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Date: Sun, 12 Mar 2000 12:16:22 -0800 (PST)
From: aron@cs.rice.edu
To: freebsd-gnats-submit@FreeBSD.org
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Subject: i386/17346: APIC cannot be enabled without turning on support for SMP
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>Number:         17346
>Category:       i386
>Synopsis:       APIC cannot be enabled without turning on support for SMP
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sun Mar 12 12:20:01 PST 2000
>Closed-Date:
>Last-Modified:
>Originator:     Mohit Aron
>Release:        FreeBSD-current
>Organization:
Rice University
>Environment:
FreeBSD aurora.cs.rice.edu 4.0-20000208-CURRENT FreeBSD 4.0-20000208-CURRENT #1: Mon Feb 14 19:59:00 CST 2000     root@vaddr.cs.rice.edu:/usr/src/sys/compile/VADDR  i386
>Description:
The FreeBSD kernel only supports enabling the APIC when SMP support
is configured in the kernel. APICs are desirable even in uniprocessor
systems because they provide 1) on chip timers 2) larger number of IRQs
and 3) low interrupt overheads. Unfortunately, if the FreeBSD kernel
is compiled with SMP support, the performance drops down - possibly
due to the various locking mechanisms that get turned on. A webserver's
performance drops down by 22% when SMP support is turnedon a an otherwise
uniprocessor machine. 

>How-To-Repeat:
Trying compiling the kernel after turning on the option APIC_IO in 
the kernel configuration file. Make sure that the option SMP is not 
turned on. Kernel wouldn't compile.
>Fix:
The FreeBSD kernel should support turning on the APIC without turning
on SMP support. This is useful in uniprocessor systems.

>Release-Note:
>Audit-Trail:
>Unformatted:


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