Performance differences with IRQ_ALL_CPUS

[Subodh Nijsure]

Hi,


I have board with MPC8572E (dual core PPC). I have one board running kernel
with IRQ_ALL_CPUS set to Y and another with that option turned off.  Kernel
version #2.6.26


With IRQ_ALL_CPUS turned off
( Here interrupts all go to CPU0 )

hdparm -tT /dev/hda
/dev/hda:
 Timing cached reads:   3048 MB in  2.00 seconds = 1523.79 MB/sec
 Timing buffered disk reads:   10 MB in  3.07 seconds =   3.25 MB/sec

cat /proc/interrupts
           CPU0       CPU1
 18:    1394100          0   OpenPIC   Edge      ide0

With IRQ_ALL_CPUS turned on I see
(Here interrupts go to CPU0 and CPU1 )

hdparm -tT /dev/hda

/dev/hda:
 Timing cached reads:   1076 MB in  2.00 seconds = 538.01 MB/sec
 Timing buffered disk reads:   10 MB in  3.08 seconds =   3.25 MB/sec

cat /proc/interrupts
           CPU0       CPU1
 18:      44951      54765   OpenPIC   Edge      ide0

I "expected" that with IRQ_ALL_CPUS -- interrupt sharing I would be able to
get higher througput but I see things otherway around. 

Would someone care to enlighten me as to why when I set IRQ_ALL_CPUS disk
I/O performance goes down so much?  Under what circumstances should one then
turn on IRQ_ALL_CPUS option on PPC platform? 

Regards,
/Subodh