Hyperthreading hurts 5.3?

Anthony Atkielski atkielski.anthony at wanadoo.fr
Wed Jan 12 09:41:06 PST 2005

Giorgos Keramidas writes:

GK> You need to enable SMP too, to allow the FreeBSD kernel to use the
GK> second (hyper-threaded) CPU.

I found it, in a file called SMP.  Why is the SMP option tucked away in
a separate file?

I stuck this into the config and rebuilt the kernel. Seems to run fine.
I see that top has a C column now, and running a program in a continuous
loop takes up exactly 50% of the machine (and two such programs take up
exactly 100%), according to top.

Boot messages looked a bit different:

The Regents of the University of California. All rights reserved.
FreeBSD 5.3-RELEASE #0: Wed Jan 12 17:55:18 CET 2005                                                                                                                                                                                                                                                               
root at freebie.atkielski.com:/usr/obj/usr/src/sys/FREEBIE-SMP                                                                                                                                                                                                                                                        
Timecounter "i8254" frequency 1193182 Hz quality 0                                                                                                                                                                                                                                                                 
CPU: Intel(R) Pentium(R) 4 CPU 3.00GHz (2998.57-MHz 686-class CPU)                                                                                                                                                                                                                                                 
Origin = "GenuineIntel"  Id = 0xf34  Stepping = 4                                                                                                                                                                                                                                                                  
Hyperthreading: 2 logical CPUs                                                                                                                                                                                                                                                                                     
real memory  = 1073414144 (1023 MB)                                                                                                                                                                                                                                                                                
avail memory = 1045061632 (996 MB)                                                                                                                                                                                                                                                                                 
ACPI APIC Table: <A M I  OEMAPIC >                                                                                                                                                                                                                                                                                 
FreeBSD/SMP: Multiprocessor System Detected: 2 CPUs                                                                                                                                                                                                                                                                
cpu0 (BSP): APIC ID:  0                                                                                                                                                                                                                                                                                            
cpu1 (AP): APIC ID:  1                                                                                                                                                                                                                                                                                             
cpu0: <ACPI CPU> on acpi0
cpu1: <ACPI CPU> on acpi0 
SMP: AP CPU #1 Launched!

So I guess it's all working now.  That was easy.

I imagine this will give me a bit more horsepower for the buck,
although--with only 0.2% of the machine busy under normal load even with
a single processor--I guess I wasn't exactly processor-bound to begin
with (I'd run out of I/O capacity long before running out of processor).
But why not profit from what's there, eh?


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