No More Apple Mysteries, Part Twoby Johan De Gelas on September 1, 2005 12:05 AM EST
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The Aftermath of the First ArticleWe received a flood of mails and posts from our readers requesting that we test the Apple machines with Linux too and questioned why we hadn't done that in the first article. We have to point out that the objective of the first article was to compare the platforms, and therefore, it is only natural to use Mac OS X on the Apple machine. Very few Apple machines run Linux, but in this article, we test this combination to shed more light on our findings.
Secondly, we spent most of our time trying out different MySQL setups to find out whether or not the poor MySQL numbers were a result of bad tuning. We tested and tried with, for example, the "skip-locking", "key_buffer" and "thread_cache" parameters, but none of them could help the Apple platform to perform significantly better. The out-of-the-box MySQL setup on Tiger is not very different from a typical SUSE Linux out-of-the-box installation, except that skip-locking is not enabled on the Apple platform. The reason seems to be that quite a few Xserves are used in clusters. Enabling "Skip-locking" gives a 1-3% performance boost to the Xserve and PowerMacs. We can say with 99% certainty that the MySQL configuration was not the cause of the poor MySQL performance.
The vast majority of the reactions of the Apple user community were very positive, despite our low server benchmark numbers. Many Apple users told us that they were glad that we had pointed out that Mac Os X still needs a bit of performance tunings. Anand reported the same thing as what many Apple users pointed out, which is that the responsiveness of the OS is not spectacular:
"The overall responsiveness of the system was decent, but go back to using a top-of-the-line PC in Windows for a few minutes, and you definitely feel a bit sluggish on the G5"We still receive suggestions because of the first article, and one question that was asked a lot was: "why not test with different compilers?" The reason was that gcc was the default compiler on both Mac OS X and Linux. Testing with compilers would widen the scope of this kind of article too much, and we wanted to use the same compiler on all CPUs. That being said, we retested with the gcc 4.0 compiler because the 3.3 version performed pretty poorly on the Power FX platform.
I would like to thank the readers for the valuable feedback. In this second part, we'll correct the inaccuracies in the first.
Scope and FocusAgain, we are focusing on workstation and server applications, especially the open source ones (MySQL, Apache) as Apple is touting heavily the importance of their move to an "open source foundation".
The 64 bit Apple Machines were running OS X Server 10.4.1 (Tiger) and Yellow Dog 4.0 Linux version 2.6.10-1.ydl.1g5-smp. The reason we chose Yellow dog is that Terrasoft, the company behind this Linux version, optimises only for the G5. So, Yellow dog is by far the most PowerPC optimized Linux distribution out there.
Our x86 machines are still running a 64 bit server version of a popular Open Source Operating Unix system: SUSE Linux SLES 9 Service Pack 1 (kernel 2.6.5).
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tthiel - Wednesday, May 24, 2006 - linkYou 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 - linkI'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 - linkInteresting 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
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 - linkIndeed, 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 - linkIf 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 - linkWell 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 - linkInteresting 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 - linkThanks 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 - linkI 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.