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Performance and Power Consumption Control Features in Intel Pentium 4 and Intel Xeon Processors – Different Names, the Same Principles


Bottom line

It's time we draw the conclusion of our today's research. Let's dwell on each of the reviewed and tested technologies and briefly enumerate and comment on its key features.

Automatic thermal monitoring mechanism #1 (TM1)

TM1 is an interesting and useful technology that prevents CPU overheating in emergency cases (for example, when a fan on the CPU cooler fails). Of course, nothing stops this technology to work in standard situations – for example, with a low-quality cooling system. To our mind, this technology has one serious drawback in this connection: TM1 effect is absolutely transparent for the operating system as well as to inexperienced users and typical sysinfo software. Why? Because an operating system as well as popular utilities like CPU-Z or WCPUID will tell users that the CPU clock in their systems is still 3.4 GHz (it's an example) and the CPU load is 100% (at full load). But in fact the CPU may be actively "throttling", that is operating at its minimum 46% and shock a user with its performance.

Automatic thermal monitoring mechanism #2 (TM2)

TM2 is a very similar (because it is actually based on the same principle – clock modulation), but still improved version of the TM1 technology. The key improvement consists in reducing voltage when TM2 snaps into action (that is when a CPU is overheated). It's useful both from the point of view of extending the CPU service life as well as its performance (which drops to a lesser extent in TM2 than in TM1). An heir to TM1, TM2 is not without the same drawback – its complete transparency to users.

Enhanced SpeedStep (DBS) technology modification for servers

Paradoxical, but true: Enhanced SpeedStep technology for servers differs little from TM2. The only difference consists in how they snap into action – TM2 operates only on CPU overheating, while DBS can be enabled or disabled "on-demand". It also allows automated performance management depending on CPU load (this function is an integral part of RMClock). From the point of view of the idea (and technology name), Enhanced SpeedStep for servers is a complete counterpart of the mobile Enhanced SpeedStep as well as AMD PowerNow! (mobile version implemented in Mobile Athlon XP) and AMD Cool`n'Quiet (desktop/mobile version implemented in Athlon 64 series processors). But from the implementation point of view it isn't. Enhanced SpeedStep for servers modulates CPU clock (which is certainly easier from the implementation point of view, taking into account that the clock modulation mechanism has been available in Pentium 4 for a long time), while the mobile Enhanced SpeedStep and proprietary AMD technologies honestly change the CPU multiplier "on the run". A nice fact – both technologies can effectively change CPU voltage, which has a much greater effect on CPU power consumption (to the simplest approximation, CPU capacity has a linear relation to its clock, and a quadratic relation to voltage). But what's so wrong with Enhanced SpeedStep for servers? Nothing's wrong actually, but it has the same drawback – CPU multiplier changes (its effective clock actually) are completely invisible to users.

On demand clock modulation (ODCM)

It's an interesting technology, which seems to allow a considerable decrease of CPU clock in idle mode and thus a considerable power consumption reduction. We have a question in this connection – why this technology is not enabled by default? The answer came rather suddenly – we decided to compare CPU temperature in idle mode "as is" (at 3.4 GHz) and at 50% modulation (at the throttled CPU clock of about 1.9 GHz). And we got unexpected results – the CPU zone had absolutely the same temperature in both cases (about 46°C)! Come to think of it, it's quite logical: when a CPU is halted (HLT), it's quite natural to expect of it to go into sleep (power saving) mode. In other words, it doesn't matter whether the clock is modulated or not. Wait... does it mean that the effect of the CPU multiplier reduction in DBS will also be that insignificant? Unfortunately, we had no opportunity to check this assumption in practice, but it may really be so...

Instead of the conclusion

When the article was still in progress, there appeared a new utility ThrottleWatch from Panopsys on the freeware web sites. It's used for detection and quantity evaluation of throttling in Pentium 4 series processors. As it's obviously appropriate to this article, we decided to review its key features and compare them with the features of new (soon to be released) RMClock 1.3.

We've performed a standard step: we took Pentium 4 processor (Prescott), ran RMClock and ThrottleWatch, applied the 100% load, and then stopped a fan on the CPU cooler.


A very interesting result! RMClock can detect precisely the drop of "effective" CPU clock, but ThrottleWatch still assures users that the CPU operates at full capacity, without throttling...

Summing it all up, ThrottleWatch is certainly a useful utility. Considering that it's the first specialized utility (from the non-specialized utilities we should first of all mention CPU Stability Test from the CPU RightMark benchmark) that can detect such a thin and user-transparent Intel technology as CPU throttling. Nevertheless, the throttling measurement methods in ThrottleWatch obviously leave much to be desired. In this connection, we can only recommend our readers to wait for the announcement of the new version of RMClock, which can detect any CPU throttling forms.


Dmitri Besedin (dmitri_b@ixbt.com)
February 9, 2005

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