true power consumption (was: disadvantages of linux cluster)

Dave Lane dlane at ap.stmarys.ca
Fri Nov 8 14:23:59 EST 2002


At 05:53 PM 11/8/2002 +0100, you wrote:
>First, your facts below are, to put it mildly, incorrect. Power max as
>given by AMD and Intel for their cpus are unfortunately not 'measured' the
>same way. To get some hard figures I grabbed my clamp meter and went into
>the computer room. Time for a reality check:
>
>dual MP2000+     idle: 0.5-0.6A (kept changing, 0.55)
>dual Xeon 2.4GHz idle: 0.2A
>dual MP2000+     load: 0.7A
>dual Xeon 2.4GHz load: 0.5-0.6A (kept changing, ~0.55)
>
>Obviously my clamp meter isn't very accurate but the relative changes
>should hold. If the numbers seem awfully low keep in mind that we are
>running on 230V here.

I haven't been doing the same thing here over the last two days and you 
have to be very careful when interpreting these measurements. Most meters 
assume that the waveform is sinusoidal and use this fact to guess what 
"RMS" current reading to display on the meter. Switching power supplies 
have nothing like a sinusoidal current waveform.

I have been measuring the supply current (120V) for an Athlon XP1700 PC 
running Linux (MSI KT266A MB, 20G drive, CD, floppy, KB, Mach64 video). For 
cpu loading I'm using the setiathome cmdline version). The measurement 
technique is inline (I made a custom cable to do this) with both normal 
(Fluke 73UIII) and true-rms (Fluke 79III) multimeters.

                 Unloaded (A)    Loaded (A)      Unloaded (W)    Loaded 
(W)
Normal          0.65A           0.74A           78W             88.8W
True RMS        1.13A           1.3A            135W            156W

Note that huge difference in readings. This even surprised me and I should 
have known better before trying this! I was also surprised how little 
difference there was between loaded and unloaded. Note that the same 
current readings were measured for unloaded when the machine was sitting in 
the setup screen of the bios.

Note that these readings don't account for the power factor that others 
have mentioned.

... Dave
Saint Mary's University
Electronics Technologist turned Solaris admin turning Linux admin

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