Information on XO-1 power efficiency
hmurray at megapathdsl.net
Wed Nov 11 15:04:00 EST 2009
pgf at laptop.org said:
> certainly -- you can point out that our latencies for waking/ sleeping
> are just a little too long (mainly due to USB device discovery) to
> make that feature comfortable to use "between key-strokes".
How well does sleep-between-keystrokes work if I ignore USB?
Has anything interesting changed in this area for XO-1.5?
Is USB device discover inherently slow? Or is the sloth just handling
strange cases? Can I speed things up if I only need to verify that devices I
knew about are still there?
Has anybody tried to work out the details of when USB needs to be
Is there a blog/wiki page on this area? If so, where, and sorry for the
My checklist would be something like:
On XO-1, WiFi is on USB, but there is a separate wakeup line for it so we
don't need to turn on USB because of WiFi unless it has a packet or we want
to send something.
Disks and SD cards don't need attention unless we want to do something...
For things like keyboards and serial adapters and probably many
toys/gizmos, we have to leave USB running in order to notice when they need
attention. That means leaving the CPU on too. We don't need to do that if
the device isn't in use (file not open).
We might need an ioctl to tell the kernel whether a device needs to be
polled (keyboard case) vs ignored unless we expect something (disk). I think
we can make a good guess at compile time, but we might need a tool to tell
the kernel how to handle strange devices. (udev might help)
User code might need tweaking to tell the kernel when a device can/can't be
powered down, either by an ioctl or by closing/reopening the device.
If the CPU is powered down, it won't notice when a new USB device gets
plugged in or one is unplugged. I think I could live with that for serious
power savings, especially since no-USB is the common case. You would have to
poke a special key or something like that to get the CPU to do USB device
1) Disks aren't really that simple. If the CPU is off you won't notice when
the disk gets unplugged. So somebody could unplug a disk, take it to another
CPU, read/write some files, bring the disk back to the still sleeping XO, and
the XO would never notice. If the XO had cached data in memory, that data
needs to get flushed. If the XO had dirty pages in memory, you are in
trouble. Open files should probably get errors. There is probably a mount
count/time that could be checked to catch this case.
These are my opinions, not necessarily my employer's. I hate spam.
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