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From "Brad Nicholes" <BNICHO...@novell.com>
Subject NetWare and protected address space (was: Re: cvs commit: httpd-2.0 STATUS)
Date Wed, 23 Jun 2004 19:02:17 GMT
   Just to provide a little background on NetWare (and if you don't
care, don't read any further).  NetWare is not your standard general
purpose operating system.  NetWare was origninally designed and built to
be a high performance, highly scalable and very secure file and print
server.  Over the years it has evolved into something that sits
somewhere between its original design purpose and a general purpose OS. 
In order to achieve the performance levels that it does, as much
overhead as possible was eliminated which includes the concept of
processes and what most people understand to be address spaces. 
Everything that runs on NetWare runs as a thread in Kernel address
space.  The idea of a separate address space or "Protected Address
Space" (as it is called around here) didn't show up in NetWare until
NetWare 5 (I think that was the first version).  Basically a protected
address space is just that.  NetWare allocates a separate address space
in which it is able to run an application.  In order for an application
to run in this address space, everything that it depends on has to be
loaded in that address space.  This mean code as well as data.  A
protected address space on NetWare could probably be thought of more as
a virtual machine than a process.  This is why it is so expensive to
spin up a CGI in a separate address space on Netware.
    But running everything in kernel address space is not as scary as
it sounds on NetWare.  If you have a wild module that causes the Apache
instance to abend (another old time term that is still widely used when
refering to NetWare), it will bring Apache down, but the OS is smart
enough to isolate the offending application and still allow the server
to continue running.  The downside is that the only way to clear an
abend in kernel address space is to restart the server. While an abend
in a protected address space can simply be unloaded and cleared.  Some
NetWare administrators will choose to run applications like Apache in a
protected address space simply for the ability to easily clean it up if
something goes wrong.  But by doing so they sacrifice some level of
performance of the web server as well as system resources.  Others may
consider applications like Apache to be so stable and the performance
hit so great, that they are comfortable in running them in kernel
address space.  For the most part, this is the case.  But Apache is a
long lived application.  Once it is spun up and running, the overhead is
gone.  CGIs, as we all know, can come and go at anytime.  Because of the
same performance reason, other platforms have been able to increase
performance by using mod_cgid rather than mod_cgi to avoid the same type
of process creation overhead.  But on NetWare, not only is starting up a
CGI in protected address space much more expensive than forking a
process on other platforms, we don't have fork/exec on NetWare.
   The bottom line is that because of the nature of the NetWare OS, it
is much more efficient and much easier to run things in kernel address
space.  It is just the nature of the OS.  What's the benefit?
Performance, scalability and yes, security. (OK, that sounds a lot like
a marketing line, but its true)

Brad

Brad Nicholes
Senior Software Engineer
Novell, Inc., the leading provider of Net business solutions
http://www.novell.com 

>>> wfrazee@wynweb.net Tuesday, June 22, 2004 8:34:54 PM >>>
Uh, then may I follow up with another stupid, obvious question, if
using
another address space insulates the parent application and, in some
cases, the server from a crash resulting from an unstable module, why
do
they all use the same address space on novell?  Does this compromise a
security measure in the case of a module fail or crash?

-- 
--------------------
Wayne S. Frazee
"Any sufficiently developed bug is indistinguishable from a feature."


On Tue, 2004-06-22 at 16:49, Jean-Jacques Clar wrote:
> >Can I ask the obvious, then?  When would a separate address space
> >be desirable for an apr-based app to invoke a child/forked process? 

> 
> 
> It is a desirable option mainly for developers using unstable modules

> 
> to ensure the child process will not kill the parent application, or
> the server, 
> 
> in case a major problem happens inside the child.
> 
> By default on NetWare, the http server and all of its child
processes
> are 
> 
> started in the same address space.
> 
> 
>  
>  


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