Here's a patch which is designed to make the kernel play more nicely
with portable computers.  I've been using it for a couple of days
and it seems to do the right thing.  I'm wondering if anyone has
any comments/suggestions/etc.

To test this code you'll also need
http://www.zip.com.au/~akpm/linux/patches/2.4.20/pdflush-sysctl.patch
(hmm.  Server seems to be dead.  So the patches are here, as attachments)

Here's the algorithm, from the Documentation/filesystems/proc.txt
section describing /proc/sys/vm/:

laptop_mode
-----------

Setting this entry to '1' will put the kernel's dirty data writeout
algorithms into a mode which is better suited to laptop/notebook
computers.  This mode is specifically designed to minimise the
frequency of disk spinups.  Laptop mode works as follows:

- Dirty data remains in memory for longer periods of time (controlled
  by laptop_writeback_centisecs).

- If there is pending dirty data and the disk is spun up for any
  reason (even for a read) then all dirty data will be written back
  shortly afterwards.  ie: when the disk is spun up, make good use of
  it.

- When the decision is made to write back some dirty data, the kernel
  will write back all dirty data.

laptop_writeback_centisecs
--------------------------

This tunable determines the maximum age of dirty data when the machine
is operating in Laptop mode.  The default value is 30000 - five
minutes.  This means that if applications are generating a small amount
of write traffic, the disk will spin up once per five minutes.

If the disk is spun up for any other reason (such as for a read) then
all dirty data will be flushed anyway, and this timer is reset to zero.

laptop_writeback_centisecs has no effect when the machine is not
operating in Laptop mode.



This implementation doesn't try to be very smart - there's a direct
call out of do_ide_request() into the writeback code.  This couldn't
be done from within ll_rw_blk.c because then a write to the ramdisk
would spin the disk up.   Even as-is, a read from the IDE CDROM
drive will cause the IDE hard disk to spin up and flush data, so
probably that call in do_ide_request() should only be made if the
device is writable.   Suggestions are sought, but let's try not to
get too fancy here...