Whereas there are a lot of tools available for overclocking in Windows, it is a fruitless task looking for similar applications for Mac OS X. This has all changed now. The ZDNet Clock tool for Mac Pro raises the frequency of the processor, FSB and memory.
ZDNet Clock for the Mac Pro is easy to operate.
Macintosh computers lack a BIOS setup entirely - and Windows tools will not run under Mac OS. So ZDNet.de developed the overclocking tool "ZDNet Clock" for Mac OS X. The current version 1.0 supports only the Mac Pro and the Apple server Xserve. An Intel processor and the current operating system version, Mac OS X 10.5.x Leopard, are also required.
ZDNet uses three Mac Pros as test machines. One comes from the first Intel/Mac Pro generation (Mac Pro 1.1) with 65-nanometer processors and 1333-MHz front side bus. The others come from the third generation with 45-nanometer processors and 1600-MHz front side bus, as sold by Apple since January 2008 (Mac Pro 3.1). The first computer is equipped with two 2.66 GHz X5355 processors[6], and runs stable at 3.10 GHz, see figure 2[7]. The other two have two 2.80 GHz E5462 processors[8]. These can be overclocked up to 3.24 GHz and remain stable, see figure 1[9].
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ZDNet Clock: Overclocking for Mac Pro[10]
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The improvement in performance is quite remarkable. For example, the cheapest Mac Pro in the current series with a 2.80 GHz processor runs significantly faster after overclocking than the most expensive at 3.20 GHz. This is because in addition to the CPU frequency, the main memory clock speed is also increased, which results in an additional increase in speed. The price difference between these two machines is 1430 Euros.
ZDNet plays it safe when overclocking. ZDNet Clock does not use any voltage increase for the CPU, the front-side bus (FSB) or the memory modules. Voltage increase leads in increased electro-erosion and shortens the life of the modules in question. In order to designate an overclocking trial as stable in the ZDNet test, the computer has to perform the mprime[11] torture test for 24 hours without any errors. And in addition to this, there must not be any reports of ECC errors from the FB-DIMM main memory modules.ZDNet Clock is easy to install. After downloading[12], you simply click on the ZDNet-Clock icon to start. Then the chipset and the clock chip are detected. The superuser password has to be entered at this point. The only computers currently supported are those equipped with Intel 5000 or Intel 5400 chipsets. That means all Mac-Pro and Xserve models with Intel processors. If the computer is not recognised the program closes with an error message.
The console can be used for diagnosing problems. It is to be found under "Applications – Utilities – Console". The ZDNet-Clock messages can be found in the "All Messages" window, see figure 3[13]. When the chipset and clock chip have been detected the application appears as shown in figure 1[9] and figure 2[7].
The main control is the slider control, with which the frequency is set. The left-hand stop is always the standard frequency of the clock chip. "Underclocking" cannot be achieved with the GUI. Three frequencies are displayed above the slider control. "Bus Clock" displays the frequency that is actually generated by the clock chip. The front-side bus is also clocked with this frequency. As current FSBs are four times as wide as the original Intel FSB, Intel always indicates the quadruple frequency. ZDNet Clock calls this frequency the "Marketing Clock".
If you move the slider control completely to the left, 1333 MHz or 1600 MHz should be displayed there, depending on the model. The "CPU Clock" value should show the standard frequency of the CPU. To raise the frequency you slide the control to the right. You should not overdo it when overclocking for the first time. A good first move is to increase the "CPU Clock" by 10 MHz.
If you push the slider all the way to the right, to the maximum frequency, and click on the "Apply" button, the computer will certainly crash. The optimum CPU frequency with which stable operation is possible has to be worked out individually for each computer.
Increasing the front-side bus frequency raises the clock speed of the CPU without having to change the multiplier. The frequency of the Northbridge[14] (MCH) is raised at the same time. So the memory is always overclocked as well. In the case of the Mac Pro, the CPU will normally tolerate higher overclocking than the MCH. As the Mac Pro uses fully buffered DDR2 memory modules (FB-DIMMs), which have error checking and correction[15] (ECC), it is easily possible to feel your way up to the maximum stable overclocking frequency.
The console is used for that purpose. The "All Messages" window contains not only the messages from ZDNet Clock but also entries relating to corrected memory errors. Immediately after changing the frequency, entries relating to corrected errors will mainly appear, that are designated as a "correctable fbd error" or as "recoverable parity errors". They are caused by the switching of the frequency. The CPU and MCH briefly encounter synchronisation problems. This is normal, and there is no need to worry about it.
If further memory errors appear in the console after successful overclocking, this is an indication that you have exceeded a stable threshold with the overclocking. It makes sense to carry out a test after switching clock speed. Longer running benchmarks can be considered in this context, such as the prime number search program mprime[11]. If no errors occur for several hours, you can assume secure operation.
In order to keep the effect of memory errors resulting from frequency switching to a minimum, ZDNet Clock overclocks the computer in steps of 1MHz (FSB). Tests have shown that an increase in the FSB by a large amount and the associated increase in the CPU frequency, for example raising the CPU clock speed from 2800 MHz to 3200 MHz, often puts the RAM modules so seriously out of synchronisation that more bits 'topple' than error correction can pick up. This leads to a controlled crash. The memory modules report the error via NMI[16], the kernel acknowledges it with panic[17], see figure 4[18].
The result of step-by-step switching is that it takes some time for confirmation to appear after clicking on the Apply button. If you prefer direct switching, this can be selected by ticking the "Use Rapid Overclocking" checkbox.
Whether the switching can be successfully carried out depends very much on the RAM modules used. The first-generation tester (Mac Pro 1.1) is equipped by Apple with modules from Positivo Informática[19]. In this case overclocking only works in very slow steps. The 2008 model (Mac Pro 3.1) comes with preinstalled Hynix[20] modules. In this case, switching also works with "Rapid Overclocking" turned on.
ZDNet has also tested modules that do not come from Apple. Tests with modules from CSX[21] equipped with memory chips from Elpida are disappointing. They run the Mac Pro clocked at 2800 MHz at a maximum of 2835 MHz. On the other hand, marginally more expensive proprietary modules from Kingston and Transcend (TS2GAPMACP8U-T[22]) display no problems.When the first IBM PC clones that were clocked faster than the original at 4.77 MHz appeared in the eighties, games freaks noticed that many of their games became faster. The Cessna 182 of the Sublogic flight simulator[23] flew at the speed of a Eurofighter. So the bus clock speed is in principle not a good source for real time.
Microsoft has now learned this, but clearly Apple has not. If you carry out benchmarks such as Cinebench[24] or Geekbench[25] after overclocking, the benchmarks do not indicate any improvement in performance at first. But if you resort to a stopwatch, the higher performance resulting from overclocking can indeed be verified. You also notice that the system clock runs significantly faster after overclocking the computer.
Mac OS with time problems after overclocking
Unlike Windows, Linux and many Hackintosh[26] versions, Mac OS uses the bus clock speed as the time source. It is compared with real time when booting up. Nor does time correction by means of the Mac OS NTP daemon[27] succeed: It simply no longer works with a large discrepancy between the bus clock and real time.
Whereas programmers of the IBM PC XT had to be forgiven for using the bus clock speed because of inadequacies of its Intel 8253[28] timer module, it should be reasonable to expect more from Apple, who now only offer computers with modern HPET[29] timers. After all, multimedia applications use the HPET module. So videos do not run faster after overclocking.
The only option for "normalising" the time of the Mac Pro again is to restart without switching off the computer. ZDNet Clock is fundamentally "reboot-proof". But there are a few snags. The latest series of the Mac Pro (Mac Pro 3.1) can indeed be overclocked in the ZDNet test up to 3241 MHz while remaining stable. But a reboot without crashing is only possible up to 3178 MHz. Carefree overclocking fun does not exist beyond this frequency.
The first-generation machine (Mac Pro 1.1) is a little more problematic. If you program the clock chip the computer cannot be rebooted any more - not even if you select the factory-set frequency. So the clock running too fast becomes a permanent annoyance.
The fact that the latest version of Mac Pro (Mac Pro 3.1) can be booted after overclocking also means that a higher frequency is possible under other operating systems. That is of particular interest because there has to date been no functioning overclocking tool for Windows. In contrast, the first-generation machine (Mac Pro 1.1) can be overclocked under Windows with SysTool[30].
Problem with standby mode
After waking up the Mac Pro from standby mode, the clock chip is reset by the operating system to the standard setting, with the result that the computer runs at the standard clock speed again, despite the overclocking frequency. Repeated overclocking with ZDNet Clock also only works again if the kernel extension associated with the utility is first loaded from memory. That can be achieved very easily via terminal, by inputting the following command:
kextunload -b de.zdnet.kext.overclock
This bug will be remedied with a later version of ZDNet Clock.Most PCs enable overclocking through the BIOS. Not so ZDNet Clock: The tool increases the bus clock speed, and thus also the frequency of the memory, only after the operating system has been started. The disadvantage of the former method with many motherboards is that a jumper has to be set if you have overclocked excessively. When overclocking by means of a tool, all it takes after a crash is to switch the computer off and on again. So it would seem that overclocking with a utility is the better option. The problem with the incorrectly running system clock demonstrates that this is not the case under Mac OS.
Safe overclocking with the Mac Pro
On the other hand, one problem that makes overclocking risky on most other computers, namely memory errors, does not occur in the Mac Pro. If you overclock a computer whose main memory does not have error correction, switching the frequency can lead to some serious malfunctions. If the memory errors, which always occur shortly after setting the new frequency, affect a few bits that contain, for example, the code of the file system driver, the result may be the complete loss of hard disk data. If, on the other hand, the computer is overclocked into RAM with the aid of the BIOS before loading the operating system and before shadowing the BIOS, this risk is eliminated.
As the Mac Pro has memory with error correction, overclocking after loading the operating system can do little damage. Either the errors are corrected or the kernel causes a controlled crash. For safety's sake a stability test should be carried out with mprime[11] after overclocking. As a rule, the memory does in fact drop out before the CPU, but you can play it safe if the stability test compares calculated results with previously known results. The best overclocking results are achieved with ZDNet Clock for the Mac Pro if the machine is equipped with brand-name memory. If you think, Apple modules are overpriced, you should use branded memory such as by Kingston or Transcend. In contrast, the no-name modules also used in the test permit only minimal overclocking.
Different models use different clock chips. These chips must be programmed in a totally different manner. ZDNet started with Mac Pro since they are the preferred computers of users who demand maximum power. Feedback from our readers shows us that there is much interest in support for more models. MacBook owners should be aware that they will not achieve a great performance increase when overclocking. Running a notebook computer under full load will push the temperature to the limit after a couple of minutes, even if not overclocked. This causes the CPU to throttle. Overclocking a notebook computer results in better performance for a few minutes only.
Is there a chance that my computer will not boot after running ZDNet clock?
No, ZDNet Clock does not load a kernel extension (kext) at boot time. After powering off, the system boots at stock speed.
Will ZDNet Clock shorten my computer's life?
Hardly at all, since ZDNet Clock does not increase the voltage. We estimate that the computer's life expectancy may be shortended from 20 years to 19.5 years. Core 2-based CPUs at stock speed run well below their theoretical limits, and one can generally expect a long lifespan.
Are there any risks involved in overclocking by a great deal?
As with any system crash, you lose unsaved files. Since HFS+ is a pretty robust filesystem, it recovers well at the next reboot.
Benchmarks, for example XBench, Cinebench and Geekbench, do not run faster after overclocking.
Yes, they do. As stated on page 3[31], Mac OS X uses the bus clock as a real time source. This causes the system clock to run faster, so benchmarks will show the same results as without overclocking. Use a stopwatch to see the performance increase.
Why does the system clock run faster? Is there anything I can do?
This is due to the fact that Apple adjusts the real time clock to bus clock ratio at boot time only. On newer Mac Pros (early 2008), simply do a warm reboot. There is currently no solution for older Mac Pros.
Apple did something wrong? This is impossible.
If you use the bus clock as a real time source, this works as long you do not change the bus clock's speed. However, especially on notebooks, the bus clock does not run at constant speed, even without overclocking. Under full load, upon reaching a certain threshold temperature, notebook processors will leave out bus cycles to cool down. This leads to known problems with USB and Firewire audio devices. Audio playback may start stuttering. The same problems occur when you overclock a Mac Pro.
On notebook computers, the problem disappears when then computer runs idle for a while and has some time to cool down. On a Mac Pro, you have to revert to stock speed. An acceptable workaround is to overclock only when you specifically benefit from it, for instance while encoding or rendering.
If you read between the lines of some of the answers in the Apple developer support forum, you will realize that Apple is obviously aware of these problems. There seem to be some integration issues between the Mach part and the BSD part of the kernel. Most likely, they are working on it.
My system crashes when I overclock by only 5 MHz. What is the cause?
Overclocking means a speed increase of the CPU, front side bus and main memory. All those components must be able to operate safely at a higher speed. If Apple install a CPU specified to run at 2.80 GHz, they cannot be blamed if it does not run at 2.81 GHz.
In ZDNet's experience, RAM is the most likely component to fail when overclocking. Pre-installed Apple memory is generally of very good quality. Cheaper RAM from Transcend or Kingston performs even better. Many other brands, however, do not meet ZDNet's expectations. Corsair even refused to supply ZDNet with samples. They state that their memory modules are not designed for overclocking.
One should also be aware that Transcend, Kingston, Corsair, OWC, iRAM and CSX are just brand names. These companies get supplies from a variety of sources. Quality may vary from batch to batch.
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