Network performance issues when writing to disk (5.2.1-RELEASE)

Erik Rothwell erik.rothwell at
Tue Aug 17 13:13:17 PDT 2004

On 8/12/04 11:17 AM, "Robert Watson" <rwatson at> wrote:

> Did you replace it with another if_dc card, or with a card using a
> different interface driver?

Well, I took the afflicted box out of production this weekend for some
further testing. After swapping out one of the if_dc NICs with an Intel card
using the if_fxp driver, the problem no longer occurs on that interface.

I can only reproduce the problem using an interface with the if_dc driver.
The difference in performance is night and day.

> - What sort of kernel configuration are you using -- GENERIC, or a custom
>   kernel, and if so, is it an SMP box and is SMP enabled in the kernel?

This is a single processor box. The kernel is compiled without SMP or APIC
and without a number of unused drivers (SCSI, ISA NICs, PCMCIA, umass, &c).
polling is not currently enabled via the sysctl however).

> - When the system performance suddenly degrades, what is the apparent load
>   and condition of the system?  In particular, at that point if you
>   measure the load average and CPU utilization, perhaps with "systat
>   -vmstat" or "top", are the CPU(s) maxed out?  How much time is in user
>   vs. system vs. interrupt (vs. idle).

if_dc: nc > /dev/null
    Load 0.79 0.25 0.13
    CPU 41.5% sys, 28.5% interrupt, 5.5% user, 24.5% idle
    Interrupts 2501 clock, 128 rtc, 11684 dc0, 83 ata
    Disk ad0: 12.33 KB/t 4 tps 0.04 MB/s
    Disk ad1: 7.80 KB/t 80 tps 0.61 MB/s 8% busy

if_dc: nc > /data/junk
    Load 0.11 0.26 0.17
    CPU 9.0% sys, 2.0% interrupt, 0.5% user, 88.5% idle
    Interrupts 2504 clock, 128 rtc, 434 dc0, 54 ata
    Disk ad0: -
    Disk ad1: 11.75 KB/t 51 tps 0.58 MB/s 100% busy

if_fxp: nc > /data/junk
    Load 0.42 0.38 0.23
    CPU 61.0% sys, 31.2% interrupt, 2.5% user, 4.5% idle
    Interrupts 2501 clock, 128 rtc, 6036 fxp0, 145 ata
    Disk ad0: -
    Disk ad1: 128 KB/t 73 tps 9.07 MB/s 37% busy

Plenty of free memory, little if any swapping at any time. Both disks are
running in udma66. devinfo/dmesg doesn't indicate any conflict between disk
controller and the network interfaces. There certainly appears to be some
contention between the if_dc cards and the disk, however.


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