Re: java thread question
On 4/3/2014 10:51 AM, mcheung63@gmail.com wrote:
Hi All
I have write a little program to keep running some threads, i got questions:
1) All threads are running within one process (ps command show me this), but it will eat up all cpu cores, how to explain this?
What needs explanation? That is, what puzzles you? The *purpose*
of threads is to break up a large task into smaller pieces and let them
execute in parallel; you *want* those smaller pieces to use whatever
machine resources they can get.
2) If thread will eat multiple cpu core, why wee need fork/join in java?
(I assume you're referring to java.util.concurrent.ForkJoinTask.)
If your large task subdivides into a million smaller pieces, you don't
want to try running each piece in a separate thread. A thread is
"lighter-weight" than a full-scale process, but its weight is non-zero:
There's a stack, there's other CPU context, there's management overhead
like schedulers and garbage collectors, ... When you multiply this by
a million, or even by several thousand, your system will collapse under
the load, most probably by throwing an OutOfMemoryException.
That's where ForkJoinPool and ForkJoinTask come in. The pool runs
a manageable number of long-lived threads, and each thread executes many
ForkJoinTasks in succession. (There are additional intricacies, but
that's the essence.) You don't have to create, start, shut down, and
garbage-collect a million threads to get the million sub-tasks done.
--
Eric Sosman
esosman@comcast-dot-net.invalid