on linux: only one cpu in use?

[Sascha Ottolski]

on a 4 core machine, running varnish exclusively, all the I/O goes to 
one core only:

# cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3
  0:  603869381          0          0          0    IO-APIC-edge  timer
  6:          3          0          0          0    IO-APIC-edge  floppy
  8:          0          0          0          0    IO-APIC-edge  rtc
  9:          0          0          0          0   IO-APIC-level  acpi
 14:         63          0          0          0    IO-APIC-edge  ide0
 50:          0          0          0          0   IO-APIC-level  libata
 58:  451845938          0          0          0   IO-APIC-level  
aacraid
 66: 1457978435          0          0          0       PCI-MSI-X  eth0
 74: 1082456599          0          0          0       PCI-MSI-X  eth0
 82:  242944858          0          0          0       PCI-MSI-X  eth0
217:          0          0          0          0   IO-APIC-level  
ohci_hcd:usb1, libata
225:        131          0          0          0   IO-APIC-level  
ehci_hcd:usb2
233:          0          0          0          0   IO-APIC-level  libata
NMI:      75579      30102       9582       7646
LOC:  603897625  603897604  603897576  603897556
ERR:          0
MIS:          0


it's debian etch with the stock 2.6.18 kernel. performance is ok, beside 
the fact that the load climbs up when the cache is about 40-50 % full 
(with a 517GB cache file); below 40 % the load stays below 3, but after 
that level it rises to over 7 and even 10 at peak times. the response 
times are still good, though.

we just installed irqbalance to see if this makes a difference. after 
some minutes (without restarting varnish), the picture according to top 
to "top" is, that 3 out of 4 core do most of the I/O:

Tasks:  89 total,   1 running,  88 sleeping,   0 stopped,   0 zombie
Cpu0  :  0.8%us,  0.8%sy,  0.0%ni, 36.7%id, 60.8%wa,  0.8%hi,  0.0%si,  
0.0%st
Cpu1  :  0.8%us,  1.7%sy,  0.0%ni,  0.0%id, 96.6%wa,  0.0%hi,  0.8%si,  
0.0%st
Cpu2  :  0.0%us,  0.0%sy,  0.0%ni, 97.5%id,  0.8%wa,  0.0%hi,  1.7%si,  
0.0%st
Cpu3  :  0.0%us,  0.8%sy,  0.0%ni,  4.2%id, 95.0%wa,  0.0%hi,  0.0%si,  
0.0%st


I'm posting mostly out of curiosity if this is an expected behavior.


Thanks, Sascha