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The Plusses And Perils of Overclocking
mblase writes "This C|Net article, published this morning, covers some of the advantages and many of the drawbacks involved for those who want to seriously overclock their PC hardware -- and why."
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When you build a new chip you build software models of the entire chip - right down to the gate and polygon level, you do a LOT of timing analysis - these days we extract the polygons and do 3d parasitic extraction - this takes a long time (days for a big chip) - but the results are by their nature statistical - because the results of building a particular chip are somewhat staistical (depends on etch rates, temp, etc etc at the fab) so we calculate the worst case fast and worst case slow process corners and use the timing tools to check all the potential timing paths (thing combinatorial explosion here). After we think we have something that will make timing we build some - at the fab at the begeinning we get the fab guys to explicitly vary the process to push some wafers into each 'corner' of the process - then we bring those die into the lab and use them to make sure that they will work at speed within the various temp ranges the chip is supposed to work at.
One of the problems with making chips is that testing them is VERY expensive - the testing machines that do die and chip sorting cost millions of $$ and the number of seconds a die spends on one effects the final cost - so you design your tests to uncover raw defects (via scan and maybe functional tests) and speed problems by using the results of your original timing imulations to identify the timing paths that are so close to the edge that they are likely to fail first - because the testers don't have access to most of the internal nodes you have to do things like overclocking by say 10% and then hoping the internal logic will fail in some manner that you can catch (you also use the previous lab work to validate this approach by identifying known bad chips and making sure they fail on the tester).
One thing you can do is 'bin' chips - test them at different frequencies and sell the ones that happen to be faster for more - because binning is a more expensive process its usually only done for CPUs and other expensive sorts of chips.
What commonly happens over the lifetime of a chip is that as the process improves the number of die that fall into the faster bins increases - however for marketting reasons a company may wish to continue to sell the 'faster' ones at a premium so it will label some fast chips as 'slower' so to keep their product mix in the market (a fancy way of saying 'so they can make more money'). I'm told the same thing happens with olive oil :-)
Now the chips are vey carefully screened and carefull spec sheets are written for them - you buy an 850MHz chip from AMD or Intel and it will work at 850 within the appropriate voltage/temp range specified on the data sheet (if not as, we've seen, Intel will recall chips that don't) - it's not in the chip manufacturer's interest to sell chips that don't work - they get soldered on to expensive boards and expensive system which have to be trashed at the OEM if they don't work - those data sheets make sure that to parts in a million those chips work as advertised.
Having said that - some chips do run faster if overclocked - you can always tell which ones - because you don't know which process corner the die was fabbed at - or what the binning policies were the day it was manufactured etc etc - even worse yet - and here's my traditional warning - WARNING - your overclocked CPU may work perfectly for months because what you're doing may not exercise the slowest timing path(s) in the design (remember combinatorial explosion!) - you might play quake for months on end without a problem .... then silently drop $1000 off your tax refund ....
If you buy a 1000 Mhz machine for a mission-critical system and simply cannot afford any downtime, you'll almost certainly want to use better cooling (the same cooling overclockers use) on the CPU, chipset, et al. You'll also want to underclock your system because by doing so, you'll reduce heat still further.
So you could still find overclocking techniques useful even if you do not overclock.
--
Oceania has always been at war with Eastasia.
Let's see what happens when I try this liquid oxyge...
134340: I am not a number. I am a free planet!
Pro:
Save $100 on a chip.
Con:
Spend $300 on cooling gear.
Trolls throughout history:
Jonathan Swift
Right now I have a 600MHZ athlon and it sounds like an airconditioner is running in my study. So if I got a 1.2 GHZ cpu and ran it at say 700-850 MHZ, I could theoretically take the fan out of my box (but keep the heatsink), use less electricity and it would last longer (besides being faster than my current cpu). Is there a downside to my logic that I am not seeing? My view of cpu's needing any/better cooling technology is to me an indication that cpu manufacturers ARE over-clocking thier cpus... just not as much as enthusiast do. Is there a reason why Intel/AMD/Cyrix etc... need better cooling besides the HZ war:
if (MyCPU.MHZ > Other.MHZ) {
BankAccount = (BankAccount + BetterSales)
}
I miss the Karma Whores.
...until you've seen a 386 boot at 400 Mhz. It won't run, but it'll boot.
God, I love the smell of silicon in the morning!
This