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Asterisk hardware interrupts
Asterisk and IRQsReference:
With multiple Zaptel cards, and sometimes just one, the most common issues are interrupt-related. The limitation with the number of cards is interrupts, not PCI slots. As the above article explains, sharing interrupts adds a signifigant amount of latency to the interrupt, causing audio droupouts and other weird stuff to happen. It is nothing specific to Digium hardware or Linux or anything like that. On a desktop running Windows (or any other OS) with a lot of devices try to play back an MP3 and generate network, disk, and other I/O activity on a machine without APIC - the audio WILL skip. Now imagine you are trying to drive 96 or more T1 channels (more than one te405/te410p), ethernet boards and who knows how much else. You can see where there might be a problem.
Apple Mac's have interrupts. They just have a lot of them. That is why they do much better with multiple Zaptel cards, or even just one. But, a modern Mac is very similar when compared to a modern x86 with an APIC. Read more.
The interrupt problem is being mitigated on the x86 architecture by the APIC - Advanced Processor Interrupt Controller. You can have more than one APIC in a machine, and each APIC has 24 available IRQs. Most new motherboards have at least one APIC/processor.
Here is some output from two of my machines demonstrating this:
Dell PowerEdge 2850 (dual Xeon):
ast1 etc # uname -a Linux ast1 2.6.9-pe2850a #2 SMP Fri Dec 3 09:23:03 CST 2004 i686 Intel(R) Xeon(TM) CPU 3.00GHz GenuineIntel GNU/Linux ast1 etc # cat /proc/interrupts CPU0 CPU1 0: 6184515 72 IO-APIC-edge timer 1: 8 1 IO-APIC-edge i8042 9: 0 0 IO-APIC-level acpi 12: 65 1 IO-APIC-edge i8042 14: 11 2 IO-APIC-edge ide0 46: 19595 1 IO-APIC-level megaraid 64: 66366 1 IO-APIC-level eth0 65: 77045 1 IO-APIC-level eth1 101: 6113521 1 IO-APIC-level wctdm NMI: 1 0 LOC: 6184694 6184698 ERR: 0 MIS: 0 ast1 etc #
Apple Macintosh G4:
[root@localhost root]# uname -a Linux localhost.localdomain 2.6.8-1.ydl.7 #1 Wed Sep 22 18:46:40 EDT 2004 ppc ppc ppc GNU/Linux [root@localhost root]# cat /proc/interrupts CPU0 19: 4540 OpenPIC Level ide0 20: 12 OpenPIC Level ide1 25: 318 OpenPIC Level VIA-PMU 26: 0 OpenPIC Level keywest i2c 27: 35 OpenPIC Level ohci_hcd 28: 97 OpenPIC Level ohci_hcd 41: 1248 OpenPIC Level eth0 42: 0 OpenPIC Level keywest i2c 47: 0 OpenPIC Level GPIO1/ADB 54: 19036943 OpenPIC Level wctdm 55: 0 OpenPIC Edge NMI - XMON 63: 3 OpenPIC Level ohci1394 BAD: 0 [root@localhost root]#
As you can see, on the Dell, the wctdm board has IRQ 101. Waaaaayyyyy past good ol' 15. On the Mac, it is at 54. Still way beyond the standard x86 15 limit.
Even in Windows, my audio card has IRQ 22 right now. This is all because of APIC.
Now, if you are using an OS that can't do APIC on x86, or have a motherboard that has a mal-functioning APIC, you are out of luck and have to try to get them all to play nice. I hope that this clears things up some.
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