What is overclocking?

[TauCeti]

Originally Posted by MrHappy

That's a contradiction.

If you need something to forget the endlessly pouring rain in england, it is "good" :)

Seriously: I loved the "bitter beer" there. There seems to be no such thing here in germany. Maybe it's under embargo like the blue pringles.

[E_tron]

Originally Posted by Citrix

How different are the Prime95 run times on not overclocked and clocked PC's?

Citrix

My Athlon's timings are 136% of its stock speed.
My older Athlon's timings are 76% of its stock speed(old motherboard, 100mhz FSB limitation)
My P4's timings are 120% of its stock speed.
My P2's timings are 150% of its stock speed.
My Via/Cyrix's (C3/EDEN) timings are 137% of its stock speed(which is slow to begin with).

Not all computers are created equal.

[jinydu]

Just wondering:

Why are personal computer speeds measured in Hz, while supercomputer speeds are measured in FLOPS? Why the diffferent units?

[optim]

A personal computer usually is a uniprocessor machine, that is, it has only one processor (CPU).

A supercomputer is usually made by connecting together thousands of CPUs (usually using standard Gigabit Ethernet or some other networking technology like Infinibad or Myrinet - http://www.myri.com/ ).

Many times the CPUs used for supercomputers do not exceed frequencies of 500MHz or 1GHz. This is because they want stability, low temperature and low electricity usage. However now many supercomputers use standard server CPUs, like AMD Opteron, with speeds starting at 1.4GHz.

Hz is frequency while FLOPS is performance. 1 MFLOPS is one million floating point operations per second. 1 GFLOPS is one billion. One TFLOPS is one trillion.

It would be innapropriate to use Hz for supercomputers. You could say that this supercomputer is 500MHz because it uses 10,000 500MHz CPUs, or you could say it is 5,000,000MHz (5 THz - terahertz) because 10,000*500=5,000,000. In either expression, the information is not very useful for a scientist who wants to run some scientific software on it (mprime? ). The scientist needs to know how fast he/she will get the results. If we express the computer power in terms of FLOPS, that is, in how many mathematical operations it can complete every second, the scientist, who presumably knows how many mathematical operations his/her software needs, can calculate when he/she will get the desired result.

Also, usually many scientists share the same supercomputer at the same time. For example, a supercomputer may serve a hundred scientists at the same time (i.e. running 100 programs at once). If I am not mistaken this is called time sharing. It is much more scientific to measure how much mathematical power each scientist gets, rather than how much frequency.

Remember: Frequency is just one factor in performance. An AMD Athlon at 2.2GHz has the same performance as an Intel Pentium4 at 3-3.2GHz.

I think we should use FLOPS for personal computers too. One might argue that most people do not know about mathematics ("floating point operations per second? what's that man?" ). However, I am sure that most people also do not know about physics (hertz). Apple recently used FLOPS to advertise its new PowerPCs. I think the PC industry may start using FLOPS after every desktop-laptop PC is based on a dual-core or dual-processor setup...

[jinydu]

So if FLOPS is a more useful measure, why not use it for PCs too? Also, how can I figure out how many FLOPS my PC has?

Thanks for the information

[optim]

You can measure the FLOPS of your PC by benchmarking. I think SiSoft Sandra can measure the FLOPS.

[optim]

You may try this:

Go to http://www.ud.com/solutions/ and click the POWER CALCULATOR image down the menu at the left.

It says an AMD Athlon at 2GHz has 3 gigaflops (3 GFLOPS) of performance, when used with the United Devices grid software at 100% utilisation.

[jinydu]

It gives 3 measurements:

Dhrystone ALU: 8161 MIPS
Whetstone FPU: 2250 MFLOPS
Whetstone iSSE2: 4110 MFLOPS

Don't know what it means...

[Uncwilly]

Hz is just speed, while FLOPS is power.

analogy
the water comes out the pipe at 15 fps vs. the water comes out the pipe at 5 MGD

One may seem fast 15 feet a second, but 5 Million Gallons a Day is a huge amount of flow.

[optim]

ALU is Areithmetic-Logic Unit inside the CPU, it processes integer mathematical operations and boolean logic. MIPS is million instuctions per second. An instuction may be the addition of two numbers or a logical operation (AND, OR, XOR etc).

FPU is a unit inside the CPU for floating point arithmetic, that is, non-integer numbers. It measures in FLOPS.

Some CPUs, including Intel Pentium 4 and AMD Athlon64, have SSE2 capability. The SSE2 capabilities of a CPU can also be measured in FLOPS, because they handle floating point arithmetic at higher speed than FPU, but they are highly specific (i.e. FPU is general-purpose, SSE2 is for special operations).

So your CPU can compute up to 8161 MIPS when used for integer arithmetic and boolean logic, 2250 MFLOPS when used for general purpose floating point arithmetic, and 4110 MFLOPS when used for SSE2 arithmetic.

Also note than SSE2 accepts 128-bit instuctions, while ALU and FPU are only 32bit, If I recall correctly.

Prime95/Mprime can utilise SSE2 when available. However, it is faster in Intel P4 than AMD Athlon64. AthlonXP doesnt have SSE2.

[patrik]

Originally Posted by Citrix

How different are the Prime95 run times on not overclocked and clocked PC's?

For Prime95 it is almost only the CPU speed that matters. This means that it can be better to lower the memory/FSB ratio (if memory fails first) so that you can get a higher CPU speed.

And don't forget to run a lot of double-checks to see that your overclock is prime-stable (not all errors are detected by Prime95). On new computers I keep lowering the FSB until I have around 20 matching double-checks.