Huge interrupt overhead reported after RAM added to Atom-based system

Brett Glass brett at
Sat Sep 3 20:38:36 UTC 2011

I've just seen something very peculiar. I have here a dual Atom 
(D525) system which was running with 1 GB of RAM, and this morning 
I put a 4 GB module into the system instead. Suddenly, the 
systat(8) and top(8) commands were both reporting bursts of 
interrupt overhead as high as 25% of total CPU capacity. Yet, in 
the display from the systat -vmstat option, no additional 
interrupts were appearing on the right hand side of the screen 
where interrupt sources were listed.

The system is running FreeBSD 8.1-RELEASE.

The documentation for several motherboards online mention that the 
Atom can be run with 4 GB of RAM with a "64 bit operating system."

I can't tell whether something non-obvious is going on under the 
hood -- either in the chipset or in the CPUs -- that's racking up 
overhead, or if the interrupt overhead doesn't exist at all and the 
reported CPU load is an artifact of some weirdness in the kernel. I 
need to know, though, before I deploy the system... so I'd 
appreciate any advice or ideas from any kernel experts who might be 
reading messages here.

--Brett Glass

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