Memory settings with thousands of threads

Question

I\'m playing around with the JVM (Oracle 1.7 64 bit) on a Linux box (AMD 6 Core, 16 GB RAM) to see how the number of threads in an application affects performance. I\'m hoping

Answer 1

I get a java.lang.OutOfMemoryError: Unable to create new native thread with more than about 31,000 jobs. I have tried setting -Xmx6000M which doesn't help. I tried playing with -Xss but that doesn't help either.

The -Xmx setting won't help because thread stacks are not allocated from the heap.

What is happening is that the JVM is asking the OS for a memory segment (outside of the heap!) to hold the stack, and the OS is refusing the request. The most likely reasons for this are a ulimit or an OS memory resource issue:

• The "data seg size" ulimit, is unlimited, so that shouldn't be the problem.

• So that leaves memory resources. 30,000 threads at 1Mb a time is ~30Gb which is a lot more physical memory than you have. My guess is that there is enough swap space for 30Gb of virtual memory, but you have pushed the boundary just a bit too far.

The -Xss setting should help, but you need to make the requested stack size LESS than the default size of 1m. And besides there is a hard minimum size.

Question #1: What do I have to do to be able to create a bigger thread pool?

Decrease the default stack size below what it currently is, or increase the amount of available virtual memory. (The latter is NOT recommended since it looks like you are already seriously over-allocating already.)

Question #2: At what stage should I expect to see context switching really reducing throughput and causing the process to grind to a halt?

It is not possible to predict that. It will be highly dependent on what the threads are actually doing. And indeed, I don't think that your benchmarking is going to give you answers that will tell you how a real multi-threaded application is going to behave.

---

The Oracle site says this on the topic of thread stackspace:

In Java SE 6, the default on Sparc is 512k in the 32-bit VM, and 1024k in the 64-bit VM. On x86 Solaris/Linux it is 320k in the 32-bit VM and 1024k in the 64-bit VM.

On Windows, the default thread stack size is read from the binary (java.exe). As of Java SE 6, this value is 320k in the 32-bit VM and 1024k in the 64-bit VM.

You can reduce your stack size by running with the -Xss option. For example:

  java -server -Xss64k

Note that on some versions of Windows, the OS may round up thread stack sizes using very coarse granularity. If the requested size is less than the default size by 1K or more, the stack size is rounded up to the default; otherwise, the stack size is rounded up to a multiple of 1 MB.

64k is the least amount of stack space allowed per thread.