No More Apple Mysteries, Part Twoby Johan De Gelas on September 1, 2005 12:05 AM EST
- Posted in
IntroductionA little bit more than a month ago, AnandTech published "No more mysteries: Apple's G5 versus x86, Mac OS X versus Linux" with the ambitious goal of finding out how the Apple platform compares, performance-wise, to the x86 PC platform. The objective was to find out how much faster or slower the Apple machines were compared to their PC alternatives in a variety of server and workstation applications.
Some of the results were very surprising and caught the attention of millions of AnandTech readers. We found out that the Apple platform was a winner when it came to workstation applications, but there were serious performance problems when you run server applications such as MySQL (Relational Database) or Apache (Webserver). The MySQL database running on Mac OS X and the Dual G5 was up to 10 times slower than on the Dual Opteron running Linux.
We suspected that Mac OS X was to blame as low level OS benchmarks (Lmbench 3.0) indicated low OS performance. The whole article was a first attempt to understand better how the Apple platform - Mac OS X + G5 - performs, and as always, first attempts are never completely successful or accurate. As we found more and more indications that the OS, not the CPU, was the one to blame, it became obvious that we should give more proof for our "Mac OS X has a weak spot" theory by testing both the Apple and x86 machines with Linux. My email was simply flooded with hundreds of requests for some Linux Mac testing...even a month after publication!
That is what we'll be doing in this article: we will shed more light on the whole Apple versus x86 PC, IBM G5 versus Intel CPU discussion by showing you what the G5 is capable of when running Linux. This gives us insight on the strength and weakness of Mac OS X, as we compare Linux and Mac OS X on the same machine.
The article won't answer all the questions that the first one had unintentionally created. As we told you in the previous article, Apple pointed out that Oracle and Sybase should fare better than MySQL on the Xserve platform. We will postpone the more in-depth database testing (including Oracle) to a later point in time, when we can test the new Apple Intel platform.
Why Bother?Why do we bother, now that Apple has announced clearly that the next generation of the Apple machines will be based on Intel? Well, this makes our research even more interesting. As you will see further in the article, the G5 is not the reason why we saw terrible, slow performance. In fact, we found that the IBM PowerPC 970FX, a.k.a. "G5", has a few compelling advantages.
As Apple moves to Intel, the only thing that makes Apple unique, and not yet another x86 PC OEM, is Mac OS X. That is why Apple will attempt to prevent you from running an x86 version of Mac OS X on anything else but their own hardware (using various protection schemes), as Anand reported in "Apple's Move to x86: More Questions Answered". Mac OS X will be the main reason why a consumer will choose an Apple machine instead of a Dell one. So, as we get to know the strengths and weaknesses about this complex but unique OS, we'll get insight into the kind of consumers who would own an Intel based machine with Mac OS X - besides the people who are in love with Apple's gorgeous cases of course....
We also gain insight into the real reasons behind the move to Intel, and what impact it will have for the IT professional. Positive but very vague statements about the move to the Intel architecture have already been preached to the Apple community. For example, it was reported that the "Speed of Apple Intel dev systems impress developers". Proudly, it was announced that the current Apple Intel dev systems - based on a 3.6 GHz Intel Pentium 4 with 2 MB L2 Cache - were faster than a dual G5 2 GHz Mac. That is very ironic for three reasons.
Firstly, Apple's own website contradicts this in every tone. Secondly, we found a 2.5 GHz G5 to perform more or less like a Pentium 4 3 - 3.2 GHz in integer tasks. So, a 2 GHz G5 is probably around the speed of a 2.6 GHz Pentium 4. It is only natural that a much faster single CPU with a better disk and memory system outpaces a slower dual CPU in single threaded booting and development tasks. Thirdly, the whole CPU industry is focused now on convincing the consumers of how much better multi-core CPUs are compared to their "old" single core brethren.
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Gandalf90125 - Friday, September 2, 2005 - linkFrom the article:
"...so it seems that IBM, although slightly late, could have provided everything that Apple needs."
I'd say not everything Apple needs as I suspect the switch to Intel was driven more by marketing than any technical aspect of the IBM vs. the Intel chips.
Illissius - Friday, September 2, 2005 - linkA few notes:
- you mention trying a --fast-math option, which I've never heard of... presumably this was a typo for -ffast-math?
- when I tried using -mcpu (which you say you used for YDL) on GCC 3.4, it told me the option had been deprecated, and -mtune has to be used instead (dunno whether it told me this latter part itself or I read it somewhere else, but it's true). I'm not sure whether GCC4 has the same behaviour (I'd think so), whether it still has the intended effect despite the warning, or whether it matters at all.
- was there a reason for using -march on one, and -mcpu/-mtune on the other? (the difference is that -mcpu/-mtune optimize the code for that processor as much as possible while still keeping the code compatible with everything else in the architecture, while -march does the same without care for compatibility -- on x86 at least, not sure whether it's the same on PPC)
- you mention using the same compiler because, err, you wanted to use the same compiler... if this was done in the hopes of it generating code of similar speed for each architecture, though, then your own results show there isn't much point -- seems GCC, 3.3 at least, is much better at generating x86 code than PPC (which isn't surprising, given much more work probably went into it due to the larger userbase). Not saying it was a bad idea to use GCC on both platforms (it's a good one, if for no other reason than most code, on the Linux side at least and OSX itself (not sure about the apps) are compiled with it), just that if the above was the reason, it wasn't a very good one ;).
- Continuing the above, I was a bit surprised at the, *ahem*, noticeable difference in speed between not even two different compilers, but two versions of the same. (I was expecting something like 1-5, maybe 10% difference, not 100). Maybe this could warrant yet another followup article, this time on compilers? :)
Pannenkoek - Friday, September 2, 2005 - linkThe reason is that GCC 4.0 incorporated infrastrucure for vector optimization (tree-ssa), which can give, especially in synthetic benchmarks, huge increase in FP performance. GCC can now finally optimize for SSE, Altivec, etc., a reason why in the past optimizing specifically for newer Pentiums did not yield much improvement.
Althougn compiler benchmarks would be interesting, I doubt it is a task for Anandtech. Normal desktop users do not have to worry about whether or not their applications are optimized optimally, and any differences between, say GCC and ICC, are small or negligible for ordinary desktop programs. (Multimedia programs often have inline assembly for performance critical parts anyway).
GCC is free, supports about any platform and improves continually while it's already a first class compiler.
javaxman - Friday, September 2, 2005 - linkWhile I generally love this article, I have to wonder...
why not write a simple benchmark for pthread(), if you think that's the bottleneck? Surely it'd be a simple thing to write a page of code which creates a bunch of threads in a loop, then issues a thread count and/or timestamp. It seems like a blindingly obvious test to run. Please run it.
I have to say that I *do* think pthread() is the likely bottleneck, possibly due to BSD4.9-derivative code, but why not test that if we think that's the problem? I understand wanting to see real-world MySQL performance, but if you're trying to find a system-level bottleneck, that's not the right type of testing to do...
Now that I metion it, Darwinx86 vs. BSD 4.9 ( on the same system ) vs. BSD 5.x ( on the same system ) vs. Linux ( on the same system ) would really be a more interesting test at this point... I'm really not caring about PPC these days unless it's an IBM blade system, to be honest... testing an Apple PPC almost seems silly, they'll be gone before too long... Apple's decision to move away from PPC has more to do with *future* chip development than *current* offerings, anyway... Intel and AMD are just putting more R&D into their x86 chips, IBM's not matching it, and Apple knows it...
but even if you are going to look at PPC systems, if you're trying to find a system-level bottleneck, write and run system-level tests... a pthread() test is what is needed here.
rhavenn - Friday, September 2, 2005 - linkIf I remember correctly, OS X is forked off of the FreeBSD 4.9 codebase. The 4.x series of BSD always had a crappy threading system and didn't handled threaded apps well at all. I doubt Apple really touched those internals all that much.
FreeBSD 5.x has a much better time of it. I'm wondering if the switch back to a Intel platform will make it easier for Apple to integrate the BSD 5.x codebase into their OS? or even if they plan on using the BSD 6.x codebase for a future release? The threading models have vastly improved.
Just a thought :)
JohanAnandtech - Friday, September 2, 2005 - linkhttp://www.apple.com/education/hed/compsci/tiger.h...">http://www.apple.com/education/hed/compsci/tiger.h... :
The upgraded kernel in Tiger, based on mach and FreeBSD, provides optimized resource locking for better scalability across multiple processors, support for 64-bit memory pointers through the System library and standards-based access control lists"
Where did you see FreeBSD 4.9?
mbe - Friday, September 2, 2005 - linkReaders also pointed out that LMBench uses "fork", which is the way to create a process and not threads in all Unix variants, including Mac OS X and Linux. I fully agree, but does this mean that the benchmark tells us nothing about the way that the OS handles threading? The relation between a low number in this particular Lmbench benchmark and a slow creating of threads may or may not be the answer, but it does give us some indication of a performance issue. Allow me to explain...
This misses the point, your claim in the last article was that MacOS X used userspace threads. Mentioning that LMbench uses processes still rules out userspace threads having any part to play. This is since processes can't in any meaningful way (short of violating some pretty basic principles) be implemented around userspace threads. The point is that a process is a virtual memory space attached to a main system thread, not a userspace thread which are not normally even considered threads on this level.
This is necessary since the virtual memory attached to the thread has to be managed when doing context switches, and by its very definition userspace code cannot directly touch the memory mappings.
JohanAnandtech - Friday, September 2, 2005 - linkYes, it could be. The interesting questions are:
- Is the only culprit for the 8 time lower performance. Microkernels are reported to be 66 to 5% slower depending on who benchmarked it. But not 8 times slower.
- What makes it still interesting for the apple devs to use it?
I hope Apple will be a bit more keen to defend their product, because their might be interesting technical reasons to keep the Mach kernel.
sdf - Friday, September 2, 2005 - linkIs Mac OS X really a microkernel? I understood it to be designed to function as a microkernel, but compiled and shipped as a macrokernel for performance reasons.
JohanAnandtech - Sunday, September 4, 2005 - linkI am sorry if I wasn't clear. As I state in the article clearly: Mac OS X is ** NOT ** a microkernel, but based on a microkernel as the Mach kernel is burried inside the FreeBSD monolithic kernel.
Most of the tasks are done by a FreeBSD alike kernel, but threading is done by the Mach kernel.