Micro CPU Benchmarks: Isolating the FPU

Although it surely wasn't the main subject of our first article, the FLOPS (Floating Point Operations Per Second) portion was one part where I clearly made a mistake. Indeed, the --noaltivec flag and the comment that Altivec was enabled by default in the gcc 3.3 compiler docs made me believe that some Altivec SIMD optimization was being done when compiling flops, a synthetic micro FPU benchmark. That was not true: flops is double precision and gcc 3.3 did not support vectorisation.

As I wrote in the article, we used -O2 and then tried a bucket load of other options like --fast-math --mtune=G5, but it didn't make any significant difference.

Again, note that benchmarking with flops is not real world, but it isolates the FPU power. 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 the following on the Opteron based PCs:
Gcc -O2 -march=k8 flops.c -o flops
And, on the G5 machines, we used:
Gcc -O2 -mcpu=G5 flops.c -o flops
The command "gcc - version" gave this output "gcc (GCC) 4.0.0 Copyright (C) 2005 Free Software Foundation, Inc."

Let us check out the results:

gcc 4.0
Powermac G5 2.7 GHz
gcc 3.3
Powermac G5 2.5 GHz
gcc 3.3
Opteron 850 2.4 GHz
gcc 3.3.3
Opteron 850 2.4 GHz
gcc 4.0
1 50% 0% 43% 7% 1158 1104 1026 1404 1319
2 43% 29% 14% 14% 607 665 618 844 695
3 35% 12% 53% 0% 3047 2890 2677 1955 1866
4 47% 0% 53% 0% 1583 522 486 1856 1850
5 45% 0% 52% 3% 1418 675 628 1831 1362
6 45% 0% 55% 0% 2163 915 851 1922 1698
7 25% 25% 25% 25% 546 284 265 562 502
8 43% 0% 57% 0% 2020 925 860 1989 1703
Average: 1568 998 926 1545 1374

As Gabriel Svelto and other readers pointed out, the problem with gcc 3.3 generating code for PowerPC CPUs is that it outputs very poorly scheduled code for these CPUs. The result is that gcc 3.3 does not make good use of the FP units of the G5 core, which are capable of FMADD instructions. This kind of instruction performs a 64-bit, double-precision floating-point multiply of an operand in floating-point register (FPR) "FRA" by the 64-bit, double-precision floating-point operand in FPR "FRC"; then add the result of this operation to the 64-bit, double-precision floating-point operand in FPR "FRB". Thus if the code allows it, you can do a multiplication and an addition while executing only one instruction. gcc 4.0 is a lot better at using these capabilities as you can see.

A bit disappointing is the fact that gcc 4.0 lowers the performance of the Opteron compared to gcc 3.3.3, but this article is not about compiler technology; rather, it is about comparing the G5 and the Apple platform to the x86 platform. With our current benchmark data, we can conclude that the G5's FPU performance is as good as the best x86 FP chip, the AMD Athlon 64 / Opteron. Using IBM's compiler for the G5 and Intel's compiler on the Opteron, there will be higher results for both platforms, but we wanted a comparison with exactly the same compiler technology.

Benchmark Configuration The Xserve Server Platform
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  • tthiel - Wednesday, May 24, 2006 - link

    You need to redo this entire test. So much has come out about how poorly this was done its hard to believe it came from Anandtech.
  • iggie - Friday, January 13, 2006 - link

    I'm surprised you didn't post the raw VM latency results from lmbench. I found http://www-128.ibm.com/developerworks/library/l-yd...">another article that did a similar performance comparison (Darwin vs. Linux on G5).
    mmap latency is 3x greater, but most tellingly, page fault latency is > 900 x greater!

    Did you observe similar results in your tests?

    I would imagine that page faults would play a greater and greater role as more and more independent clients connect to a server. I have experienced a huge disparity in http://www.openmicroscopy.org/api/omeis/">our own server software implementation for scientific imaging. In our case, all disk access is done via mmap and page faults (its a shared-VM-based image server system meant to serve many terabytes of image data)
  • asifyoucare - Sunday, September 4, 2005 - link

    Interesting article.

    If you suspect that thread performance is the bottleneck, why not write a short program to measure how many threads can be created and destroyed per second?

  • DoctorBooze - Saturday, September 3, 2005 - link


    In the case of Linux, creating a thread is very similar to creating a process. [...] So, if you test fork() on Linux, you also get a rough idea of how fast threads are created

    I'm no guru but I don't think that's true now with Native Posix Threads, which you get in 2.6 kernels with a suitable libc (and some distros with 2.4 kernels). Check what your program's linked with: on my Fedora Core 3 system `ldd /usr/libexec/mysqld` shows me MySQL is linked with /lib/tls/libc.so.6 and running that shows it has NPTL. The API may be similar but what happens in the kernel isn't and it makes a big, big difference to MySQL. Still, Linux now has fast native POSIX threads and it looks like OS X doesn't.
  • ikruusa - Saturday, September 3, 2005 - link

    Indeed, as mentioned previously there was some mistakes in gcc options. And SIMD optimization is really basic in 4.0.x - only certain loops can be vectorized automatically. But loops around arrays are most significant part in signal processing and that is where SIMD really matters :)
    As we know for NetBurst arch it is recommended to use XMM registers (that is registers for SSE/SSE2) for FP calculations. And that is what gcc 3.x does (4.x too): -mfpmath=sse triggers all x87 stuff to run as scalar math using SSE command-set. As I know AltiVec is SIMD unit which is smoothly added to PowerPC pipeline. How useful there is scalar math instead of usual FP - I have no idea.
    What I want to say - my opinion is that if MySQL team has something to say about compiler options then they have documents about it. Using SIMD style processing in DB engine is very challenging exercise for coders. Dont expect magic from compiler here. Hint: maybe Intel's own icc compiler provide some magic but you have to prove it ;) I still believe that the most useful options can be -O[2,3] -funroll-loops and -ffast-math (as you mentioned) with -arch=[processor]. The last one should provide basic branching elimination (e.g. using cmov for x86) and correct instr. ordering.
    About testing Linux. I have some skills in Apache testing with JMeter. I have been quite stuck but kernel developers were kind enough to help: http://marc.theaimsgroup.com/?l=linux-kernel&m...">http://marc.theaimsgroup.com/?l=linux-kernel&m...
    Then I discovered all OS tuning possibilities in /proc Well, most are still unknown for me but I just want to get your attention here. Oracle talks about shared memory and number of semaphores and some particular Linux /proc parameters. Of course there should be all written in MySQL manual too if any parameter needs tuning. But is it enough to read MySQL manual and create profile for OS'es IPC and process management if we need to stress test MySQL on e.g 8-way SMP?
    But still - good start of interesting investigation, anandtech.com!! Thank you and keep going!
  • kvs - Saturday, September 3, 2005 - link

    If thread-creation is extremely slow in Darwin, maybe MySQL-performance could be helped by enabled the thread cache? A look at 'mysqladmin extended-status' would show how many threads had been created and cached, and should reveal if thread_cache would be needed.
  • tester2 - Friday, September 2, 2005 - link

    Well if ab on Mac OS X was the problem you could have easily tested this from a Linux box over the network.

    Because you probably did this as well, and found out that performance tuning done by Apple outperformed the Linux/PPC and Linux/Opteron system by a substantial amount you keept this out of the story ...

    So I did some testing, and yes when using ab from a Mac OS X I find the exact figures you report. Using a Linux Pentium 4 based system over Gb network gave me 6150 req/sec substantially faster then anything out there.
    Look here for numbers from another source; http://www.pcmag.com/article2/0,1895,1637655,00.as...">http://www.pcmag.com/article2/0,1895,1637655,00.as...

    The webserver runs around 60 threads ... go figure.

    Yes there is a problem with the Mac OS X - Mysql combo if you are looking for performance, but jugging this as Mac OS X for server applications is a nono is drawing the wrong conclusion. I hope someone with good development skills will look at the mysql code and tune it to work well with Mac OS X.

  • benh - Friday, September 2, 2005 - link

    Interesting article ! One thing that is worth looking into however is wether the YDL kernel is actually a 32 or a 64 bits kernel. This would probably have an impact on some of the numbers. I would expect the ppc64 kernel to perform faster overall on a 64 bits CPU with a small overhead on syscalls from 32 bits applications due to the argument size translation.

    Also, the problem with the 2.7Ghz on linux is indeed a slight change in the firmware. It in fact looks like a bug in Apple Open Firmware device tree on those machine where they left out the properties providing the interrupt routing of the i2c controller in the north bridge used to drive the fan controller among others. The OS X driver silently falls back to a polled mecanism, while the linux driver doesn't and (shame on me!) used to have a small bug that would cause it crash when unable to locate those properties.

    I posted a patch a while ago fixing that up, I would expect YDL to have an updated kernel/installer available by now.

    Finally, you are right about the U3 northbridge having a quite high memory latency, that is definitely not helping the G5. There have been rumours floating around that Apple now has a new bridge that improves that significantly, though it's pretty much impossible to tell if/when they will release a machine using it. IBM also had multicore G5s available for some time now, though Apple is still not releasing any machine using them.

  • JohanAnandtech - Friday, September 2, 2005 - link

    Thanks for the very helpful feedback.

    Do you have any idea why the U3 came with such high latency. Lack of development time? Lack of expertise? A inherent problem with the FSB of the G5? Rather old technology? You see I am very curious, and couldn't find much info on it.

  • benh - Friday, September 2, 2005 - link

    I don't know for sure. I wouldn't blame the FSB though. I remember reading somewhere that the memory controller in U3 was similar if not identical to the old one they used in U2 on G4 machines and was to blame but I can't guarantee the reliability of that information.

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