No More Apple Mysteries, Part Twoby Johan De Gelas on September 1, 2005 12:05 AM EST
- Posted in
The Xserve Server PlatformThe most surprising and even astonishing results of the previous article were, of course, the MySQL and Apache server benchmarks. A powerful Windows XP based client (see above: "Client Configuration: Dual Opteron 250") fires off an enormous amount of Select, grouping and ordering read intensive queries and simulates 1 to 50 concurrent clients. All that query data is sent over a direct Gigabit Ethernet link to the tested server; in this case, a PowerMac Dual G5 2.5 GHz running OS X Server (Tiger). In part I, we discovered that performance of the Apple machine completely collapsed once there were more than 2 concurrent clients.
The solution? Install a Linux distribution to verify our suspicion that the OS is to blame is on the mark. We chose Yellow Dog Linux (YDL). Terra Soft, the company behind Yellow Dog, is an Apple Authorized OEM Value Added Reseller, so you could say that Apple has no objection to installing YDL on your Apple machines. There is more: Terra Soft is specialized in optimizing for the G5 processor. The version that we used, Yellow Dog Linux 4.0.1, is based on the Linux Kernel version 2.6.10-1.ydl.1g5-smp.
Let us see how the Dual 2.5 GHz G5 performed in MySQL when running Yellow Dog Linux. Please note: YDL 4.0 wouldn't run on the 2.7 GHz Apple machine, so we do not have results for that platform.
The difference between the PowerMac running Linux and Mac OS X Server is absolutely striking. Mac OS X server shows better performance going from one to a second connection (and thus thread) because the second CPU steps in and helps carry the load. After that, however, performance completely collapses and stabilizes at around 50 queries per second.
While the G5 is not the best integer processing unit out there, it is not the one to blame for the poor performance that we experienced in our first tests. Running Yellow Dog Linux, the Dual G5 was capable of performing similar to a 3 GHz Xeon. Notice that more concurrent connections gives better performance from 1 to 20. At 5 concurrent simulated users, YDL simply wipes the floor with Mac OS X: 411 versus 113 queries per second. It gets worse at 10 concurrent users: 443 queries per second on Linux versus 62 on Mac Os X. Around 20 connections, performance declines only very slowly just like all the x86/Linux machines.
With the MySQL performance woes now clearly caused by OS X, let us see if Apache tells us the same story. We tested with Apachebench, with "n" being the total of number of connections and "c" the total of concurrent connections:
ab -n 100000 -c 100 http://localhostSome people suggested that we should test with both Apache 1.3 and 2.0, so we gave Apache 2.0 a test run.
|Unit: Requests per second||Powermac Dual G5 2.5 GHz OS X||Powermac Dual G5 2.5 GHz YDL||Dual Xeon 3.6 GHz|
On OS X, we noticed that the activity monitor was telling us that the CPUs were not working very hard and were underutilized. This seems to indicate that the problem with Apache is somewhat different from MySQL, as MySQL showed a CPU load between 165% and 190%. (200% is the maximum, as there are 2 CPUs in the system.)
Apple told us that the problem lies in Apachebench (the client side), which stalls from time to time and thus generates too low of a load on the (Apache) server. The weird thing is that this does not happen with few connections (up to 10,000). When we repeated the test, Apachebench on Mac OS X gets in trouble again. Version 2.0 is slightly faster on OS X, but it still trails by a significant margin. On the other hand, YDL and the Xeon platform are roughly 3X as fast with version 2.0.
According to Apple, this is a bug in Apachebench. Now, we can accept that explanation, as it is clear that the server is not loaded and can still accept a lot more web requests. However, the Apachebench problem is still interesting. Why exactly does the client stall? Is it really a bug or is it running out of some resources? We didn't delve deeper, as we are developing a less synthetic, closer to the real world benchmark to test web servers.
Even if we ignore the Apache results, our MySQL tests - and the queries used in these tests - are based on a real world usage pattern of a real world database. The G5 is partially crippled by a chipset that takes a long time to access the memory, and it's not the fastest integer CPU; still, it performs like a 3 GHz Xeon on Linux. The problem clearly lies in Mac OS X, and is worth further investigation.
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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.