What Makes Apple's Power Mac G5 Processor So Hot

An anonymous reader writes "58 million transistors can drive a lot of power. Apparently, Apple appreciated the choices IBM processor architects made when designing the 970 family. This article provides the 64-bit architecture big picture for the 970 family (A.K.A. the Power Mac G5) and the critical issues in IBM's 64-bit POWER designs, covering 32-bit compatibility, power management, and processor bus design."

Comments

Circular Logic

[WormholeFiend]

If it's so hot, maybe it's not cool enough.

What Makes Apple's Power Mac G5 Processor So Hot?

[Anonymous Coward]

Insufficient cooling?

64 bit integers

[Xpilot]

From the article:

...64-bit processors also accelerate complex mathematical calculations through their ability to perform calculations directly on 64-bit numbers...

Don't they mean 64-bit integers? Since floating point registers in most modern CPU's are 64-bit wide already.

Re:64 bit integers

[francisew]

More importantly, it doesn't have to break the 64 bit operations into many successive 32 bit operations. 64 bit operation are not simply 2x32 bit operations, but can be several dozen operations.

An 8-bit microcontroller can perform 64 bit floating point operations correctly. It just takes a long time.

Re:64 bit integers

[Waffle Iron]

Since floating point registers in most modern CPU's are 64-bit wide already.

Actually, since most modern CPUs are x86 variants, the floating point registers are usually 80 bits wide (and have been since the 1981 introduction of the 8087).

As far as "complex mathematical calculations" go, 64-bit integers aren't really that big a deal. It's pretty rare to need integers bigger than 2^32 but no bigger than 2^64; floating point usually handles big numbers more flexibly.

The big deal with 64-bit CPUs is 64-bit address pointers and operations on them (which usually aren't more complex than adding and shifting).

Re:anyone else noticed how COOL the AMD-64 chips r

[Noah Adler]

I was able to hold my hand on the heatsink and it was barely warm.

It could be because there's inadequate conduction between the CPU core and the heatsink. Check the temperature monitors to make sure it's actually as cool as you hope it to be. It could be that just most of the heat is staying in on the CPU, which would be a bad thing. Hopefully you've already checked this though.

ob Memory

[GillBates0]

18 exabytes ought to be enough for anybody

-GillBates0, 2004.

Re:ob Memory

[Mark_in_Brazil]

18 exabytes ought to be enough for anybody

-GillBates0, 2004

So where does it end?

This page makes a fairly convincing argument that 256 bit CPUs should be enough (basically, there would be no way to exhaust the amount of memory a 256 bit CPU could access, because the number of memory locations is about the same as the number of atoms in the universe).

--Mark

Re:What Makes Apple's Power Mac G5 Processor So Ho

[sky289hawk1]

The fan belt on its search engine doesn't drive the radiation air emission unit fast enough! SHE NEEDS MORE POWER!

Teflon underside

[suso]

They should make the G5 powerbooks have a teflon underside.

"Turn it over, and you can cook dinner".

Re:Teflon underside

[Reducer2001]

Ah ha! The REAL cause of global warming. OC'd CPU's.

Increased Pointer size

[trigeek]

One side-effect of 64-bit computing that I don't hear a lot of discussion about is the increase in the size of a pointer. A standard implementation of a linked list of integers will now be 50 to 100% larger (depending on if you use 32 or 64 bit integers), simply because the pointers take up more space. If I bought a 64 bit system, simply because it's the "Best", but only got 1GB of RAM, I have less useful memory, because the pointers take up all of my physical RAM. Do the architects of these systems take this into account?

Re:Increased Pointer size

[Eccles]

In most meaningful, sizeable programs, pointers aren't a significant chunk of memory usage. (And for small programs, it doesn't matter.) I would think most modern apps consume most of their memory storing images, which aren't affected by the 32->64 change.

Also, 64-bit pointers allow you to go from a max of 4GB of RAM to 16 billion GB, so the assumption is memory prices will keep dropping and you'll have much more than twice as much RAM on your 64-bit system anyway.

Re:Increased Pointer size

[HeghmoH]

On Mac OS X, dynamic memory has a granularity of 16 bytes. That means that if your linked list node is only 8 bytes (4 bytes data, 4 bytes pointer) then it will get a 16-byte allocation anyway, and you'll waste the extra space. Using 16 bytes per node won't hurt at all. Allocation overhead makes the standard linked list a fairly wasteful way to store data anyway.

Re:Increased Pointer size

[spaceyhackerlady]

On Silicon Graphics 64-bit machines, this was solved by having two ABIs, one 32-bit, one 64-bit.

Sun machines with UltraSPARC processors do this too. They run 64 bit kernels, and applications are 32 bits. Unless you actually need 64 bits, in which case you feed the compiler some differnet options and it makes a 64 bit executable for you.

Both Solaris and Linux do it the same way. When you build a kernel for Linux on an UltraSPARC machine the part about kernel support for different kinds of executables offers you (among other options) 32 bit ELF (which you need), 64 bit ELF (optional), and Solaris emulation (never tried it...).

...laura

Re:Increased Pointer size

[SnapShot]

The C++ standard template library uses MAGIC PIXIE DUST and MOONBEAMS for the data structures. Java Containers, on the other hand, are held together with PURE THOUGHTS and KITTEN WHISKERS. That's how they avoid using pointers... ;-)

Re:Heat Problem Back Ground

[jrockway]

I'll probably get modded down for saying this, but I have Karma to burn...

That article is some crybaby whining about how expensive the G5s are. "Apple is so dumb. Why would anyone pay that much for a CRAP computer," is what the article sounds like. I think that guy needs to take his superior knowledge elsewhere and try some benchmarks ("512K of cache isn't competitive for $3000")... apparently it is because it's winning benchmarks and people are buying them. Just because you can't afford it doesn't mean it's a bad computer (doesn't mean it's a good computer either). This guy needs to grow up and write an article with facts instead of emotions...

Addressing

[vlad_petric]

Most importantly, it can address up 2^64 bytes of memory. And yes, that generally implies 64-bit integer GPRS. BTW, vector operations on x86 (MMX) also operate with 64 bit registers, but it can only access 4G (32G if you use the extended bits "hack").

Re:The Athlon64 is very cool

[strictfoo]

Holy fuck dude... your room temp is 109.4 degrees Fahrenheit (thank you google)?

Open a window or buy an air conditioner or something!

WATT figures for G5 vs AMD-64?

[green pizza]

Does anyone have the numbers to compare how many watts of power the G5 uses vs a similar AthlonXP or AMD64? Ie, I'd like to see how a 2.0 or 2.5 GHz G5 compares to a 2.0 or 2.5 GHz AMD processor.

Re:WATT figures for G5 vs AMD-64?

[Anonymous Coward]

It's hard to make a comparison because for some reason IBM/Apple doesn't want to release official measurements for power usage. Which is strange because they should do really well in that measurement compared to AMD and Intel. Here's their official numbers:

2.4 GHz A64- 89 W
3.4 GHz P4(Northwood)- 89 W
3.4 GHz P4(Prescott)- 103 W

Best guess on the 2.5 GHz G5 is around 65 W.

Nitpicking...

[Tristandh]

Capable of addressing an astronomical 18 billion GB, or 18 exabytes, of memory,

I know the first 2 digits are 1 and 8, but 2^64 bytes is still 'only' 16 exabytes...

Re:Nitpicking...

[TonyZahn]

If you had read the sidebar you'd see the article defined the issues with base-2 and base-10 number names, and introduced the prefixes "mibi-" and "gibi", which should be familiar to /.ers.

When they say 18 exabytes, they're talking base-10, otherwise they would have used the "gibi-" equivalent (exibytes?)

Re:So sick of it

[BobWeiner]

Obligatory PC Weenies link that fits the topic at hand.

Re:Heat Problem Back Ground

[the_2nd_coming]

umm, what heat problem? the 9 fans and liquid cooling are in their respective models to solve a SOUND problem. not a heat problem. but then if you bothered looking up statistics on the G5 you would not be able to bash the mac would you?

Fans and cooling

[2nd Post!]

Your iMac G5 has two fans. Not much space left for additional cooling, really, without interfering with the current cooling setup.

Your PowerMac G5 has nine fans. Again, not much space left for additional cooling without interfering.

And get this, the PowerMac G5 already uses a liquid cooling setup. The only possible additional mod is to hook the current setup to a resevoir and radiator on the outside of the case, as the inside already has a radiator per CPU and something like a 120mm fan per CPU.

Good followup link from the article

[C.Batt]

Introduction to 64-bit computing

There's an informative link at the bottom of the article for those requiring a bit more insight into the effect of 64-bit computing. /wishes he had exa-bytes of memory right now... VS.NET on WinXP is a PIG!

Re:What makes it so hot (abridged)

[malfunct]

Its not that apples (and most likely some of the x86 based computers too though I haven't really looked into it) aren't supercomputers. Its just that given the current definition of a supercomputer its just not that amazing anymore. Maybe its the definition that needs to change rather than you being upset at people that follow the definition.

Re:atoms in one universe

[pclminion]

So, what happens if you want to use your fancy computer to model two universes?

Simple. Since we've got a computer capable of simulating the entire universe, we'll just use it to simulate a universe which contains a computer which is capable of simulating more than one universe.

Re:What makes it so hot (abridged)

[John Whitley]

Wow. Artful and elegant rebalancing of engineering tradeoffs for very diferent markets reduced to a knee-jerk oversimplification in one fell swoop. And it got a +5 Insightful for that, too boot. Here are some reasons why the "stripped it down a bit so we don't cut into our own market" statement is ridiculous:

1. If selling POWER series chips to Apple was going to undermine IBM's server business, IBM would have a hell of a lot more to worry about from the plain 'ol x86 market.
2. IBM's POWER-series chips are designed to trade away ultra-high-speed clock rates in favor of low failure rates. The design rule (feature size on chip) is pulled back from the bleeding edge and other layout techniques are employed to make these processors rock solid, to avoid costly downtime from hardware failures in business servers.
3. These days Apple is well known for its forays into the cluster computing space -- but that's a far cry from the sort of transactional throughput capacity of IBM's high-end servers. I.e. not the same markets!

it's hot. really. (a little perspective)

[Mr. Slurpee]

for literal heat, this puppy is pretty hot.

my dual 2.5GHz PowerMac G5 idles at 52C (125F) on CPU A and 50C (122F) on CPU B. the memory controller is actually one of the hotter things, it idles at 62C (143F). however, it's not the hottest thing, of course: at full load (DVD rip+encode or playing 15 videos at once + MP3 + tasks + flicking around Exposé) both CPUs have hit a max of 83C (181F) (the computer is supposed to automatically sleep around 90C or so).

so why so effing hot? i mean, this idles at the max temp my athlon 2500 peaks at! it certainly idles at a hotter temp than it needs to, but i have no problem with that: the system runs the fans dynamically to keep the noise down, so at idle it's not as cool as it could be. the difference in noise in my room when i sleep the athlon is ridiculous - the G5 sounds like a slightly loud external hard drive that's spun up. the system also has a liquid cooling system to quench the processors. this seems to just keep the processors within their range. the value that i see in it is response to new heat - the CPU temps flick around a lot and are very responsive to load and the loss of load. after ramping up the CPUs to >80C, it take about three or four seconds after the load drops for the CPU temps to drop 15-20C, then maybe a total of ten or twelve seconds to drop to idle temp.

for some real-world perspective... a DVD rip+encode with HandBrake with using ffmpeg engine, MP3 audio, 2-pass encoding, and gunning for your average 700MB movie time (800-1300kbps?) takes slightly less than the length of the DVD. an hour and a half long movie took about and hour and fifteen minutes to get on to my hard drive. MP3 ripping in iTunes will run up to 28x, but it's not fully loading the processors so i wonder about a drive read bottleneck. the first night i got it, i was at a loss for how to really test the speed on it, so i just decided to open up a shitload of videos. basically i played a DVD (fluff, the GPU does that), opened up something in VLC, opened up about 13 videos in QuickTime of various sizes and formats, played some MP3 music (fluff again, that's ball sweat of a cutting edge proc), and still had enough processing power to comfortably navigate files, chat, browse web pages, and flick around Exposé. around all of these things plus one is when a few of the videos would start stuttering and expose would start dropping frames to keep collapse speed uniform. anything past this would really start robbing time from videos.

all in all? it's fast. it's quiet. it gets hot, but it takes care of itself. coming from a 375MHz G3-upgraded PowerMac 7600 (vintage '98), i'm not doing too shabby. i just decided i'd scramjet at mach 7 to the top of the pack and then sit there for another few years.