Experimental Physics and
Industrial Control System

"Jeff Hill" <[email protected]> · Mon, 18 Nov 2002 10:04:34 -0700

>    -- VxWorks NFS uses UDP; all told there is not much overhead.
> (CA,
> which is very efficient in most respects, uses TCP; and Channel
> Watcher
> may be additionally using NFS to access the file system from
> whichever
> host it is actually runnning on)
> 

A considerable amount of effort has gone into making TCP efficient. This
is especially true in high throughput situations and in situations where
there is network congestion. While similar things can be accomplished
with UDP you are always at a disadvantage because TCP is scheduled
within the operating system, and because TCP is so fundamental that is
has had more research dollars thrown at it. Making and breaking TCP
circuits is not particularly efficient, but after the circuit is
established it is hard to beat. In contrast, I seem to recall that the
implementation of NFS on vxWorks is resource intensive. It uses quite a
bit of stack space, and it uses a lot of dynamically allocated memory.
There is nothing inherently wrong with dynamic memory allocation on
vxWorks. However whenever an action is running periodically that must
repeatedly allocate and free memory then there may be concerns about
memory fragmentation. I seem to also recall reading that NFS was not
particularly efficient in wide area network situations because of less
than optimal packet schedualing, but that may have changed in recent
years.

I completely agree with the goal of distributing the system and thereby
avoiding a single point of failure. However, the NFS server, while
capable of some robust configurations, should probably not be entirely
released from scrutiny when identifying single points of failure. 

In a perfect world, the backup and restore data would be maintained in a
battery backed up RAM resident within the IOC. All other boot data would
be maintained in a flash ram also resident within the IOC.

Jeff

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System