No more mysteries: Apple's G5 versus x86, Mac OS X versus Linux
by Johan De Gelas on June 3, 2005 7:48 AM EST- Posted in
- Mac
Workstation, yes; Server, no.
The G5 is a gigantic improvement over the previous CPU in the PowerMac, the G4e. The G5 is one of the most superscalar CPUs ever, and has all the characteristics that could give Apple the edge, especially now that the clock speed race between AMD and Intel is over. However, there is still a lot of work to be done.First of all, the G5 needs a lower latency access to the memory because right now, the integer performance of the G5 leaves a lot to be desired. The Opteron and Xeon have a better integer engine, and especially the Pentium 4/Xeon has a better Branch predictor too. The Opteron's memory subsystem runs circles around the G5's.
Secondly, it is clear that the G5 FP performance, despite its access to 32 architectural registers, needs good optimisation. Only one of our flops tests was " Altivectorized", which means that the GCC compiler needs to improve quite a bit before it can turn those many open source programs into super fast applications on the Mac. In contrast, the Intel compiler can vectorize all 8 tests.
Altivec or the velocity engine can make the G5 shine in workstation applications. A good example is Lightwave where the G5 takes on the best x86 competition in some situations, and remains behind in others.
The future looks promising in the workstation market for Apple, as the G5 has a lot of unused potential and the increasing market share of the Power Mac should tempt developers to put a little more effort in Mac optimisation.
The server performance of the Apple platform is, however, catastrophic. When we asked Apple for a reaction, they told us that some database vendors, Sybase and Oracle, have found a way around the threading problems. We'll try Sybase later, but frankly, we are very sceptical. The whole "multi-threaded Mach microkernel trapped inside a monolithic FreeBSD cocoon with several threading wrappers and coarse-grained threading access to the kernel", with a "backwards compatibility" millstone around its neck sounds like a bad fusion recipe for performance.
Workstation apps will hardly mind, but the performance of server applications depends greatly on the threading, signalling and locking engine. I am no operating system expert, but with the data that we have today, I think that a PowerPC optimised Linux such as Yellow Dog is a better idea for the Xserve than Mac OS X server.
References
Threading on OS Xhttp://developer.apple.com/technotes/tn/tn2028.html
Basics OS X
http://developer.apple.com/documentation/macosx/index.html
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Joepublic2 - Monday, June 6, 2005 - link
Wow, pixelglow, that's an awesome way to advertise your product. No marketing BS, just numbers!pixelglow - Sunday, June 5, 2005 - link
I've done a direct comparison of G5 vs. Pentium 4 here. The benchmark is cache-bound, minimal branching, maximal floating point and designed to minimize use of the underlying operating system. It is also single-threaded so there's no significant advantage to dual procs. More importantly it uses Altivec on G5 and SSE/SSE2 on the Pentium 4, and also compares against different compilers including the autovectorizing Intel ICC.http://www.pixelglow.com/stories/macstl-intel-auto...
http://www.pixelglow.com/stories/pentium-vs-g5/
Let the results speak for themselves.
webflits - Sunday, June 5, 2005 - link
"From the numbers, it seems like gcc was only capable of using Altivec in one test,"Nonsense!
The Altivec SIMD only supports single (32-bit) precision floating point and the benchmark uses double precision floating point.
webflits - Sunday, June 5, 2005 - link
Here are the resuls on a dual 2.0Ghz G5 running 10.4.1 using the stock Apple gcc 4.0 compiler.[Session started at 2005-06-05 22:47:52 +0200.]
FLOPS C Program (Double Precision), V2.0 18 Dec 1992
Module Error RunTime MFLOPS
(usec)
1 4.0146e-13 0.0163 859.4752
2 -1.4166e-13 0.0156 450.0935
3 4.7184e-14 0.0075 2264.2656
4 -1.2546e-13 0.0130 1152.8620
5 -1.3800e-13 0.0276 1051.5730
6 3.2374e-13 0.0180 1609.4871
7 -8.4583e-11 0.0296 405.4409
8 3.4855e-13 0.0200 1498.4641
Iterations = 512000000
NullTime (usec) = 0.0015
MFLOPS(1) = 609.8307
MFLOPS(2) = 756.9962
MFLOPS(3) = 1105.8774
MFLOPS(4) = 1554.0224
frfr - Sunday, June 5, 2005 - link
If you test a database you have to disable the write cache on the disk on almost any OS unless you don't care about your data. I've read that OS X is an exception because it allows the database software control over it, and that mySql indeed does use this. This would invalidate al your mySql results except for OS X.Besides all serious database's run on controllers with write cache with batteries (and with the write cache on the disks disabled).
nicksay - Sunday, June 5, 2005 - link
It is pretty clear that there are a lot of people who want Linux PPC benchmarks. I agree. I also think that if this is to be a "where should I position the G5/Mac OS X combination compared to x86/Linux/Windows" article, you should at least use the default OS X compiler. I got flops.c from http://home.iae.nl/users/mhx/flops.c to do my own test. I have a stock 10.4.1 install on a single 1.6 GHz G5.In the terminal, I ran:
gcc -DUNIX -fast flops.c -o flops
My results:
FLOPS C Program (Double Precision), V2.0 18 Dec 1992
Module Error RunTime MFLOPS
(usec)
1 4.0146e-13 0.0228 614.4905
2 -1.4166e-13 0.0124 565.3013
3 4.7184e-14 0.0087 1952.5703
4 -1.2546e-13 0.0135 1109.5877
5 -1.3800e-13 0.0383 757.4925
6 3.2374e-13 0.0220 1320.3769
7 -8.4583e-11 0.0393 305.1391
8 3.4855e-13 0.0238 1258.5012
Iterations = 512000000
NullTime (usec) = 0.0002
MFLOPS(1) = 736.3316
MFLOPS(2) = 578.9129
MFLOPS(3) = 866.8806
MFLOPS(4) = 1337.7177
A quick add-n-divide gives my system an average result of 985.43243.
985. On a single 1.6 G5.
So, the oldest, slowest PowerMac G5 ever made almost matches a top-of-the-line dual 2.7 G5 system?
To quote, "Something is rotten in the state of Denmark." Or should I say the state of the benchmark?
Eug - Saturday, June 4, 2005 - link
BTW, about the link I posted above:http://lists.apple.com/archives/darwin-dev/2005/Fe...
The guy who wrote that is the creator of the BeOS file system (and who now works for Apple).
It will be interesting to see if this is truly part of the cause of the performance issues.
Also, there is this related thread from a few weeks back on Slashdot:
http://hardware.slashdot.org/article.pl?sid=05/05/...
profchaos - Saturday, June 4, 2005 - link
The statement about Linux kernel modules is incorrect. It is a popular misconception that kernel modules make the Linux kernel something other than purely monolithic. The module loader links module code in kernelspace, not in userspace, the advantage being dynamic control of kernel memory footprint. Although some previously kernelspace subsystems, such as devfs, have been recently rewritten as userspace daemons, such as udev, the Linux kernel is for the most part a fully monolithic design. The theories that fueled the monolithic vs. microkernel flame wars of the mid-90s were nullified by the rapid ramping of single-thread performance relative to memory subsystems. From the perspective of the CPU, it take years for a context switch to occur since modifying kernel data structures in main memory is so slow relative to anything else. Userspace context switching is based on IPC in microkernel designs, and may require several context switches in practice. As you can see from the results, Linux 2.6 wipes the floor with Darwin just the same as it does with several of the BSDs (especially OpenBSD and FreeBSD4.x) and its older cousin Linux 2.4. It's also anyone's guess whether the Linux 2.6 systems were using pthreads (from NPTL) or linuxthreads in glibc. It takes a heavyweight UNIX server system, which today means IBM AIX on POWER, HP-UX on Itanium, or to a lesser degree Solaris on SPARC, to best Linux 2.6 under most server workloads.Eug - Saturday, June 4, 2005 - link
Responses/Musings from an Apple developer.http://ridiculousfish.com/blog/?p=17
http://lists.apple.com/archives/darwin-dev/2005/Fe...
Also:
They claim that making a new thread is called "forking". No, it’s not. Calling fork() is forking, and fork() makes processes, not threads.
They claim that Mac OS X is slower at making threads by benchmarking fork() and exec(). I don’t follow this train of thought at all. Making a new process is substantially different from making a new thread, less so on Linux, but very much so on OS X. And, as you can see from their screenshot, there is one mySQL process with 60 threads; neither fork() nor exec() is being called here.
They claim that OS X does not use kernel threads to implement user threads. But of course it does - see for yourself.
/* Create the Mach thread for this thread */
PTHREAD_MACH_CALL(thread_create(mach_task_self(), &kernel_thread), kern_res);
They claim that OS X has to go through "extra layers" and "several threading wrappers" to create a thread. But anyone can see in that source file that a pthread maps pretty directly to a Mach thread, so I’m clueless as to what "extra layers" they’re talking about.
They guess a lot about the important performance factors, but they never actually profile mySQL. Why not?
orenb - Saturday, June 4, 2005 - link
Thank you for a very interesting article. A follow up on desktop and workstation performance will be very appreciated... :-)Good job!