why "need_resched" per process?

why "need_resched" per process?

[Daniel Rodrick]

Hi,

I was determining a reason for the need_resched flag to be per process
rather than being a global variable. I read that the sole reason is
for performance.

I could not understand how would having it (need_resched) per process
result in a better performance? Having it as a global variable in the
kernel address space would be just a matter of getting the value at a
known address.

On the other hand, having a per process copy sure results in
unecessary memory occupancy.

Thanks,

Dan
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Re: why "need_resched" per process?

[Rik van Riel]

Daniel Rodrick wrote:
> Hi,
> 
> I was determining a reason for the need_resched flag to be per process
> rather than being a global variable. I read that the sole reason is
> for performance.
> 
> I could not understand how would having it (need_resched) per process
> result in a better performance? Having it as a global variable in the
> kernel address space would be just a matter of getting the value at a
> known address.
> 
> On the other hand, having a per process copy sure results in
> unecessary memory occupancy.

1) not every CPU needs to reschedule simultaneously, think
    about an 8-cpu core SMP system where one of the current
    tasks needs to reschedule

2) the cacheline with the current task_struct pointer in
    it is probably in the CPU's L2 cache, since it is used
    very frequently - a global variable would probably not
    be in the CPU cache

3) the thread struct (IIRC need_resched lives there, please
    correct me if I misremember) is the first few bytes of a
    task's kernel thread, so it doesn't really take any extra
    memory

-- 
"You don't have to be crazy to do this... but it helps." -- Bob Ross
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