very common kernel modules slow down the boot process

Will Woods wwoods at redhat.com
Wed Apr 2 04:27:18 UTC 2008


On Apr 1, 2008, at 7:22 PM, Harald Hoyer wrote:
> Compiling these modules, which are loaded on nearly every PC, in the  
> kernel cuts down my boot time from 42s to 32s on my computer:

It's pretty well known that modprobe is slow. Heck, you've reported  
bugs about this before:
https://bugzilla.redhat.com/show_bug.cgi?id=249270 - modprobe is slow

Will Cohen (here at Red Hat) did some profiling and found one of the  
major problems was that modprobe takes a long time to search /lib/ 
modules/`uname -r`/modules.deps:
http://sourceware.org/ml/systemtap/2007-q1/msg00140.html

IIRC one of the proposed solutions was to have 'modprobed' - a  
modprobe daemon which would read modules.deps at system startup and  
handle all subsequent modprobe requests from udev etc.

Mac OS X does a similar thing; see
http://www.osxbook.com/book/bonus/misc/optimizations/#TWO
and its man pages for kextd(8) and kextcache(8).

The way they do early boot (the parts in our kernel & initrd) is  
essentially the same as us. One interesting optimization is that they  
support booting from a 'kernelcache' - a big fat image that has the  
kernel with the modules needed to find the root device *already linked  
in*. Weird. Other than that, as far as I can tell, it's basically just  
a kernel+initrd with modules in it.

Once they're into the full system, they start up kextd, which loads  
kernel module metadata and handles all further requests for module  
loading.

We, on the other hand, re-read the entire module dependency list every  
time anything requests a module be loaded. Which happens a *lot* at  
system startup. Yuck.

So, yes. There's a fun summer project for someone: modprobed.

-w




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