Atlantic Linux Blog

Passing kernel module parameters in Ubuntu 8.10

Sorry for the mouthful of a title but I wanted to use something that would show up for the kind of queries I was firing into Google yesterday in a vain attempt to solve my problem.

A little background first: I’m working with some SuperMicro Twin systems (basically, two system boards in a single 1U chassis sharing a power supply – not as compact as a blade but not bad) which includes a nVidia MCP55V Pro Chipset Dual-port LAN / Ethernet Controller.  On Ubuntu 8.10 at least, this uses the forcedeth driver (originally a cleanroom implementation of a driver which competed with a proprietary offering from Nvidia – it now seems to have superceded that driver).

I noticed while doing large network transfers to or from one of these machines that the load on the machine seemed to spike. Running dmesg show a lot of these messages,

[1617484.523059] eth0: too many iterations (6) in nv_nic_irq.
[1617484.843113] eth0: too many iterations (6) in nv_nic_irq.
[1617484.869831] eth0: too many iterations (6) in nv_nic_irq.
[1617485.101377] eth0: too many iterations (6) in nv_nic_irq.
[1617485.855067] eth0: too many iterations (6) in nv_nic_irq.
[1617485.896692] eth0: too many iterations (6) in nv_nic_irq.

Google returns lots of results for this message – some people seem to have experienced complete lockups while others noted slowdowns. The proposed solution seems to be to pass the max_interrupt_work option to the forcedeth kernel module to increase the maximum events handled per interrupt. The driver seems to default to running in throughput mode where each packet received or transmitted generates an interrupt (which would presumably ensure the fastest possible transfer of the data) but can also be configured to operate in CPU mode (aka poll mode) where the interrupts are controlled by a timer (I’m assuming this makes for higher latency of transfers but smoother before for high network loads – the documentation on this is a little thin). This behaviour is controlled by the optimization_mode option. You can investigate the options which can be passed to any kernel module by running,

modinfo <module name>

for example,

modinfo forcedeth

So, as an initial pass at tuning the behaviour on the server, I decided to pass the following options to the forcedeth driver.

max_interrupt_work=20 optimization_mode=1

The standard way on Ubuntu 8.10 to set kernel module parameters is to add a line to /etc/modprobe.d/options like the following

options forcedeth max_interrupt_work=20 optimization_mode=1

I tried this and then tried rebooting my system but found it didn’t have any effect (I started seeing the same error messages after running a large network transfer and the max iterations were still referenced as 6 rather than the 20 I should be seeing if my options were parsed).

After trying a few things and brainstorming with the good people on #ubuntu-uk I figured out that you need to run the following command, after modifying /etc/modprobe.d/options

sudo update-initramfs -u

This presumably updates the initramfs image to include the specified options. It is possible this is only needed for modules that are loaded early on in the boot process (such as the network driver) – I haven’t had the need to modify other kernel modules recently (and this stuff seems to change subtly between distributions and even versions).

Following another reboot and a large network transfer, the error messages suggesting that the options are now being parsed (some kernel modules allow you to review the parameters currently being used by looking in the file /sys/modules/<module name>/parameters but forcedeth doesn’t seem to support this).

I figured the trick with update-initramfs was worth publishing for the next person who runs into this problem. I’d also love to hear from others using the forcedeth driver as to what options they find useful to tune the throughput.

Tags: forcedeth, kernel module, linux, nvidia, tuning, ubuntu 8.10