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SubjectRe: [PATCH 02/18] xstat: Add a pair of system calls to make extended file stats available [ver #6]
Date
Arnd Bergmann <arnd@arndb.de> wrote:

> You could also define the tv_gran_units to be power-of-ten nanoseconds,
> making it a decimal floating point number like
>
> enum {
> XSTAT_NANOSECONDS_GRANULARITY = 0,
> XSTAT_MICROSECONDS_GRANULARITY = 3,
> XSTAT_MILLISECONDS_GRANULARITY = 6,
> XSTAT_SECONDS_GRANULARITY = 9,
> };

Are you thinking, then, of having tv_nsec be in terms of those units?

> That would make it easier to define an xstat_time_before() function, though
> it means that you could no longer do XSTAT_MINUTES_GRANULARITY and
> higher directly other than { .tv_gran_units = 10, .tv_granularity = 6, }.

So you're thinking of indicating time (in)equality based on overlapping time
granules?

Your suggestion would suffice, I think. With a 2:2 split between exponent
(tv_gran_units) and mantissa (tv_granularity), you can do:

UNIT SECONDS/UNIT EXPONENT MANTISSA
nanoseconds 0.000000001 -9 1
microseconds 0.000001 -6 1
millseconds 0.001 -3 1
seconds 1 0 1
minutes 60 1 6
hours 3600 2 36
days 86400 2 864
weeks 604800 2 6048

Any units beyond that are variable length and not worth considering, IMO.

And if you don't want negative numbers in your exponent, you can make the base
unit nS instead of S.

Is it worth allowing a filesystem to indicate that it has granularity smaller
than nS, even if the resolution can't be handled here? We could even have:

struct xstat_time {
signed long long tv_sec; /* seconds */
unsigned int tv_nsec; /* nanoseconds */
unsigned char tv_psec4; /* picoseconds/4 */
signed char tv_gran_exp; /* exponent */
unsigned short tv_gran_mant; /* mantissa */
};

Though it's probably still an unnecessary extravagance to have the pS field.
It's probably best left as padding for now; we can always change our minds
later...

David


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