Enterprise-level applications often need more CPU power than a
single-CPU machine can provide, but getting applications to
effectively use multiple CPUs can be tricky. If
you're lucky, you may be able to run an application on separate
single-CPU
machines; Web servers scale this way. But many
applications, such as databases, need to run on a single machine, which
will
require additional CPUs to be scaled.
In this article, we'll examine tests that show
that Java threads in Sun's JRE 1.1.7 do not use more than two processors
by
default, regardless of how many are in the
machine. Java is advertised as automatically scaling to use all
available processors,
but we'll see below that this is not entirely
true. We'll also look at some simple ways to maximize Java
multithreading performance
on multi-CPU machines.
Note:
My tests indicate that
JRE 1.1.7 does not use more than two CPUs when running pure Java
programs. Under other conditions,
such as with native code that has native
OS-level threads, JRE 1.1.7 may utilize all of the CPUs.
The scoop on symmetric multiprocessing
Getting multiple equivalent CPUs to cooperate in
one machine is known as symmetric multiprocessing (SMP); running a
single-CPU
machine is known as uniprocessing. SMP machines'
CPU modules usually plug into a bus. In theory, you add capacity by
plugging
in more CPUs.
Unfortunately, because coordinating CPU activity
is difficult and most applications spend a lot of time waiting for I/O
rather
than using the CPU, using two processors doesn't
necessarily double your throughput. Performance may even degrade with
multiple
CPUs. Indeed, for software not designed for an
SMP system, adding a CPU may increase performance by no more than 30
percent.
For CPU-intensive programs designed for SMP, a
second CPU may increase performance by 80 or 90 percent, depending on
what
the application is doing. CPU-intensive programs
that can run as parallel processes or threads benefit more from
additional
CPU power than programs that spend most of their
CPU time idle, waiting for a network or disk.
SMP options
One way to use multiple CPUs on a single machine
is to run multiple processes that communicate via the various forms of
interprocess
communication, such as semaphores and shared
memory. The operating system will automatically allocate different CPUs
to different
processes. Interprocess communication techniques
are not especially portable, though. One alternative is using native
thread
programming in C or C++: this is difficult, but
is a way to use multiple CPUs in a single process with better
portability.
Java thread programming is simple by comparison
and provides high portability. Java threads have also been advertised as
scaling
well on multi-CPU machines. You can run Java
threads in parallel on multiple CPUs, but only if you use a
native-threads library
for Java instead of the green
threads
option. (The green thread implementation does all thread scheduling
within the JVM. On Unix, that means all threads
will run in a single process and never use
multiple CPUs.)
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