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In The Lab
Overclocking - a Closer Look at System Stability
by chris

Last month I discussed the basic overclocking process and touched on some of the issues you could encounter. To continue with this theme, let's take a closer look at some of the items that can impact your results.

Cooling: One of the side effects of running a CPU at higher bus speeds and core voltages is an increase in the amount of heat produced. It is critical to be able to remove the heat consistently and keep the CPU and surrounding components within a reasonable temperature range. I mention the surrounding components, because you will typically find the motherboard chipset and several power regulators located near the CPU socket. Some boards, like the Asus P5W DH Deluxe, have a passive (no fan) heat sink on both the chipset and regulators, connected together with a conductive heat pipe. The taller heat fins on the regulator depend on air movement from the CPU cooler to move air across the fins to keep these components at a reasonable temperature. While water cooling provides much superior cooling for the CPU, most water cool kits have no auxiliary fans located on or near the heat blocks. The result is that your CPU stays at a much lower temperature at all times, but the chipset and regulators will run at much higher temperatures than recommended. The Asus kit includes a special fan just for this purpose, in case you install a water cooling solution.

During testing with an Asus P5W motherboard with an Intel Pentium D 805 (2.66 GHz) CPU that was overclocked to 3.76 GHz (188 MHz x 20), I compared the cooling capabilities of several high-performance CPU coolers and one water cooling kit. To obtain a realistic comparison of their efficiency, nothing was changed in either the system configuration or overclock settings. MG brand white heat transfer compound was used on all heat sinks. Ambient air temperature remained in the 22-23 degree C range during testing.

Heatsink Idle temp Stress temp
Zalman CNPS9500LED-Cu 32 C 58 C
Zalman CNPS7700-Cu 35 C 65 C
Scythe Katana 775 37-38 C 66 C
Zerotherm BTF90 40 C 68 C
Intel boxed heatsink (Pentium D 805 retail package) 44-45 C 69 C
Corsair Nautilus 500 Water Cooling* 26 C** 48 C
*Possible issue with system board overheating can result from no CPU fan air movement. Nearby motherboard chipset and power regulators are typically passive in nature.
** The 26 degree C idle temperature was the stable temperature achieved after installation ("cold start"). During normal operation of the computer with the CPU running for a period of time long enough to bring the reservoir liquid up to an equal level, I would expect a higher reading more consistent with the air-cool solutions, and in the 30-32 C range.

You need to maintain a reasonable cooling level to prevent damage to the CPU or other components. Even if the system is not immediately damaged by over voltage or heat, degradation and a shorter operation life will be the result of exceeding either. A consideration with most Intel CPUs is the thermal protection feature where the CPU will "throttle back" to reduce overheating above pre-set levels. Since the point of overclocking is to gain additional performance, having the CPU slow itself down to stay cool defeats the purpose entirely.

A spare fan sits on the power regulator heat fins during testing to provide cooling that is not being provided by the water cooling kit. A fan mounted in the side panel of your case might provide adequate ventilation. Use the PC Health monitoring feature in the BIOS or one of the Asus utilities to monitor temperature of CPU and motherboard levels. Some boards will specifically monitor the power regulator temperature as well.

CPU: The main component being stressed during overclocking is the CPU; previous testing of several different CPUs identified different potentials in terms of how fast I could reliably push them. Using an Asus P5WD2 motherboard, the same memory, power supply, and Zalman 9500 CPU cooler. I compared five different Intel processors, and got wildly different capabilities. Both Asus Overclocking Profiles and Manual overclock settings were tried.

CPU Base GHz Overclock %
Pentium 505 2.66 4.00 50%
Pentium 532 3.06 4.14 35%
Pentium D 805 2.66 4.00 50%
Pentium D 820 2.80 3.08 10%
Pentium D 830 3.00 3.00 0%

One interesting thing I noted with these results was that both 2.66 GHz CPUs could be apparently be pushed to a very high level. My question at this stage was if it was reproducible or if individual CPUs each had a unique performance range of operation. To test this, I obtained five more of the Pentium D 805 processors, and ran them through the Asus P5W DH motherboard.

While the previous testing showed a capability of running at speeds up to 4GHz, stability testing was only done using a bootable diagnostic disk with CPU burn-in routines running overnight. To achieve slightly faster (and probably more significant) stability results, I installed Windows XP with all of the current service pack updates, and driver support, and used a combination of S&M, SuperPi, and Prime95 programs to stress the CPU at 100% load for several test cycles.

Degrees of instability: On the Asus board, the worst overclock failure caused the system to hang and a no-POST startup failure. Other overclock failures usually turn up in the form a blue screen error and the system rebooting, assuming I could get Windows to start at all. If any errors turned up, speed settings were reduced and then tried again. Overnight testing was then used to eliminate any subtle intermittent errors. CPU core voltage was never raised above about 1.3875v, although some improved stability has been achieved by raising this as high as 1.4375v.

Results of Pentium D 805 overclock testing on the Asus P5W DH Deluxe motherboard:

CPU Base GHz Overclock %
Pentium D 805 #1 2.66 3.46 30%
Pentium D 805 #2 2.66 3.60 35%
Pentium D 805 #3 2.66 3.46 30%
Pentium D 805 #4 2.66 3.46 30%
Pentium D 805 #5 2.66 3.70 39%
Pentium D 805 #6 2.66 3.80 43%

What I found with this particular board was that I could not duplicate my earlier 4.0GHz result, although I suspect this is only because the testing was more stressful and better detected any instability in the CPU or memory. Indeed, I could set several (but not all) of these CPU to run at 4.0GHz and boot into Windows, as long as I kept the CPU activity to low levels (i.e. less than 30-40% utilization), the system would run for hours or even overnight without issue. But as soon as I would launch one of the heavy-load stress test programs, either the test would fail, or more often, the system would reboot spontaneously (sometimes with a blue-screen error). Every one of these CPUs would run stable at 30% overclocking using the Asus Overclock Profile settings. Manual settings were much more sensitive, and do seem to show that there are differences in CPUs, although not as great as you might expect. Of the six CPUs, I could not get three of them to perform better then the 30% profile settings and still pass overnight CPU stress testing at 100% load.

Power Supply: Power supplies can be a definite limitation to system stability, especially when overclocking or loading down the system with high-end video, lots of memory and storage drives. Most supplies will have a specification table on the side of the unit or on the box that details the power available on each of the different voltage lines. As you load down your system, and the components draw more power, the ability of a given power supply must be enough that the voltage level does not vary or drop below a certain range. If you run close to the maximum available, you also increase the risk of the power supply overheating or failing prematurely. (Although the odds are that you will see other stability issues long before this occurs.) More likely, the system will not even power on, will continually reboot, or shut itself down if you are operating too close to its available capacity. Check motherboard and video manufacturer power recommendations especially, and then go at least the next level higher if you want to try to overclock on top of this.

The key value I watch when overclocking is the CPU core voltage. The power levels that can be maintained when idle and when stressed are going to be different, as are the values when overclocking a CPU. For example, on the P5W DH motherboard, an Intel Pentium D 805 operating at the system default of 2.66 GHz idled at 27 C (ambient 24.7 C) with a core voltage maintained at 1.30 volts. Under stress, the temperature jumped to 46 C, and the core voltage dropped to 1.27v. During overclocking with manual settings, the core voltage was set to as high as 1.3875v in the BIOS menus; the CPU temperature increased to 57 C, and the core voltage still stabilized at 1.28 volts under stress.

As a rule, this power level is not generated directly from the power supply, but by adjustable regulators on the motherboard. However, if the power level supplied to the regulators drops below a certain level, the output voltage will also drop. The stability of the CPU can be improved in many overclocking situations by increasing the core voltage level. This must be done with care, because while the CPU may have thermal protection, protection from under or over-voltage conditions must be done externally. Even intermittent fluctuations of over-voltage can degrade the processor and shorten its operating life, a situation you may have no indication of until your computer stops working.

Once you have all of the parts to build your system, then you can look forward to spending hours twiddling with the settings to overclock it to the highest level possible. That, or just take the easy way out and settle for the 10-30% boost over the factory settings by using the "overclock profile" settings. One way to look at this is that overclocking a system really is like a challenging game that you must juggle half a dozen settings just right to "win."

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