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.

IBM PowerPC 970FX: Superscalar monster Benchmark configuration
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  • Rosyna - Friday, June 3, 2005 - link

    Actually, for better or worse the GCC Apple includes is being used for most Mac OS X software. OS X itself was compiled with it.
  • elvisizer - Friday, June 3, 2005 - link

    rosyna's right.
    i'm just not sure if there IS anyway to do the kind of comparison you seem to've been shooting for (pure competition between the chips with as little else affecting the outcome as possible). you could use the 'special' compilers on each platform, but those aren't used for compiling most of the binaries you buy at compusa.
  • elvisizer - Friday, June 3, 2005 - link

    why didn't you run some tests with YD linux on the g5?!?!?!?!?!?!? you could've answered the questions you posed yourself!!!!!
    argh.
    and you definitly should've included after effects. "we don't have access to that software" what the heck is THAT about?? you can get your hands on a dual 3.6 xeon machine, a dual 2.5 gr, and adual 2.7 g5, and you can't buy a freaking piece of adobe software at retail?!?!?!?!?!
    some seroiusly weird decisions being made here.
    other than that, the article was ok. re-confirmed suspicions i've had for awhile about OS X server handling large numbers of thread. My OS X servers ALWAYS tank hard with lots of open sessions, so i keep them around only for emergencies. They are so very easy to admin, tho, they're still attractive to me for small workgroup sizes. like last month, I had to support 8 people working on a daily magazine being published at e3. litterally inside the convention center. os x server was perfect in that situation.
  • Rosyna - Friday, June 3, 2005 - link

    There appears to be either a typo or a horrible flaw in the test. It says you used GCC 3.3.3 but OS X comes with gcc version 3.3 20030304 (Apple Computer, Inc. build 1809).

    If you did use GCC 3.3.3 then you were giving the PPC a severe disadvantage as the stock GCC has almost no optimizations for PPC while it has many for x86.
  • Eug - Friday, June 3, 2005 - link

    "But do you really think that Oracle would migrate to this if it wasn't on a par?"

    [Eug dons computer geek wannabe hat]

    There are lots of reasons to migrate, and I'm sure absolute performance isn't always the primary concern. We won't know the real performance until we actually see tests on Oracle/Sybase.

    My uneducated guess is that they won't be anywhere near as bad as the artifical server benches might suggest, but OTOH, I could easily see Linux on G5 significantly besting OS X on G5 for this type of stuff.

    ie. The most interesting test I'd like to see is Oracle on the G5, with both OS X and Linux, compared to Xeon and Opteron with Linux.

    And yeah, it would be interesting to see what gcc 4 brings to the table, since 3.3 provides no autovectorization at all. It would also be interesting to see how xlc/xlf does, although that doesn't provide autovectorization either. Where are the autovectorizing IBM compilers that were supposed to come out???
  • melgross - Friday, June 3, 2005 - link

    As none of us has actual experiance with this, none of us can say yes or no.

    But do you really think that Oracle would migrate to this if it wasn't on a par? After all Ellison isn't on Apple's board anymore, so there's nothing to prove there.

    I also remember that going back to Apple's G4 XServes, their performance was better than the x86 crowd, and the Sun servers as well. Those tests were on several sites. Been a while though.
  • JohanAnandtech - Friday, June 3, 2005 - link

    querymc: Yes, you are right. The --noaltivec flag and the comment that altivec was enabled by default in the gcc 3.3.3 compiler docs made me believe there is autovectorization (or at least "scalarisation"). As I wrote in the article we used -O2 and and then tried a bucket load of other options like --fast-math --mtune=G5 and others I don't remember anymore but it didn't make any big difference.
  • querymc - Friday, June 3, 2005 - link

    The SSE support would probably also be improved by using GCC 4 with autovectorization, I should note. There's a reason it does poorly in GCC 3. :)
  • querymc - Friday, June 3, 2005 - link

    Johan: I didn't see this the first time through, but you need to make a slight clarification to the floating point stuff. There is no autovectorization capability in GCC 3.3. None. There is limited support for SSE, but that is not quite the same, as SSE isn't SIMD to the extent that AltiVec is. If you want to use the AltiVec unit in otherwise unaltered benchmarks, you don't have a choice other than GCC 4 (and you need to pass a special flag to turn it on).

    Also, what compiler flags did you pass on each platform? For example, did you use --fast-math?
  • JohanAnandtech - Friday, June 3, 2005 - link

    Melgross: Apple told me that most xserves in europe are sold as "do it all". A little webserver (apache), a database sybase, samba and so on. They didn't have any client who had heavy traffic on the webserver, so nobody complains.

    Sybase/oracle seems to have done quite a bit of work to get good performance out of Mac OS-x, so it must be interesting to see how they managed to solve those problems. But I am sceptical that Oracle/Sybase runs faster on Mac OS x than on Linux.

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