WOL: how does it work?

Donald Becker becker at scyld.com
Fri Sep 7 22:31:00 PDT 2001

On Fri, 7 Sep 2001, Martin Siegert wrote:

> I am trying to get wake-on-lan (WOL) to wirk on my Beowulf cluster
> Unfortunately, I have been unable to wake up a node. Here is what I do:
> "halt" a node. Detach the power cable. Reattach the power cable.
> At this point the lights on the two onboard NICs (the Tyan web site
> and the printing on the chips say that those are 3c920, the 3c59x driver
> identifies them as 3c980; I don't know whether that is relevant; the NICs
> work fine) come on.

This is likely a ethercard-specific problem.  Pre-CX 3Com cards don't
automatically go into wake-on-LAN mode.  The driver must first be
loaded, and the card left in the correct state (TotalReset + ACPI-D3).
My 3c59x.c driver takes care to do this properly.

I believe that the new 3c905CX cards do have a setting for automatically
configuring the card for WOL with just stand-by power.

Most other Ethernet adapters enable wake-on-magic-packet when stand-by
power is first applied.

> A Tyan technician told me that WOL on the Thunder K7 is
> always on, no special BIOS setup would be needed.

Likely true.  If you use a WOL cable, the ethernet adapter almost
literally pushes the power butter.  If you rely on standby power from
the PCI slot, the chipset must default to treating the PME signal as a
power-on signal.

> They also told me that I have to use a 2.4.x kernel because only those
> would support APCI.

That's false, and mostly not relevant.
My pci-scan code adds PCI power management state control to the 2.2
kernel, which is part of the ACPI spec.

The only aspect which is relavent is the ability to soft power down the
system.  That might require an ACPI Control Language interpreter if your
motherboard does not have APM functions.

> I don't understand why the kernel is important here: when the node is
> halted what difference does the kernel make for the receiving of the
> magic WOL packet that is supposed to wake up the box?

Yup.  After restoring power, the OS has never had a chance to run.  Only
the power-down procedure depends on the kernel support.

Donald Becker				becker at scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993

More information about the Beowulf mailing list