Re: Finegrained a/c/mtime was Re: Directory notification problem

[Padraig Brady <padraig@antefacto.com>]

Padraig Brady wrote:

> Andi Kleen wrote:
>
>> On Fri, Oct 05, 2001 at 04:00:08PM +0100, Padraig Brady wrote:
>>
>>> Andi Kleen wrote:
>>>
>>>>>> 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.
>>>>>
>>>>> ??
>>>>> if (file->mtime != mtime || file->gen_count != gen_count)
>>>>>    file_changed=1;
>>>>>
>>>> And how would you implement "newer than" and "older than" with a 
>>>> generation
>>>> count that doesn't reset in a always fixed time interval (=requiring
>>>> additional timestamps in kernel)? 
>>>> -Andi
>>>>
>>> Well IMHO "newer than", "older than" applications have until now
>>> done with second resolution, and that's all that's required?
>>>
>>
>> No they haven't. GNU make supports nsec mtime on Solaris and apparently
>> some other OS too, because the second granuality mtime can be a big 
>> problem with make -j<bignumber> on a big SMP box. make has to distingush
>> "is older" from "is newer"; "not equal" alone doesn't cut it.
>>
>> [If you think it is modify your make to replace the "is older" check
>> for dependencies with "is not equal" and see what happens]
>>
>
> OK agreed, in this case the, complete state/relationship between 
> files, must be
> maintained independently of the userspace app, i.e. in the filesystem. 
> But wont
> you then have the same problem with synchronising nanosecond times 
> between
> the various processors (which could be the other side of a network 
> cable in some
> configurations)? So perhaps the best solution is to maintain both a 
> generation
> count which would do for many apps who just care if the file has 
> changed relative
> to some moment it time and not relative to another file(s) on the 
> filesystem .
> Then for make type applications you could maintain the full resolution 
> timestamp,
> however this will still have the synchronisation/portability/CPU 
> expense issues
> discussed previously.


Just thinking that it's VERY hard to synchronise timings to nanosecond 
or even millisecond
resolution over distributed or even within the same filesystem, how 
about you synchronise
the timestamps to the particular filesystem and not the universe. I.E. 
Instead of incrementing
a "generation count" in each inode you could increment a global 
filesystem count everytime
a file is modified in the filesystem, and then this count is stored in 
the particular inode being
modified. This would allow you to have exact order relationships between 
files in the same
filesystem, and would work perfectly every time for both "types" of apps 
mentioned above.
Outside the filesystem you can then resort to just the (second 
resolution) timestamp.

Padraig.