Messages in this thread |  | | Date | Fri, 5 Oct 2001 15:01:44 +0200 | From | Andi Kleen <> | Subject | Re: Finegrained a/c/mtime was Re: Directory notification problem |
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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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