Why is creating a Thread said to be expensive?
Because it >>is<< expensive.
Java thread creation is expensive because there is a fair bit of work involved:
- A large block of memory has to be allocated and initialized for the thread stack.
- System calls need to be made to create / register the native thread with the host OS.
- Descriptors need to be created, initialized and added to JVM-internal data structures.
It is also expensive in the sense that the thread ties down resources as long as it is alive; e.g. the thread stack, any objects reachable from the stack, the JVM thread descriptors, the OS native thread descriptors.
The costs of all of these things are platform specific, but they are not cheap on any Java platform I've ever come across.
A Google search found me an old benchmark that reports a thread creation rate of ~4000 per second on a Sun Java 1.4.1 on a 2002 vintage dual processor Xeon running 2002 vintage Linux. A more modern platform will give better numbers ... and I can't comment on the methodology ... but at least it gives a ballpark for how expensive thread creation is likely to be.
Peter Lawrey's benchmarking indicates that thread creation is significantly faster these days in absolute terms, but it is unclear how much of this is due improvements in Java and/or the OS ... or higher processor speeds. But his numbers still indicate a 150+ fold improvement if you use a thread pool versus creating/starting a new thread each time. (And he makes the point that this is all relative ...)
The above assumes native threads rather than green threads, but modern JVMs all use native threads for performance reasons. Green threads are possibly cheaper to create, but you pay for it in other areas.
Update: The OpenJDK Loom project aims to provide a light-weight alternative to standard Java threads, among other things. The are proposing virtual threads which are a hybrid of native threads and green threads. In simple terms, a virtual thread is rather like a green thread implementation that uses native threads underneath when parallel execution is required.
As of now (Jan 2021) the Project Loom work is still at the prototyping stage, with (AFAIK) no Java version targeted for the release.
I've done a bit of digging to see how a Java thread's stack really gets allocated. In the case of OpenJDK 6 on Linux, the thread stack is allocated by the call to
pthread_create that creates the native thread. (The JVM does not pass
pthread_create a preallocated stack.)
pthread_create the stack is allocated by a call to
mmap as follows:
mmap(0, attr.__stacksize, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
man mmap, the
MAP_ANONYMOUS flag causes the memory to be initialized to zero.
Thus, even though it might not be essential that new Java thread stacks are zeroed (per the JVM spec), in practice (at least with OpenJDK 6 on Linux) they are zeroed.