Chris:

> I am about to add a second processor to my machine.  I have compiled several
> applications on it since installation (redhat 7.2), but have not yet
> recompiled the kernel.  These applications include:

> bind
> openssl
> apache
> php
> mysql
> qmail
> gd
> freetype
> ssh

> I imagine that the second proc will not be used until after I recompile the
> kernel with SMP support, given that RedHat ships with an straight kernel and
> an SMP kernel and selects the appropriate one during install.  I only have
> the straight kernel installed and am missing the 2.4.10SMP kernel.

If the Red Hat install CD contains the 2.4.10 kernel that you are using now,
then the 2.4.10SMP kernel should be on there as well.  I guess I thought
that 2.4.9 was the most recent Red Hat kernel, but maybe that's just what
Dell provides on their servers.

> Should I expect any problems with the software after I add the second
> processor and recompile the kernel?  Should I expect the second processor to
> be used before I recompile the kernel?  Will the kernel handle the use of
> both, or do I need to recompile each application before they can make use of
> the two processors?

The SMP kernel will make both processors available to the applications.  Most
applications do not make any use of more than one CPU, so they would be
bound to whatever CPU the kernel assigns them to (usually CPU#0).  In the
case of these applications that do not make use of more than one CPU, a
recompile will not cause them to start using more than one CPU.

On the other hand, if the application was written to begin with to be SMP-
aware, then it will use both processors if it finds them, and will use only
one processor if it finds only one.  In this case, a recompile will not
change the behavior either.

We have done this sort of upgrade and have had very few problems.  Most
things are either not SMP-aware (so it's irrelevant) or they are SMP-aware
(so they work fine either way).  We had one of our own apps quit working
from time to time on an SMP kernel but it worked OK on the UP kernel, but
we still aren't sure if that was an SMP problem or if it was just bugs in
our code that were manifesting themselves.

Most of the programmers thought the bugs were ours, since generally the
Linux kernel has fewer bugs than other code.

--
Jim Ockers (ockers@ockers.net)
Contact info: please see http://www.ockers.net/

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