[asterisk-users] Scaling Asterisk: Dual-Core CPUs not yielding gains at high call volumes

[Matthew J. Roth]

Mark Coccimiglio wrote:
> Sounds like you are running into the hardware limitations of your 
> systems PCI or "Front Side Bus" (FSB) and not necessarily an issue of 
> asterisk.  In short there is a limited amount of bandwidth on the 
> computer's PCI Bus (33 MHz)  and the FSB (100-800MHz).  One thing to 
> remember is that ALL cores and data streams need to share the PCI and 
> FSB.    Asterisk is very processor and memory intensive.  At the 
> extreme level of usage more cores won't help if data is "stuck in the 
> pipe".  So the performance planing you described would be expected.
Mark,

That is a great theory and I'd like to follow up on it.  Do you know if 
the PCI or FSB buses are instrumented by Linux?  If not, are you aware 
of any way to gather statistics about their utilization?   I'd like to 
see if the numbers support your idea and, if so, which bus is saturated.

Let me add a little bit of extra information to this discussion.  The 
CPU utilization does not flatten out at 50%.  In fact, as more calls are 
added, Asterisk will eventually drive the idle percentage down to single 
digits with surprisingly few problems.  If PCI or FSB bandwidth were the 
limiting factor, wouldn't the CPU utilization top out at the point that 
the available bandwidth was used?

Thank you,

Matthew Roth
InterMedia Marketing Solutions
Software Engineer and Systems Developer