A question about Thread and Process

I read some tutorial about threads and processes, it is said that the processes be scheduled by operating system kernel, and the threads can be managed and scheduled in a user mode.

I do not understand the saying "threads can be managed and scheduled in a user mode", for example: the producer and consumer problem? is this a example for "scheduled in a user mode"? o开发者_如何学编程r can anyone explain me?

Not sure what tutorial you're looking at, but there are two ways that threads can be scheduled.

The first is user-mode scheduling, which basically mean that one process, using Green threads or perhaps fibers, schedules different threads to run without involving the operating system in its decision. This can be more portable across operating systems, but usually doesn't allow you to take advantage of multiple processors.

The second is kernel scheduling, which means that the various threads are visible to the kernel and are scheduled by it, possibly simultaneously on different processors. This can make thread creation and scheduling more expensive, however.

So it doesn't really depend on the problem that you are trying to solve. User-mode just means that the scheduling of threads happens without involving the operating system. Some early Java versions used Green/user-mode threads, but I believe most now use native/kernel threads.

EDIT: Coding Horror has a nice overview of the difference between user and kernel mode.

Get a better tutorial? The official Java tutorials are quite good, and contain a lesson on concurrency, that also defines what process and thread mean.

PS: Whether threads are managed/scheduled in user mode is an implementation detail of the Java Virtual Machine that generally need not concern the application programmer.

Scheduled in user mode means you have control over the threads of your software but they are managed by the operating system kernel. So yes, the producer consumer problem is an example you normally handle yourself (but it is not directly related to user mode scheduling) by having two threads, a producer thread and a consumer thread. Both threads access the same shared recource. This resource has to be thread-safe, this means you have to make sure the shared resource does not get corrupted becouse both threads access it at the same time. Thread safety can either be guaranteed by using thread-safe data types or by manually locking or synchronizing your resource.

However, even if you have some control over your threads e.g. starting threads, stopping threads, make threads sleep etc. you do not have full control. The operating system is still managing which threads are allowed cpu time etc.