack space (at least to my experience).

Answer 4

Once you create your 7k threads, you're not going to have any memory to do anything useful. Perhaps you should have a rethink about the design of your application?

Anyway, isn't 512Mb quite small? Perhaps you could provide a bit morer information about your application or perhaps the domain?

Answer 5

Use asynchronous IO (java nio) and you'll don't need 7k threads to support 7k clients, a few threads for handling io (5?) will be enough.
Take a look at Netty ;)

One thread for each client is a really bad design.

Answer 6

You don't necessarily need one thread per client session. If you look at the way that a J2EE (or JavaEE) server handles multiple connections it uses a mixture of strategies including concurrency, queuing and swapping. Usually you can configure the maximum number of live concurrent instances and idle time-out values at deployment time to tune the performance of your application.