Bug 219457

Summary: CONFIG_NR_CPUS is insanely high for fedora
Product: [Fedora] Fedora Reporter: Arjan van de Ven <arjan>
Component: kernelAssignee: Dave Jones <davej>
Status: CLOSED ERRATA QA Contact: Brian Brock <bbrock>
Severity: medium Docs Contact:
Priority: medium    
Version: 6CC: pfrields, wtogami
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OS: Linux   
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Fixed In Version: Doc Type: Bug Fix
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Last Closed: 2006-12-19 03:40:03 UTC Type: ---
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Description Arjan van de Ven 2006-12-13 10:01:21 UTC 
Description of problem:

The Fedora kernel has CONFIG_NR_CPUS set to 255, which is insanely high.
First of all, no systems exist that can have that many processors or cores (you
run out of apics well before that).
Second, even if they would exist they'd probably not run fedora. (and would cost
a milion bucks or more:0

The downsides are a massive waste of memory; for example just the irq_desc array
alone (in 2.6.19) is more than 2 megabytes just because of this, and a lot of
other things in memory scale likewise with CONFIG_NR_CPUS.

I would suggest a value of no larger than 64 (that also means that cpu bitmasks
fit in a long) for this value ... it wastes a lot less memory and generates a
lot better code in all the cpu mask operations..
Comment 1 Dave Jones 2006-12-19 03:40:03 UTC 
fixed in update.