Re: Pin-pointing the root of unusual application latencies

[John Sigler]

John Sigler wrote:

> Len Brown wrote:
>
> > John Sigler wrote:
> >
> > > # cat /proc/interrupts
> > >             CPU0
> > >    0:         37    XT-PIC-XT        timer
> > >    1:          2    XT-PIC-XT        i8042
> > >    2:          0    XT-PIC-XT        cascade
> > >    7:          0    XT-PIC-XT        acpi
> > >   10:        175    XT-PIC-XT        eth2, Dta1xx
> > >   11:       1129    XT-PIC-XT        eth0
> > >   12:          4    XT-PIC-XT        eth1
> > >   14:      21482    XT-PIC-XT        ide0
> > > NMI:          0
> > > LOC:     161632
> > > ERR:          0
> > > MIS:          0
> > >
> > > IRQ 10 is shared between a NIC and an I/O board.
> > >
> > > For eth2, the kernel said:
> > > ACPI: PCI Interrupt 0000:00:0a.0[A] -> Link [LNKC]
> > >    -> GSI 10 (level, low) -> IRQ 10
> > >
> > > For Dta1xx, the kernel said:
> > > ACPI: PCI Interrupt 0000:02:0e.0[A] -> Link [LNKC]
> > >    -> GSI 10 (level, low) -> IRQ 10
> > >
> > > Is it possible to avoid the two boards sharing IRQ 10?
> >
> > Maybe.  In this configuration, INTA of the two devices
> > is physically connected to the same wire on the device-side
> > of the interrupt re-mapper -- so you'd have to change the configuration.
> > If you have an IOAPIC and can enable it, that will not hurt --
>
> I believe this board does not provide an IO-APIC.
> Even the LAPIC is disabled in the BIOS.
> (Why would they do that??)
>
> > though unless something else changes, these devices are still
> > tied together on the device-side of the mapper.
> > So if you can physically move one of the devices to another slot
> > that is your best bet.

The NICs are on-board, therefore it is not possible to move them.

The motherboard only has one PCI slot, so the manufacturer includes
a backplane (is that what it's called?) to provide two PCI slots.

The results I've given so far were with the I/O board inserted in
the bottom slot. If it is inserted in the top slot, the results are
different indeed.

# cat /proc/interrupts
           CPU0
  0:         37    XT-PIC-XT        timer
  1:          2    XT-PIC-XT        i8042
  2:          0    XT-PIC-XT        cascade
  5:      20270    XT-PIC-XT        Dta1xx
  7:          0    XT-PIC-XT        acpi
 10:          4    XT-PIC-XT        eth2
 11:       2639    XT-PIC-XT        eth0
 12:          4    XT-PIC-XT        eth1
 14:      13984    XT-PIC-XT        ide0
NMI:          0
LOC:     518501
ERR:          0
MIS:          0

$ diff dmesg.adlink dmesg2.adlink
208c208,210
< ACPI: PCI Interrupt 0000:02:0e.0[A] -> Link [LNKC] -> GSI 10 (level, 
low) -> IRQ 10
---
> ACPI: PCI Interrupt Link [LNKD] enabled at IRQ 5
> PCI: setting IRQ 5 as level-triggered
> ACPI: PCI Interrupt 0000:02:0f.0[A] -> Link [LNKD] -> GSI 5 (level, 
low) -> IRQ 5

$ diff lspci.adlink lspci2.adlink
121c121
< 02:0e.0 Multimedia controller: PLX Technology, Inc. 9056 PCI I/O 
Accelerator
---
> 02:0f.0 Multimedia controller: PLX Technology, Inc. 9056 PCI I/O 
Accelerator
126c126
<       Interrupt: pin A routed to IRQ 10
---
>       Interrupt: pin A routed to IRQ 5

I'll give a 4-port PCI NIC a spin.

Regards.
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