Re: [LAU] buying a new laptop - Linux-audio-user

9 Sep 2011

andy baxter wrote:
...
  The specific problem I'm trying to solve is that
the firewire card in
 the laptop is on the same irq as the onboard sound card and one of the
 usb ports:

   17:        271        120   IO-APIC-fasteoi   uhci_hcd:usb6, HDA Intel, ohci1394

 [USB] Buses 1 and 2 are used by plug-in devices; 3 is used by onboard
 peripherals, and the rest seem to be unused, so I assume that it doesn't
 matter that usb6 is shared; only the sound-card. 
When an interrupt on line 17 arrives, the kernel asks all attached
drivers whether their device needs servicing.

Big latencies usually are the result of long-running interrupt handlers,
so an unused device does not matter much, but these interrupt checks
still are unneeded work.

Please note that UHCI and EHCI controllers share ports; UHCI is for
USB 1.x devices, EHCI, for 2.0 ones.  Therefore, one of your ports
will be usb6 when you connect a full- or low-speed device.  (Only very
incompetent laptop makers wouldn't deactivate unused controllers.)

...
  Is what you're saying that the only way round the
problem is to disable
 the other devices on that irq altogether? 
I'm not sure whether those other interrupt handlers are much of
a problem, but if they are, yes.

Please note that shared interrupts are not the only latency source;
interrupt handlers for any device (or system management interrupt
handlers installed by the BIOS) could block the CPU for too long.

And just to show off: here is my /proc/interrupts, with four PCI(e) cards:
:-)

CPU0       CPU1       CPU2       CPU3       CPU4       CPU5       
  0:        135          0          0          7        779     205506   IO-APIC-edge     
timer
  1:          1          0          0          0          9       7300   IO-APIC-edge     
i8042
  4:          0          0          0          0          0          2   IO-APIC-edge    
  7:          0          0          0          0          0          0   IO-APIC-edge     
parport0
  9:          0          0          0          0          0          0   IO-APIC-fasteoi  
acpi
 14:          0          0          0          0          1         53   IO-APIC-edge     
pata_atiixp
 15:          0          0          0          0          0          0   IO-APIC-edge     
pata_atiixp
 16:          0          0          0          0          3        885   IO-APIC-fasteoi  
hda_intel
 17:          0          0          0          0          0          5   IO-APIC-fasteoi  
firewire_ohci
 18:          0          0          0          0          2       1712   IO-APIC-fasteoi  
radeon
 19:          0          0          0          0          2         18   IO-APIC-fasteoi  
hda_intel
 20:          0          0          0          0        134      42418   IO-APIC-fasteoi  
oxygen
 21:          0          0          0          0          0          0   IO-APIC-fasteoi  
EMU10K1
 22:          0          0          0          0          0          0   IO-APIC-fasteoi  
YMFPCI
 23:          0          0          0          0          0          5   IO-APIC-fasteoi  
firewire_ohci
 40:     565565          0          0          0          0          0  HPET_MSI-edge     
hpet2
 41:          0          0          0          0          0          0   PCI-MSI-edge     
aerdrv, PCIe PME
 42:          0          0          0          0          0          0   PCI-MSI-edge     
aerdrv, PCIe PME
 43:          0          0          0          0         36      15254   PCI-MSI-edge     
ahci
 44:          0          0          0          0          0          0   PCI-MSI-edge     
eth0
 45:          0          0          0          0         16        713   PCI-MSI-edge     
ehci_hcd:usb1
 46:          0          0          0          0          0          0   PCI-MSI-edge     
ehci_hcd:usb2
 47:          0          0          0          0          0        804   PCI-MSI-edge     
ohci_hcd:usb3
 48:          0          0          0          0          0         40   PCI-MSI-edge     
ohci_hcd:usb4
 49:          0          0          0          0          0          1   PCI-MSI-edge     
ohci_hcd:usb5
 50:          0          0          0          0          0          1   PCI-MSI-edge     
ohci_hcd:usb6
 51:          0          0          0          0          0          0   PCI-MSI-edge     
ohci_hcd:usb7
NMI:          0          0          0          0          0          0   Non-maskable
interrupts
LOC:        159     538453     575689     528275     507949     690032   Local timer
interrupts
SPU:          0          0          0          0          0          0   Spurious
interrupts
PMI:          0          0          0          0          0          0   Performance
monitoring interrupts
IWI:          0          0          0          0          0          0   IRQ work
interrupts
RES:      34599      25532      23354      12203      15350      13619   Rescheduling
interrupts
CAL:       1122       7777        981        718       1188      11005   Function call
interrupts
TLB:        279       8399        908        624        485        550   TLB shootdowns
THR:          0          0          0          0          0          0   Threshold APIC
interrupts
MCE:          0          0          0          0          0          0   Machine check
exceptions
MCP:         23         23         23         23         23         23   Machine check
polls
ERR:          0
MIS:          0

Regards,
Clemens