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

Micro CPU benchmarks: isolating the FPU

But you can't compare an Intel PC with an Apple. The software might not be optimised the right way." Indeed, it is clear that the Final Cut Pro, owned by Apple, or Adobe Premiere, which is far better optimised for the Intel PC, are not very good choices to compare the G5 with the x86 world.

So, before we start with application benchmarks, we performed a few micro benchmarks compiled on all platforms with the same gcc 3.3.3 compiler.

The first one is flops. Flops, programmed by Al Aburto, is a very floating-point intensive benchmark. Analyses show that this benchmark contains:

• 70% floating point instructions;
• only 4% branches; and
• Only 34% of instructions are memory instructions.

Note that some of those 70% FP instructions are also memory instructions. Benchmarking with Flops is not real world, but isolates the FPU power.

Al Aburto, about Flops:

" Flops.c is a 'C' program which attempts to estimate your systems floating-point 'MFLOPS' rating for the FADD, FSUB, FMUL, and FDIV operations based on specific 'instruction mixes' (see table below). The program provides an estimate of PEAK MFLOPS performance by making maximal use of register variables with minimal interaction with main memory. The execution loops are all small so that they will fit in any cache."

Flops shows the maximum double precision power that the core has, by making sure that the program fits in the L1-cache. Flops consists of 8 tests, and each test has a different, but well known instruction mix. The most frequently used instructions are FADD (addition), FSUB (subtraction) and FMUL (multiplication). We used gcc -O2 flops.c -o flops to compile flops on each platform.

MODULE	FADD	FSUB	FMUL	FDIV	Powermac G5 2.5 GHz	Powermac G5 2.7 GHz	Xeon Irwindale 3.6 GHz	Xeon Irwindale 3.6 w/o SSE2*	Xeon Galatin 3.06 GHz	Opteron 250 2.4 GHz
1	50%	0%	43%	7%	1026	1104	677	1103	1033	1404
2	43%	29%	14%	14%	618	665	328	528	442	843
3	35%	12%	53%	0%	2677	2890	532	1088	802	1955
4	47%	0%	53%	0%	486	522	557	777	988	1856
5	45%	0%	52%	3%	628	675	470	913	995	1831
6	45%	0%	55%	0%	851	915	552	904	1030	1922
7	25%	25%	25%	25%	264	284	358	315	289	562
8	43%	0%	57%	0%	860	925	1031	910	1062	1989
Average:					926	998	563	817	830	1545

The results are quite interesting. First of all, the gcc compiler isn't very good in vectorizing. With vectorizing, we mean generating SIMD (SSE, Altivec) code. From the numbers, it seems like gcc was only capable of using Altivec in one test, the third one. In this test, the G5 really shows superiority compared to the Opteron and especially the Xeons.

The really funny thing is that the new Xeon Irwindale performed better when we disabled support for the SSE-2, and used the "- mfpmath=387" option. It seems that the GCC compiler makes a real mess when it tries to optimise for the SSE-2 instructions. One can, of course, use the Intel compiler, which produces code that is up to twice as fast. But the use of the special Intel compiler isn't widespread in the real world.

Also interesting is that the 3.06 GHz Xeon performs better than the Xeon Irwindale at 3.6 GHz. Running completely out of the L1-cache, the high latency (4 cycles) of the L1-cache of Irwindale hurts performance badly. On the Galatin Xeon, which is similar to Northwood, Flops benefits from the very fast 2-cycle latency.

The conclusion is that the Opteron has, by far, the best FPU, especially when more complex instructions such a FDIV (divisions) are used. When the code is using something close to the ideal 50% FADD/FSUB and 50% FMUL mix and is optimised for Altivec, the G5 can roll its muscles. The normal FPU is rather mediocre though.

Micro CPU benchmarks: isolating the Branch Predictor

To test the branch prediction, we used the benchmark " Queens". Queens is a very well known problem where you have to place n chess Queens on an n x n board. The catch is that no single Queen must be able to attack the other. The exhaustive search strategy for finding a solution to placing the Queens on a chess board so they don't attack each other is the algorithm behind this benchmark, and it contains some very branch intensive code.

Queens has about:

• 23% branches
• 45% memory instructions
• No FP operations

On a PIII, the Branch misprediction rate is up to 19%! (Typical: 9%) Queens runs perfectly in the L1-cache.

	RUN TIME (sec)
Powermac G5 2.5 GHz	134.110
Xeon Irwindale 3.6 GHz	125.285
Opteron 250 2.4 GHz	103.159

At 2.7 GHz, the G5 was just as fast as the Xeon. It is pretty clear that despite the enormous 31 stage pipeline, the fantastic branch predictor of the "Xeon Pentium 4" is capable of keeping the damage to a minimum. The Opteron's branch predictor seems to be at the level of G5's: the branch misprediction penalty of the G5 is 30% higher, and the Opteron does about 30% better.

The G5 as workstation processor

It is well known that the G5 is a decent workstation CPU. The G5 is probably the fastest CPU when it comes to Adobe After Effects and Final Cut Pro, as this kind of software was made to be run on a PowerMac. Unfortunately, we didn't have access to that kind of software.

First, we test with Povray, which is not optimised for any architecture, and single-threaded.

	Povray Seconds
Dual Opteron 250 (2.4 GHz)	804
Dual Xeon DP 3.6 GHz	1169
Dual G5 2.5 GHz PowerMac	1125
Dual G5 2.7 GHz PowerMac	1049

Povray runs mostly out of the L2- and L1-caches and mimics almost perfectly what we have witnessed in our Flops benchmarks. As long as there are little or no Altivec or SSE-2 optimisations present, the Opteron is by far the fastest CPU. The G5's FPU is still quite a bit better than the one of the Xeon.

The next two tests are the only 32 bit ones, done in Windows XP on the x86 machines.

	Lightwave 8.0 Raytrace	Lightwave 8.0 Tracer Radiosity
Dual Opteron 250 (2,4 GHz)	47	204
Dual Xeon DP 3,6 GHz	47.3	180
Dual G5 2,5 GHz PowerMac	46.5	254

The G5 is capable of competing in one test. Lightwave rendering engine has been meticulously optimised for SSE-2, and the " Netburst" architecture prevails here. We have no idea how much attention the software engineers gave Altivec, but it doesn't seem to be much. This might of course be a result of Apple's small market share.

	Cinema 4D Cinebench
Dual Opteron 250 (2.4 GHz)	630
Dual Xeon DP 3.6 GHz	682
Dual G5 2.5 GHz PowerMac	638
Dual G5 2.7 GHz PowerMac	682

Maxon has invested some time and effort to get the Cinema4D engine running well on the G5 and it shows. The G5 competes with the best x86 CPUs.