No more mysteries: Apple's G5 versus x86, Mac OS X versus Linux

Summary: the cores compared

Below, you find a comparison of the Intel Xeon/Pentium 4, the Opteron/Athlon 4, the G5 and the previous CPU in the Apple Power: the G4 of Motorola.

CPU feature	Motorola G4+	G5 (IBM PowerPC 970)	Intel Xeon P4 Irwindale	AMD Opteron Troy
Process technology	0.18 µ CU SOI	0.09 µ CU SOI	0.09 µ CU	0.09 µ CU SOI
GP Register Width (bit)	32	64	64	64
Number of transistors (Million)	33	58	169	106
Die Size (mm²)	106	66	+/-130 (112   for 1 MB L2)	115
Maximum Clockspeed (MHz)	1400	2700 (liquid cooled)	3800	2600
Pipeline Stages ( fp)	7	16 (21)	31 - 39*	12 (17)
issue rate (Instruction per clockcycle)	3 + 1 Branch	4 + 1 branch	4 ports, max. 6 (3 sustained)	6 (3 sustained)
Integer issue rate (IPC)	3 + 1 Branch	2	4 (3 sustained)	3
Floating point issue rate (IPC)	1	2	1	3
Vector  issue rate (IPC)	2-4 ( Altivec)	2-4 ( Altivec, velocity)	4  Single(SSE-2/3)	4  Single(SSE-2/3)
			2 Double (SSE-2/3)	2 Double (SSE-2/3)
Load & Store units	1	2	2	2
"instructions in flight" (OOO Window)	16	215 (100)	126	72
Branch History Table size (entries)	2048	16384	4096	16384
L1-cache (Instruction/Data)	32 KB/32 KB	64 KB/32 KB	12k µops (+/- 8-16 KB)/16 KB	64 KB/64KB
L2-cache	256 KB	512 KB	2048 KB	1024 KB
L3-cache	2 MB DDR SRAM 64 bit at 1/4 th of core clock	none	None	none
Front Side Bus (MHz)	166	1350 (675 DDR)	800 (200 Quad)	N/A
Front Side Bus (GB/s)	1.3 Half Duplex	10,8 Full Duplex	6.4 Half Duplex	N/A
Memory Bandwidth (GB/s)	2.7	6.4	6.4	6.4
Core Voltage	1.6V	1,1V ?	1.38V	1.4V
Power Dissipation	30W at 1 GHz	+/- 59 (Typical) -80 Watt (max)	110 W (Typical)	92,6W (Max)

*31 is branch misprediction pipeline length, 39 is the length of the total pipeline including decoding stages before the trace cache.

Let us summarize: in theory, the PowerPc 970FX is a very wide, deeply pipelined superscalar monster chip, with excellent Branch prediction and fantastic features for streaming applications. And let us not forget the two parallel FPUs and the SIMD Altivec unit, which can process up to 4 calculations per clock cycle.

The disadvantages are the rather coarse way that the 970FX handles the instruction flow and the high latency to the RAM.

Enough theory. Let us see how the G5 2.5 GHz and 2.7 GHz compares to the 3.6 GHz Xeon Irwindale and Opteron 250 (2.4 GHz). The Opteron 852 arrived just a day before my deadline, but I think that you will know how the 252 performs compared to the 250. Before we tackle performance, here are a few quick notes about power dissipation.

Power to the PowerPC

How power thirsty is this PowerPC 970FX? His predecessor, the 0.13µ SOI PowerPC 970 was a pretty cool chip. It consumed about 42W at 1.8 GHz (1.3v). The newer 0.09µ SOI PowerPC 970FX CPU is reported to dissipate about 55-59W at 2.5 GHz. However, a few annotations must be made.

First of all, IBM and Apple tend to increase the core voltage when running at higher clock speed. This makes the needed power increase more than linearly. For example, the 1.8 GHz PowerPC 970 consumed 42 Watt, but the 2 GHz version (both 0.13µ CPUs) needed 66 Watt.

Secondly, the TDP IBM talks about is typical , not maximum like AMD's.

Let us clarify this by checking IBM's and Apple's numbers. For the 90 nm, IBM's own documents tell us that the PowerPC 970FX only consumes 24.5 Watt at 2 GHz (1V). However, the same 0.09µ SOI PowerPC970FX is reported to consume about 55W at 2.3 GHz (1.1V?) in the Xserve, according to Apple's own website. Typically, you would expect the G5 to consume about 28 Watt (24.5 * 2.3 / 2) at 2.3 GHz, when using the 24.5 Watt at 2 GHz as a reference. Apple talks about "at most" (maximum), and IBM about "typical".

Still, that is a huge gap between "typical" and "maximum" power dissipation. The 55 Watt number seems to indicate that the core voltage must have been increased significantly at 2.3 GHz. The maximum power dissipation of the 2.5/2.7 GHz G5 inside the liquid-cooled PowerMacs might thus be quite a bit higher than in the 1U Xserve, probably around 80 Watt for the 2.7 GHz. That is a lot of power for a 66 mm² CPU, and it probably explains why Apple introduced liquid cooling. The liquid cooling system inside our PowerMac wouldn't get warm and wouldn't be necessary at all if the two 2.5 GHz CPUs were only dissipating a 59 Watt maximum.