Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: [2.4.17/18pre] VM and swap - it's really unusable | Date | Tue, 8 Jan 2002 22:57:28 +0100 |
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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.
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