Fwd: XO 1.5 frequency scaling
Paul Fox
pgf at laptop.org
Sat May 8 09:03:59 EDT 2010
tiago wrote:
> ---------- Forwarded message ----------
> From: Tiago Marques <tiagomnm at gmail.com>
> Date: Sat, May 8, 2010 at 12:17 PM
> Subject: Re: XO 1.5 frequency scaling
> To: "Richard A. Smith" <richard at laptop.org>
>
>
> Hi Richard,
>
> On Wed, May 5, 2010 at 12:28 AM, Richard A. Smith <richard at laptop.org>wrote:
>
> > On 05/04/2010 07:01 PM, Tiago Marques wrote:
> >
> > > Let me see if I understand what you said. I understand how it works on
> > > desktop and regular laptops. You load the module for the specific power
> > > saving feature and either a kernel module to do the job or the userspace
> > > module which then allows a daemon to do the frequency scaling.
> > > Now the XO 1.5, AFAIK, isn't doing it or powerd is doing it(or openfw).
> > > I tried loading the C7 power saver module but it can't find the device.
> > > Is it already being taken and should I not worry with frequency scaling
> > > although /proc/cpuinfo always shows 1000MHz?
> >
> > The module is not enabled in the kernel build and in our case appears to
> > only offer saving in very limited cases. Yes, you could cap your max
> > frequency and max power draw but in general you end up using more
> > power*time that way because you keep all the other components that don't
> > have sort of power scaling up longer then they would have been because
> > it takes longer to do the task. If you decrease the power draw 2x but
> > then extend the time 2x you have gained nothing.
> >
> >
> Not my experience, but I'm desktop biased I guess. I was thinking that you
> could further lower the core voltage on the XO and get something like 50%
> the clock with 25% or less power. But, as you describe below, if VIA managed
> such an agressive power gating, it's the way to go.
>
>
> > Feel free to experiment though. The latest versions of powerd have
> > power logging built in and if you want a more specific measurement my
> > power logging scripts should be useful.
> >
> > IIRC I had to hack on the driver a bit to make it work. The following
> > is a summary email I posted from when I worked on it.
> >
> > Notes:
> > - Ignore the comment about C5. Our CPU does not support C5.
> >
> > - Ignore the comment about being scared to burn up the CPU. We now have
> > thermal throttling enabled and have tested it extensively. Unless you
> > turn that off you should not be able to burn up the CPU.
> >
>
> This is on olpc-powerd or the VIA C7 powersaver driver? I couldn't find
> anything related to over/underclocking in powerd.
you're right. there's nothing in powerd related to clocking. powerd
limits itself to managing the display, wlan on/off, and system suspend.
> Did I miss some firmware or kernel update, as I can't load the c7 powersaver
> kernel module in a kernel I built myself.
are you sure you're running the kernel you built? (i.e., it, and
whatever symlinks it needs to vmlinuz, need to be in /boot and
/bootpart/boot.)
paul
>
>
> >
> > ======================
> > I spent the day/night today working on getting our C states and P states
> > enabled.
> >
> > The good news is that I got C4,C5 and frequency/voltage scaling (P
> > states) working.
> >
> > The bad news is that C5 causes memory corruption and P states don't help
> > much.
> >
> > Enabling C4 seems to save us about 170mW in idle.
> >
>
> Any measurement on how low it goes in C4?
>
>
> >
> > C5 should save us a bit more but with it enabled the system won't boot.
> > It gets all sorts of funky ext4 errors. C5 turns off the L2 cache and
> > the docs say you should flush before entering. I suspect thats not
> > happening.
> >
> > P states currently don't seem to save us enough to be measured. One
> > reason is that our core voltage is set by default to be very close to
> > the minimum. Its at .796V and the minimum is .7V with scaling enabled
> > (+ code hack) the minimum setting drops Vcore to .73V. Its supposed to
> > go to .7 but the volt meter says otherwise. 60mV diff doesn't offer a
> > whole lot of savings.
> >
>
> I see, I thought they could drop it even further.
>
>
> >
> > The CPU frequency slides between 400Mhz and 1GHz and you would think
> > that it would make a large difference but the meter says otherwise. How
> > can that be you ask? The answer is because Linux issues a hlt when
> > idle. If you run the test under OFW then you can create up to 1.5W of
> > power difference by sliding the freq from min to max [1] and holding the
> > Vcore constant. But in idle not so much. The processor already does a
> > very good job of gating the clocks.
> >
> >
> Nice, kudos for VIA.
>
>
> > So this brings us back to what we already knew. The big money on power
> > savings is in our special sauce idle suspend.
> >
> > [1] Turns out you can overclock the processor. Via lists the max
> > multiplier at 16x FSB (100Mhz) which is 1.6Ghz even though its listed as
> > a max of 1Ghz. However if you continue to put values into the multiplier
> > register the power draw continues to increase. I stopped when the
> > system draw had hit 9W cause the XO on the power meter does not have a
> > heat spreader and I didn't want to take the chance of burning it up.
> > =================
> >
>
> The heatspreader I can hack with a heatpipe and some coolers, I'm going to
> do it anyway since it is already going to 85ºC in load. The speed is of some
> use to me most of the time, I'm just worried that the VRM can't handle the
> extra current.
>
> Best regards,
> Tiago
>
>
> >
> > --
> >
> > Richard A. Smith <richard at laptop.org>
> > One Laptop per Child
> > _______________________________________________
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> > Devel at lists.laptop.org
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> >
> part 2 text/plain 129
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paul fox, pgf at laptop.org
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