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

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

[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

[Anonymous Coward]

This is an excellent observation. Please forgive me if this seems offtopic -- my primary development platform is PC, however I've got quite a bit of experience working on 64 bit.

From a developer perspective (at least in software I've worked on) programmers often do not realize that a vast number of projects took dependencies on 4 byte pointers.

Structure alignment, pointer size, etc have plagued all projects I've worked on, even after a cursory perusal at the code indicating "It looks good". Lots of bugs in porting code to 64 bit can be hidden until the right things happen.

Your particular example probably overexaggerates the point (I doubt many modern apps use a majority of memory in integer linked lists). When you think of standard server apps, most data is serialized request data which is a significant constant overhead. For the average desktop user, typing documents & email is their primary utilization of a PC, in which case it shouldn't have a tremendous effect.

That being said, there is no way your memory usage will become more effecient :-).

One additional reason that your memory will drop -- on 64 bit platforms compiled binaries become much larger. This plagues the IA64 chipset more than any other -- due to their "four simultaneous operations" (forgive me if I'm misrepresenting -- this is only through hallway chatter) binaries get an order of 2 to 4 times larger (mostly full of NOPs, no less).

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...

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.

Add instruction sets size too

[Gopal.V]

The instruction sets are also generally 64 bit... so you end up with lesser disk space as well :) .. add the space in loading these into RAM ..

The notable exception is the Arm's thumb instruction set (it's cool).

The sad part "my address bus is bigger than you" is going the "I have more MHz than you" way soon as parallel CPUs (mulit-core or otherwise) become cheaper.. 90% of our tasks are better done parallel than using a single fast chip . Hell , half of the tasks really don't need anything beyond a 300/400 mhz clocks.

Re:you want heat data

[Anonymous Coward]

The power coming out of a chip is marginal; it's pretty much all converted to heat.

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.

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.

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.