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SubjectRe: Finegrained a/c/mtime was Re: Directory notification problem
On Fri, Oct 05, 2001 at 01:44:20PM +0100, Padraig Brady wrote:
> Andi Kleen wrote:
> >On Wed, Oct 03, 2001 at 11:15:04AM -0400, Alex Larsson wrote:
> >
> >>Is a nanoseconds field the right choice though? In reality you might not
> >>have a nanosecond resolution timer, so you would miss changes that appear
> >>on shorter timescale than the timer resolution. Wouldn't a generation
> >>counter, increased when ctime was updated, be a better solution?
> >>
> >
> >Near any CPU has a cycle counter builtin now, which gives you ns like
> >resolution. In theory you could still get collisions on MP systems,
> >but window is small enough that it can be ignored in practice.
> >
> >-Andi
> >
> But the point is you, only ever would want nano second resolution to make
> sure you notice all changes to a file. A more general (and much simpler)
> solution would be to gen_count++ every time a file's modified. What other
> applications would require better than second resolution on files?

The main advantage of using a real timestamp instead of a generation
counter is that we would be compatible to Unixware/Solaris/... Their
API is fine, so I see no advantage in inventing a new incompatible one.

Another advantage of using the real time instead of a counter is that
you can easily merge the both values into a single 64bit value and do
arithmetic on it in user space. With a generation counter you would need
to work with number pairs, which is much more complex.
[or alternatively reset the generation counter every second in the kernel
to get a flat time range again,
which would be racy and ugly and complicated in the kernel because it
would need additional timestamps]

Also a rdtsc/get_timestamp or in the worst case a jiffie read is really
not complex to code in kernel, what makes you think it is?

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