[lug] q: multithreading on smp weirdness

[D. Stimits]

...
> server D:
> (another new one i found for testing. the best part is
> i managed to test on this server twice.. once when it
> was running RH AS 2.1, another after it was upgraded
> to RH AS 3; & in both cases, it had that multithreading
> weirdness i described earlier).
> has 2 itanium CPUs.
> doesn't have IRQ requests distributed across CPUs.
> (i didn't think to get the kernel version when it was running
> RH AS 2.1)
> when running RH AS 3, the kernel version is
> "2.4.21-4.EL #1 SMP Fri Oct 3 17:29:39 EDT 2003 ia64 ia64 ia64 GNU/Linux"
...

I forgot to mention, on this machine without SMP APIC enabled: It is 
probably a fair test if it is not under a large hardware IRQ load, e.g., 
not huge disk access, any IDE drives have DMA enabled, and network is 
not under huge load. If any IDE drive does not have DMA enabled, or if 
networking is saturated, or if any other device could starve the 100 Hz 
hardware IRQ rate of the 2.4 kernels, then it is not a fair test. Even 
when fair it is possible that threads will suffer since hardware will 
always force any required driver handlers to constantly fill cpu#0 cache 
with it's own data (assuming there is enough pressure on cache that 
software and hardware drivers compete). Most of the time a machine like 
this which is not a heavily loaded machine is at least ok with this. One 
of the things about 2.6 kernels that is supposed to be more efficient is 
the ability to use a higher IRQ rate than 100 Hz, but I have not looked 
into exactly what was done, or whether it is purely software IRQ or 
hardware IRQ also.

D. Stimits, stimits AT comcast DOT net