Subject: [linux-audio-user] [Fwd: Re: [ardour-dev] Ardour, IRQs and xruns]
From: Patrick Shirkey (pshirkey_AT_boosthardware.com)
Date: Mon Sep 16 2002 - 19:12:40 EEST
People here who are not on the ardour list should find this interesting
and helpful.
Mark. I have an Nvidia agp card which uses irq9 and a usb audio card
that I can put on 11. Now I ust have to figure out how to tell which
USB port is using 11 and which is using 4.
Thanks.
-- Patrick Shirkey - Boost Hardware Ltd. For the discerning hardware connoisseur http://www.boosthardware.com http://www.boosthardware.com/LAU/guide/ ========================================"Um...symbol_get and symbol_put... They're kindof like does anyone remember like get_symbol and put_symbol I think we used to have..." - Rusty Russell in his talk on the module subsystem
attached mail follows:
Steve and Neil, This is a simple, or difficult, subject. I used to design chipsets (I'm a hardware chip designer in Silicon Valley) for AMD early on after they purchased NexGen and got started on building the Athlon line. Unfortunately, my experience is getting a bit dated. It's still valid for single processor machines, but dual-processor/APIC I'm not at all sure about.
Due to the history of the PC architecture, PC's first had 1 interrupt controller, and then with the AT architecture, added a second. This accounts for the sort of strange numbers I'm going to give you next.
Here's the order of interrupt priority on a non-APIC machine:
0, 1, 8, 9, 10, 11, 12, 13, 14, 15, 3, 4, 5, 6, 7
0 is the highest priority 7 is the lowest priority
Potentially 9, 10, 11, 3, 4, 5, 6, 7 are made available to PCI slots on many motherboards. You MUST look at your SPECIFIC motherboard to know what's available to you.
I won't go into (unless someone asks, in which case I'd be happy to) why the order is this way, but it is.
In my experience under Windows all audio cards want to be on IRQ's 9, 10 or 11. It isn't actually that important WHAT IRQ a card is on, but it IS IMPORTANT what devices are on higher IRQs. For instance, if Steve's audio card is on IRQ 11, and nothing is on IRQ 9 or 10, then no problem. But if Steve was to insert a PCI graphics adapter on IRQ9, his audio card would start having trouble because the graphics adapter would have higher priority.
Again, I won't go into all the reasons why, but just assume that any device with a higher priority number (from the list above) gets in the way of devices with lower priorities.
One last thing about non-APIC, and then I'll let anyone ask anything specific they want. Interrupts may be shared. Just because your card is on IRQ9 doesn't mean that another device isn't sharing IRQ9. What matters in this case is how many interrupts the other device is going to generate, and how well it's driver is written.
With the APIC model, Intel and Microsoft introduced a way to have more interrupts in the PC, and a way for the PC to route them without the user having to deal with this stuff. Unfortunately, I personally think the jury is still out about APIC mode FOR AUDIO PCs. It works great from standard work PCs. It's intention was to abstract the IRQ hardware layer so that IRQs could be directed to the 'correct' processor in a multi-processor system. I don't know much about it, but clearly when PTLE switched over to XP support, and the default HAL layer in Win XP was APIC, many, many people started having problems.
I hope this sort of info is valuable to you guys. I get so much from all you software gurus it's nice to feel like I'm making a contribution once in awhile.
Mark
-----Original Message----- From: ardour-dev-admin_AT_lists.sourceforge.net [mailto:ardour-dev-admin_AT_lists.sourceforge.net]On Behalf Of Steve Harris Sent: Monday, September 16, 2002 5:44 AM To: ardour-dev_AT_lists.sourceforge.net Subject: Re: [ardour-dev] Ardour, IRQs and xruns
On Mon, Sep 16, 2002 at 12:53:17 +0100, Neil McBennett wrote: > think? I could disable APIC, but how do I force my Hammerfall card to > use a specific interrupt, and which ones are "good"? Is this a trial and > error thing?
I think higher is better. I got best results by forcing it to be on IRQ11. It's sharing with a couple of USB controllers, but it doesn't seem to hurt too much. IRQ10 was free on my machine, but I got slightly worse results in my very unscientific trials.
On my previous machine I had to juggle the IRQs around so that the hammerfall was on its own, otherwise the audio was garbled or something. This was a year or more ago though, so it may have been a driver problem.
On most (all?) motherboards you can control what IRQ gets assigned to what PCI slot. I've no idea if APIC effects latency.
- Steve
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