Communicator e3000 MPE/iX Release 6.0 Express 1 (C.60.01) (30216-90286)

50 Chapter3

Technical Articles

Java Developers Kit for MPE/iX Version 1.1.7B Release Notes

Performance Improvements and Resource Requirement Reductions

With 1.1.7B, several improvements were made to the MPE/iX implementation to reduce

the resources required for each instance of the Java Virtual Machine, and to reduce the

amount of time and overhead required to start a VM. These changed have resulted in the

elimination of approximately half of the start-up overhead.

Here is a summary of differences between 1.1.5 and 1.1.7B:

1.1.7 1.1.5

----------------------- -------------------- ------------------------

Thread stack allocation dynamic static (39MB stack req.)

# of Thread stacks dynamic fixed, ~35

max # of Threads limited by ;NMSTACK= fixed, ~35

Thread stack size 128KB 1MB

Min ;NMSTACK for shell default (2MB) works NMSTACK=40000000

Default min heap 256KB 64MB

Default max heap 64MB 64MB

Out of memory result Exception thrown VM aborts with SIGBUS

# of fork/exec to start 1 multiple, perhaps many

Can fork VM? yes no

Thread Stacks

Since the Java Virtual Machine is a multithreaded environment, a stack must be allocated

for each thread. Currently Java/iX uses a “Green Threads” package which simulates

multiple threads within a single process.

In 1.1.5, storage for thread stacks was statically allocated on the stack at start-up. This

resulted in several problems:

• The number of threads that was supported was a ﬁxed number.

• The VM needed to run with ;NMSTACK=40000000, which required that the user enter

the POSIX environment using at least this large of an NMSTACK so that it would be

inherited by the eventual Java VM.

• The ﬁrst thread used the highest addresses in the stack, resulting in an apparent 39MB

stack size for even the most trivial Java program.

• Functions invoked from within the VM that required executing a fork and exec

sequence would fail. This included executing shell commands and processes from within

Java, and the jdb Java debugger.

• Running out of memory could cause an abort rather than throwing an

OutOfMemoryException.

• The JIT required a large stack size, and could potentially run on any thread, so each

thread stack was allocated a full megabyte of storage.

In 1.1.7, the following changes have been made to address these problems:

• Thread stacks are now allocated as needed, starting at low addresses in the stack. This

has reduced the minimum ;NMSTACK= requirements for the VM from nearly 40MB to

around 1.5MB. Because the shell now has a 2MB default NMSTACK, simple Java

programs can now be run without any special ;NMSTACK= parameter being speciﬁed

anywhere.