Re: [webmin-l] Urgent Help Needed - Kernel Issues

[Joe C. <jo...@sw...>]

> Vern wrote:
> 
>> In case anyone is interested it has to do multiprocessing which I 
>> don't even have so disabling it does nothing. It's been working for 
>> over a year and all of a sudden... sometimes I hate computers! Anyway, 
>> that is resolved.

I'll add a few minor bits of info to this thread:

P4 processors over 3.0 GHz, P4-based Xeon processors (and not the older 
PIII-based Xeons), and a few other Intel processors have a feature 
called hyper-threading.  This is treated by Linux as an additional 
processor and so if you have one of those processors, Linux will treat 
it as a dual processor SMP machine if you boot into an SMP kernel.  This 
isn't a bug, it's a feature, and increases performance by a small amount 
for some types of task (primarily tasks that require multiple 
simultaneous processes rather than a very demanding single process 
task--a server with a database-backed website would see an improvement 
while a Squid caching proxy server would lose performance).  That's not 
to say there aren't some systems that might get misdetected as multi-CPU 
when they don't have hyperthreading or additional physical CPUs, but I'm 
not aware of any that do.  It would be a bug for a non-hyperthread 
cabable single-core single-CPU system to be detected as SMP-capable. 
Intel has an overview of hyper-threading, including a specific list of 
processors that have this feature, here: 
http://www.intel.com/technology/hyperthread/

An SMP kernel should operate on a single CPU machine without disatrous 
consequences.  Performance will likely suffer, as there is additional 
overhead in an SMP system due to the additional locking required to have 
more than one CPU share a single pool of memory.  If you find 
instability when running an SMP kernel on a single CPU system (that is 
stable when run under a uniprocessor kernel), a bug report to that 
effect to your operating system vendor would be appropriate.  But, there 
is also no reason to run an SMP kernel on a machine that does not have 
either multiple CPUs or hyper-threading.  It does not improve 
performance (it does the opposite due to the extra locks), and increases 
memory usage by a small amount.

Finally, kernels are way outside of the scope of the Webmin list (as 
clearly evidenced by the well-meaning but misleading and/or incomplete 
replies to this query to date).  The newbie or general technical 
discussion list of your Linux distribution would be the place to start 
with a problem like this, possibly escalating to a bug report to your OS 
vendor.