The G5 a.k.a. Power 970FX
You might not have noticed it, but there is in fact a lot of good news in this article for owners of current Apple systems. Gcc 4.0 promises a lot better (FP) performance in open source software. The improvement from gcc 4.0 over gcc 3.3.3 and 3.3 is amazing on the PowerFX family: almost a 70% improved FP performance!Now that the open source community finally has a decent compiler for the Apple platform, Apple management decides to step over to another architecture. Ironically, right now, the Intel architecture needs a super-optimized compiler (Intel's own) to reach the FP performance that the G5 now reaches with a very popular but far less aggressive compiler (gcc).
Combined with the data from our first article, we can safely say that the G5 2.7 GHz FP performance is at least as good as the best x86 CPUs. Integer performance seems to be between 70% and 80% of the fastest x86 CPUs, while FP/SIMD performance can actually surpass x86 in certain situations.
With the dual-core Power 970MP available and IBM's current outstanding track record when it comes to multi-core CPUs, big question marks can be placed on whether or not the switch to Intel CPUs will - from a technical point of view - be such a big step forward as Steve Jobs claims. There is more: each core has 1 MB cache instead of the current 512 KB, which will improve integer performance quite a bit as it lessens the impact of the biggest problem of the G5 - the high latency access to the memory system.
Xserve, silently cooled. Below the G5 with the cover, you can see the heatsink.
A low power Power 970FX is also available and consumes about 16 Watts at 1.6 GHz; so it seems that IBM, although slightly late, could have provided everything that Apple needs. The G5 with its 58 million transistors and 66 mm² die size is not really a hot CPU. The Xserve (2 x 2.3 GHz G5) was by far the quietest 1U air-cooled server that ever entered our lab in Kortrijk.
The Usual Suspects
The Mac OS X kernel environment includes the Mach kernel, BSD, the I/O Kit, file systems, and networking components. Some of these components slow down MySQL significantly. While our rough profiling has not identified the true culprits, we think that we can narrow the possible suspects to:- Relatively high TCP Latency that we measured
- The implementation of the threading system. Does the pthread to Mach threads mapping involve some overhead, or is this the result of the traditional performance problem of the micro kernel, namely the high latency of such a kernel on system calls? While Mac Os X is not a micro-kernel, the problem might still exist as the Mach core is deep inside that kernel. Is there IPC overhead? Lmbench signaling benchmarks seem to suggest that there is.
- The finer grained locking in the current version of Tiger does not appear to be working for some reason and we still have the "two lock" system of Panther.
We look forward to testing other database and server apps on the Mac OS X platform. Critical reports that point out weaknesses can only help the Apple community move forward and keep the Apple people on their toes.
References
[1] Threading on OS Xhttp://developer.apple.com/technotes/tn/tn2028.html
[2] Lmbench: Portable Tools for Performance Analysis
Larry McVoy, Silicon Graphics
Carl Staelin, Hewlett-Packard Laboratories
[3] Performance Characterization of a Quad Pentium Pro SMP Using OLTP
Kimberly Keeton*, David A. Patterson*, Yong Qiang He+, Roger C. Raphael+, and Walter E. Baker
Computer Science Division
University of California at Berkeley
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Gandalf90125 - Friday, September 2, 2005 - link
From 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 - link
A 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 - link
The 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 - link
While 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 - link
If 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 - link
http://www.apple.com/education/hed/compsci/tiger.h...">http://www.apple.com/education/hed/compsci/tiger.h... :"FreeBSD 5.0
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 - link
Readers 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 - link
Yes, 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 - link
Is 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 - link
I 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.