Comments on: Forthcoming p4-clockmod changes.

By: gary.trakhman

gary.trakhman — Wed, 28 Jan 2009 17:58:16 +0000

I don’t know what else to say except to give you a data point. If my celeron M is using 100% cpu usage to do a task and I let it run at 1.6GHz, it runs hotter than doing the same task at 100% cpu usage at 800Mhz throttled down with p4_clockmod. The battery lasts about 20% longer.

More heat= more power/minute being drained from the battery, means less battery life.

However, I don’t even care, as I sold it for a core 2 duo laptop recently that has c-states and can use cpufreqd. I’m just arguing b/c it could affect some people who rely on p4_clockmod’s function like I did.

Celeron M does not have c-states or speedstep, unlike pentium M, which can lower its own voltage. So for that processor, p4_clockmod was the best you could do to achieve marginal power savings during a scenario like what I describe.

By: davej

davej — Wed, 28 Jan 2009 17:47:58 +0000

Where do you that that ‘more heat’ is coming from? Whilst the CPU is being throttled it’s being prevented from entering lower C states. So you’re effectively disabling runtime power management.

C states are used even on non idle systems, it’s why there are multiple of them.

By: gary.trakhman

gary.trakhman — Wed, 28 Jan 2009 17:37:43 +0000

the above statement is a consequence of thermodynamics and energy balance… all the power gets converted to heat. More heat=more watts used.

By: gary.trakhman

gary.trakhman — Wed, 28 Jan 2009 17:35:07 +0000

nah, man. If it’s getting hotter at a higher rate, that means it’s spending more power.

By: gary.trakhman

gary.trakhman — Wed, 28 Jan 2009 17:34:10 +0000

I guess what you don’t understand is, in the case of a 3d game, the workload depends only on how long you’re playing, and it will use all the resources it can ALL the time.

It doesn’t stop processing sooner at a faster cpu rate like other workloads such as compiling, etc.

By: davej

davej — Wed, 28 Jan 2009 17:33:53 +0000

logic state changes aren’t even on the radar when it comes to power saving. The only way to get noticable results is by either reducing the amount of current going to those gates (through frequency adjustment), or by switching a power domain off completely. Clock modulation does neither of these.

A thermal event is defined by the platform in ACPI. See /proc/acpi/thermal_zone/*/* for details.

By: gary.trakhman

gary.trakhman — Wed, 28 Jan 2009 17:29:58 +0000

you sure do save power… extra power is spent every time a flip-flop or other logic changes state. This happens less often when there’s less work being done. A 3d game tends to do as much work as it can to provide the best user experience, but it’s really not necessary for it to take 100% of the cpu. They’re just programmed to do so.

Now, I guess the question is, with this change we users lose flexibility, yes? how do you define a thermal event? Can we users define a thermal event through the acpi interface?

Could I write somewhere in a config file… if the temp gets above X, then throttle down to 25%?

By: davej

davej — Wed, 28 Jan 2009 17:22:09 +0000

Exactly the same answer. If a thermal event occurs, it will automatically adjust for you. Also, re-read the part where I mention that you’re not saving power. The effect on battery life from throttling is negligible.

By: gary.trakhman

gary.trakhman — Wed, 28 Jan 2009 17:19:21 +0000

How about this case, I used it frequently in the past on my celeron M laptop.

3d games tend to take 100% cpu to do non-critical processing which is a battery sucker on laptops. It’s nice to be able to throttle down the duty cycle of the cpu in this case for prolonged use during battery discharge.

By: davej

davej — Mon, 26 Jan 2009 17:27:00 +0000

The thermal monitoring code does it for you.