Re: [2.4.17/18pre] VM and swap - it's really unusable

[Daniel Phillips]

On January 8, 2002 10:17 pm, Robert Love wrote:
> On Tue, 2002-01-08 at 15:59, Daniel Phillips wrote:
> 
> > And while I'm enumerating differences, the preemptable kernel (in this 
> > incarnation) has a slight per-spinlock cost, while the non-preemptable kernel 
> > has the fixed cost of checking for rescheduling, at intervals throughout all 
> > 'interesting' kernel code, essentially all long-running loops.  But by clever 
> > coding it's possible to finesse away almost all the overhead of those loop 
> > checks, so in the end, the non-preemptible low-latency patch has a slight 
> > efficiency advantage here, with emphasis on 'slight'.
> 
> True (re spinlock weight in preemptible kernel) but how is that not
> comparable to explicit scheduling points?  Worse, the preempt-kernel
> typically does its preemption on a branch on return to interrupt
> (similar to user space's preemption).  What better time to check and
> reschedule if needed?

The per-spinlock cost I was refering to is the cost of the inc/dec per 
spinlock.  I guess this cost is small enough as to be hard to measure, but
I have not tried so I don't know.  Curiously, none of the people I've heard
making pronouncements on the overhead of your preempt patch seem to have 
measured it either.

--
Daniel