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Gigabit ethernet, PowerMac G5, and CPU usage

Do you share or copy files over gigabit ethernet?  Perhaps you have a central file server and store your photographs on a server shared by multiple photographers? Perhaps you perform backups, as I do, over gigabit ethernet to another machine?

If so, you should be aware of a little detail that won’t be mentioned in any marketing materials extolling the dual gigabit ethernet ports on the latest PowerMac G5 models.  You might guess what that secret is by taking a look at the CPU usage graphs below (taken from a quad-core PowerMac G5 Quad):

The red area represents “system time”, or time that the operating system spends on behalf of programs.  In this case, 90% or more of the red area represents CPU time spent doing nothing more than transferring data over a gigabit ethernet connection to another PowerMac G5 at the rate of 90-100 megabytes/sec (roughly 800 megabits/second). [Both machines have high-speed striped RAID disk arrays].   The red area represents roughly 100% of a core, or 25% of the available processing power on the Quad.  Similar usage (100% of a CPU) may be observed on the PowerMac G5 (dual 2.5GHz/single core, which was the recipient of the data being transferred).  I have also confirmed similar CPU usage on a dual CPU (single core) 2.0 GHz PowerMac G5.   So it seems that the entire PowerMac G5 model line shares this trait (those machines cover all three generations of the PowerMac G5).

Of course, if you have a single-core machine, anything involving high-speed network transfers will run as slow as molasses, since data transfer will be competing for CPU time with productive tasks.   The utility of gigabit ethernet is severely restricted on such wimpy machines—don’t edit large image files over a network if performance matters.

Imagine what would happen if both of the built-in PowerMac Quad gigabit ethernet ports were fully utilized—fully half of the available CPU power would be sucked up simply to transfer data!  And that’s with the 4-core Quad—the mundane dual-core (single CPU) 2.0 and 2.3 GHz PowerMac G5 models not only have just two cores, but run at a slower clock speed too.  They simply don’t have enough horsepower to do anything compute-intensive while a high-speed transfer is in progress.   So stick to the Quad if your work involves high-speed transfers of large amounts of data with simultaneous demands on CPU time.

If you envision using a PowerMac as a server (Apple sells MacOS X Server after all), don’t even think about using it in any application requiring high bandwidth without first testing combined network and CPU performance.  A PCI ethernet card capable of offloading all the ethernet-processing overhead would be ideal, thus freeing the CPU to do real work.  I don’t know if such a card exists for PCI-Express for Macs yet.

Apple’s Technology Overview for the PowerMac G5 makes claims about “server class” features for the dual gigabit ethernet (see page 5 and 12).   I haven’t figured out how to use “jumbo frame support” to “reduce system overhead” yet; perhaps that would cut down the system CPU time usage shown above.  From what I can tell, such features are only supported within Mac OS X Server, but maybe there is a way to use them in plain Mac OS X.

UPDATE:  a reader informed me (thank you) that jumbo frame support can be enabled by manually configuring ethernet in the Network control panel in System Preferences (click to see a larger version):

I haven’t yet tested this, nor have I checked if my dual 2.0GHz G5 supports jumbo frames; if not, then perhaps another reader will be able to make that determination.  Update: the dual 2.0GHz G5 does not support jumbo frames.


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