Re: Pin-pointing the root of unusual application latencies

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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
With a 4-port NIC (instead of the I/O board) inserted in the top slot.

# cat /proc/interrupts
           CPU0
  0:         37    XT-PIC-XT        timer
  1:          2    XT-PIC-XT        i8042
  2:          0    XT-PIC-XT        cascade
  5:          0    XT-PIC-XT        eth3
  7:          0    XT-PIC-XT        acpi
 10:          4    XT-PIC-XT        eth1, eth5
 11:          4    XT-PIC-XT        eth2, eth6
 12:        265    XT-PIC-XT        eth0, eth4
 14:      12726    XT-PIC-XT        ide0
NMI:          0
LOC:      12704
ERR:          0
MIS:          0

With the 4-port NIC inserted in the bottom slot.

# cat /proc/interrupts
           CPU0
  0:         37    XT-PIC-XT        timer
  1:          2    XT-PIC-XT        i8042
  2:          0    XT-PIC-XT        cascade
  5:          4    XT-PIC-XT        eth2, eth3
  7:          0    XT-PIC-XT        acpi
 10:         99    XT-PIC-XT        eth0, eth5
 11:          4    XT-PIC-XT        eth1, eth6
 12:          0    XT-PIC-XT        eth4
 14:      12138    XT-PIC-XT        ide0
NMI:          0
LOC:       3554
ERR:          0
MIS:          0

As far as I understand, this board has 4 interrupt lines, and they are mapped (by the BIOS? by Linux?) to ISA IRQs 5, 10, 11, and 12.
If I insert another 4-port NIC in the remaining slot, eth7 through eth10
will spread to these IRQs. Correct?
Suppose 3 NICs share the same interrupt line, and each receives 1500
packets per second. (I suppose that translates to 1500 interrupt
requests per second.) Will the system cope with 4500 interrupt requests
from 3 different devices on the same interrupt line? How different is it
from 4500 interrupt requests from a single device not sharing the line?
Regards.
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