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From: hmp@cive.ri.cmu.edu (Henning Pangels)
Newsgroups: comp.realtime
Subject: Memory management under VxWorks
Keywords: VxWorks, memory fragmentation, programmer frustration
Message-ID: <8136@pt.cs.cmu.edu>
Date: 23 Feb 90 15:33:29 GMT
Organization: Carnegie-Mellon University, CS/RI
Lines: 35


The problem of memory fragmentation under vxWorks has been discussed in this
group before, I know, but I'm curious if anyone has found a workaround, if
not an outright solution by now. The general problem I'm seeing is that
frequent use of malloc() and free() leads to more and more fragmentation,
eventually causing an exception when the largest chunk of memory isn't large
enough for a particular operation. While it is somewhat possible to control
explicit use of these calls in application code, they also seem to be used
by many system calls that create and delete system objects like sockets,
semaphores etc. A simple experiment shows that
socket(AF_INET,SOCK_STREAM,0), for example, makes 6 calls to malloc(), using
up a total of 140 bytes. 

Now, if the answer is that you just shouldn't be creating and deleting these
things on a regular basis, what good is having dynamic memory allocation in
the first place? If you knew beforehand exactly what and how many data
objects your application needed, you'd just allocate them statically in your
code.

An approach I've thought about is to create the objects you need when you
need them, and reuse them from then on whenever possible. In a way, that
amounts to writing you own memory manager, which is something an application
really shouldn't have to deal with. 

Another idea is to play with memory pools, but as far as I can see, there is
no way to have system calls allocate memory out of anything but the system
pool.

Do others out there continue to run into this problem, too? How about Wind
River? Any comments or promises of future enhancements in this area?

-- 
Henning Pangels                                   Field Robotics Center
ARPAnet/Internet: hmp@cive.ri.cmu.edu             Robotics Institute
(412) 268-7088                                    Carnegie-Mellon University