do Pentium4 & PentiumM consume about the same at their lowest possible freq?
Hi all,
I am not sure this is the right place where posing my question, if not please suggest me a better one!
I come to the question.
I happily used the speedstep kernel module with my centrino laptop and it really made a difference in terms of battery duration (actually now I have a celeron-m based laptop and the difference does not seem so evident, but I still have little statistic on it).
Now I need to replace my main home 'desktop' PC and I am tempted to buy a PentiumM-based minit-itx MB (e.g. the AOpen i945GTt-VFA) in order to decrease the power bill (my PC is on 24/7), but since mini-itx MBs are quite expensive I'd prefer first to know the following: can the "Intel Pentium 4 clock modulation" kernel module (or maybe the "AMD Opteron/Athlon64 PowerNow!" one) bring the corresponding CPUs to consume as little as the "Intel Enhanced SpeedStep" does for a centrino? This info is quite important but I could not find it anywhere!
Thanks,
Roberto
Quoted 4 lines · Roberto De Leo
Hi all,
I am not sure this is the right place where posing my question, if not please suggest me a better one!
Quoted 13 lines
I come to the question.
I happily used the speedstep kernel module with my centrino laptop and it really made a difference in terms of battery duration (actually now I have a celeron-m based laptop and the difference does not seem so evident, but I still have little statistic on it).
Now I need to replace my main home 'desktop' PC and I am tempted to buy a PentiumM-based minit-itx MB (e.g. the AOpen i945GTt-VFA) in order to decrease the power bill (my PC is on 24/7), but since mini-itx MBs are quite expensive I'd prefer first to know the following: can the "Intel Pentium 4 clock modulation" kernel module (or maybe the "AMD Opteron/Athlon64 PowerNow!" one) bring the corresponding CPUs to consume as little as the "Intel Enhanced SpeedStep" does for a centrino? This info is quite important but I could not find it anywhere!
In general, no -- you are better off with a mobile-optimized processor.
indeed, under many conditions, using pentium 4 clock modulation will actually increase energy use, rather than decrease it.
-Len
Quoted 5 lines
indeed, under many conditions, using pentium 4 clock modulation will actually increase energy use, rather than decrease it.
Jeez, that's odd!
Then maybe this is the reason why I don't notice any particular improvement when I slow down my celeron-m, which uses indeed the P4 modulation sw rather than the speedstep centrino. But then, why slowing down the CPU at all if it doesn't save you any watt? I guess I'll stop using it. Thanks a lot for the preciuos info,
Roberto
Quoted 15 lines
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Roberto De Leo
Sent: Friday, August 03, 2007 1:16 PM
To: [email protected]
Subject: Re: do Pentium4 & PentiumM consume about the same at their lowest possible freq?Quoted 3 lines
indeed, under many conditions, using pentium 4 clock modulation will actually increase energy use, rather than decrease it.
Quoted 7 lines
Jeez, that's odd!
Then maybe this is the reason why I don't notice any particular improvement when I slow down my celeron-m, which uses indeed the P4 modulation sw rather than the speedstep centrino. But then, why slowing
down the CPU at all if it doesn't save you any watt? I guess I'll stop using it.
It is meant to be used under thermal condition where slowing down CPU will result in reducing the temperature and not for power savings in normal conditions.
Thanks,
Venki
Quoted 8 lines
Quoted 4 lines
why slowing down the CPU at all if it doesn't save you any watt? I guess I'll stop using it.
It is meant to be used under thermal condition where slowing down CPU will result in reducing the temperature and not for power savings in normal conditions.
I thought that power consumption and heat dissipation in the CPU were more or less proportional but evidently my belief was too naif :-)
I guess that the same thing holds for the Athlon64 so prolly centrino is the only CPU were I can get a substantial power saving by decreasing the freq when I don't need much of it (which is almost always), is this correct?
Thanks,
Roberto
Quoted 8 lines · Roberto De Leo
Quoted 8 lines
Quoted 4 lines
why slowing down the CPU at all if it doesn't save you any watt? I guess I'll stop using it.
It is meant to be used under thermal condition where slowing down CPU will result in reducing the temperature and not for power savings in normal conditions.
Quoted 7 lines
I thought that power consumption and heat dissipation in the CPU were more or less proportional but evidently my belief was too naif :-)
I guess that the same thing holds for the Athlon64 so prolly centrino is the only CPU were I can get a substantial power saving by decreasing the freq when I don't need much of it (which is almost always), is this correct?
No. AMD64 does the right thing too. The P4 only throttles, the AMD64 and centrino chips actually scale their frequencies down. Throttling and frequency scaling are different beasts, with freq scaling being far more worthwhile.
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Quoted 8 lines
Quoted 4 lines
It is meant to be used under thermal condition where slowing down CPU will result in reducing the temperature and not for power savings in normal conditions.
I thought that power consumption and heat dissipation in the CPU were more or less proportional but evidently my belief was too naif :-)
I guess that the same thing holds for the Athlon64
Most processors reduce power consumption faster than they reduce frequency, because reducing frequency allows the processor to also reduce voltage. There is a corner case that if you only reduce frequency, you do not get much power reduction, but the standard powernow-k[7,8] driver reduces both voltage and frequency.
The only numbers I have handy are a bit old, but here's the original Opteron/Athlon FX at 2.2 GHz:
Frequency (GHz) Voltage (V) Power (W)
2.2 1.5 89
2.0 1.4 69
1.8 1.3 50
1.0 1.1 22
A drop of 50% in frequency cuts maximum power
by more than 66%. Actual measured power may be
lower, since idle blocks consume less power,
and a processor at minimum pstate may also be
going into C1 (Halt) a good deal of the time.
-Mark Langsdorf
Operating System Research Center
AMD
FYI, I work at AMD currently, and worked at Intel in the past. The model discussed below is an approximation, and some circuit or process geek can correct me if they care to. ;-)
Quoted 7 lines · Roberto De Leo
I thought that power consumption and heat dissipation in the CPU were more or less proportional but evidently my belief was too naif :-)
I guess that the same thing holds for the Athlon64 so prolly centrino is the only CPU were I can get a substantial power saving by decreasing the freq when I don't need much of it (which is almost always), is this correct?
-- "Frequency Scaling" is when the freqency is increased or decreased,
usually by fixed factors or to fixed step values. In an ideal world, power consumption is linear with frequency given you keep the voltage(s) the same. In practice, you have leakage, so, roughly, at a given set of voltages the eq, looks like:
Power = Fixed_Power + Freq * (watt/Hz)
NOTE: The Fixed_Power and (watt/Hz) values vary based on the processor and voltage type.
-- "Throttling" is a bit like frequency scaling. The number of clock
edges is reduced, often by stuttering (times the clock is running vs. times it is not), with similar power results.
-- "Voltage Scaling" roughly changes power as a square with another
semi-constant:
Power = Fixed_Power_2 + V^2 * (watt/V^2)Now, with all that intro, here's the mechanisms used by the different chips involved to my knowledge:
-- Pentium 4 supported Throttling - used to keep the part from
overheating, and not really useful for power savings in general.
-- Pentium M, Core, and Core 2 support freq/voltage scaling and maybe (?)
also Throttling. Don't know when they started supporting it in server configs.
-- (don't remember what Athlon (K7) supported, I think the mobile
version had freq/voltage scaling)
-- Opteron/Athlon64 support freq/voltage scaling across the whole line.
I think the multi-socket (i.e. SMP server) support for freq/voltage scaling only came into products since about 2004-2005. I know Opterons/Athlon64's had it in 2004, but don't know/remember where it intersected into Intel products.
Results of all this says that you get *less* than linear benefits from scaling frequency back or throttling alone. Luckily, you can often reduce voltage when scaling back the frequency, and in that case it's really the voltage reduction which gives you most of the power savings.
So, whenever possible use a freq/voltage scaling combination, which is what Enhanced Speedstep and Powernow have been doing for a while.
--
Erich Stefan Boleyn <[email protected]> http://www.uruk.org/
"Reality is truly stranger than fiction; Probably why fiction is so popular"
Quoted 8 lines · Erich Boleyn
Results of all this says that you get *less* than linear benefits from scaling frequency back or throttling alone. Luckily, you can often reduce voltage when scaling back the frequency, and in that case it's really the voltage reduction which gives you most of the power savings.
So, whenever possible use a freq/voltage scaling combination, which is what Enhanced Speedstep and Powernow have been doing for a while.
the situation is more complex than this though....
The CPU uses a LOT less power when idle. In practice on most cpus (except when you overclock like crazy and go to high voltages) it's more power efficient to go full speed to be idle as quickly as possible, so that you get the power saving from idle.
This is especially the case for things like throttling; where you basically keep the cpu just at a higher voltage for a longer time than you would have otherwise...
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